Global Information 2050

Apple

2008-10-30T13:45:00.000-07:00

This article is about the fruit. For the consumer electronics company, see Apple Inc.. For other uses, see Apple (disambiguation).

Apple

Blossoms, fruits, and leaves of the apple tree (Malus domestica)

Scientific classification

Kingdom:	Plantae
Division:	Magnoliophyta
Class:	Magnoliopsida
Order:	Rosales
Family:	Rosaceae
Subfamily:	Maloideae
Tribe:	Maleae
Genus:	Malus
Species:	M. domestica

Binomial name

Malus domestica
Borkh.

The apple is the pomaceous fruit of the apple tree, species Malus domestica in the rose family Rosaceae. It is one of the most widely cultivated tree fruits. The tree is small and deciduous, reaching 5 to 12 metres (16 to 39 ft) tall, with a broad, often densely twiggy crown.^[1] The leaves are alternately arranged simple ovals 5 to 12 cm long and 3–6 centimetres (1.2–2.4 in) broad on a 2 to 5 centimetres (0.79 to 2.0 in) petiole with an acute tip, serrated margin and a slightly downy underside. Flowers are produced in spring simultaneously with the budding of the leaves. The flowers are white with a pink tinge that gradually fades, five petaled, and 2.5 to 3.5 centimetres (0.98 to 1.4 in) in diameter. The fruit matures in autumn, and is typically 5 to 9 centimetres (2.0 to 3.5 in) diameter. The centre of the fruit contains five carpels arranged in a five-point star, each carpel containing one to three seeds.^[1]Apples are very nutritious.

The tree originated from Central Asia, where its wild ancestor is still found today. There are more than 7,500 known cultivars of apples resulting in range of desired characteristics. Cultivars vary in their yield and the ultimate size of the tree, even when grown on the same rootstock.^[2]

At least 55 million tonnes of apples were grown worldwide in 2005, with a value of about $10 billion. China produced about 35% of this total.^[3] The United States is the second leading producer, with more than 7.5% of the world production. Turkey, France, Italy and Iran are among the leading apple exporters.

Apple

2008-10-30T13:43:00.000-07:00

Greek mythology

Heracles with the apple of Hesperides

Apples appear in many religious traditions, often as a mystical or forbidden fruit. One of the problems identifying apples in religion, mythology and folktales is that the word "apple" was used as a generic term for all (foreign) fruit, other than berries but including nuts, as late as the 17th century.^[7] For instance, in Greek mythology, the Greek hero Heracles, as a part of his Twelve Labours, was required to travel to the Garden of the Hesperides and pick the golden apples off the Tree of Life growing at its center.^[12]^[13]^[14]

The Greek goddess of discord, Eris, became disgruntled after she was excluded from the wedding of Peleus and Thetis.^[15] In retaliation, she tossed a golden apple inscribed Καλλιστή (Kalliste, sometimes transliterated Kallisti, 'For the most beautiful one'), into the wedding party. Three goddesses claimed the apple: Hera, Athena, and Aphrodite. Paris of Troy was appointed to select the recipient. After being bribed by both Hera and Athena, Aphrodite tempted him with the most beautiful woman in the world, Helen of Sparta. He awarded the apple to Aphrodite, thus indirectly causing the Trojan War.

Adam and Eve
Showcasing the apple as a symbol of sin.
Albrecht Dürer, 1507

Atalanta, also of Greek mythology, raced all her suitors in an attempt to avoid marriage. She outran all but Hippomenes (a.k.a. Melanion, a name possibly derived from melon the Greek word for both "apple" and fruit in general),^[13] who defeated her by cunning, not speed. Hippomenes knew that he could not win in a fair race, so he used three golden apples (gifts of Aphrodite, the goddess of love) to distract Atalanta. It took all three apples and all of his speed, but Hippomenes was finally successful, winning the race and Atalanta's hand.^[12]

Apple

2008-10-30T13:38:00.000-07:00

Apple cultivars

See List of apple cultivars for a listing.

Different kinds of apple cultivars in a supermarket

There are more than 7,500 known cultivars of apples. Different cultivars are available for temperate and climates. Reputedly the world's biggest collection of apple cultivars is housed at the National Fruit Collection in England.^[2] Most of these cultivars are bred for eating fresh (dessert apples), though some are cultivated specifically for cooking (cooking apples) or producing cider. Cider apples are typically too tart and astringent to eat fresh, but they give the beverage a rich flavour that dessert apples cannot.

Apple

2008-10-30T13:34:00.000-07:00

Apple production

"Apple Blossom" redirects here. For other uses, see Apple Blossom (disambiguation).

Apple breeding

Like most perennial fruits, apples ordinarily propagate asexually by grafting. Seedling apples are an example of "Extreme heterozygotes", in that rather than inheriting DNA from their parents to create a new apple with those characteristics, they are instead different from their parents, sometimes radically.^[20] Most new apple cultivars originate as seedlings, which either arise by chance or are bred by deliberately crossing cultivars with promising characteristics.^[21] The words 'seedling', 'pippin', and 'kernel' in the name of an apple cultivar suggest that it originated as a seedling. Apples can also form bud sports (mutations on a single branch). Some bud sports turn out to be improved strains of the parent cultivar. Some differ sufficiently from the parent tree to be considered new cultivars.^[22]

Pollination

See also: Fruit tree pollination

Apple tree in flower

Apples are self-incompatible; they must cross-pollinate to develop fruit. During the flowering each season, apple growers usually provide pollinators to carry the pollen. Honeybee hives are most commonly used. Orchard mason bees are also used as supplemental pollinators in commercial orchards. Bumble bee queens are sometimes present in orchards, but not usually in enough quantity to be significant pollinators.^[22]

There are four to seven pollination groups in apples depending on climate:

Group A – Early flowering, May 1 to 3 in England (Gravenstein, Red Astrachan)
Group B – May 4 to 7 (Idared, McIntosh)
Group C – Mid-season flowering, May 8 to 11 (Granny Smith, Cox's Orange Pippin)
Group D – Mid/Late season flowering, May 12 to 15 (Golden Delicious, Calville blanc d'hiver)
Group E – Late flowering, May 16 to 18 (Braeburn, Reinette d'Orléans)
Group F – May 19 to 23 (Suntan)
Group H – May 24 to 28 (Court-Pendu Gris) (also called Court-Pendu plat)

One cultivar can be pollinated by a compatible cultivar from the same group or close (A with A, or A with B, but not A with C or D).^[25]

Apple

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A wide range of pests and diseases can affect the plant; three of the more common diseases/pests are mildew, aphids and apple scab.

Mildew: which is characterized by light grey powdery patches appearing on the leaves, shoots and flowers, normally in spring. The flowers will turn a creamy yellow colour and will not develop correctly. This can be treated in a manner not dissimilar from treating Botrytis; eliminating the conditions which caused the disease in the first place and burning the infected plants are among the recommended actions to take.^[29]^[29]

Feeding aphids

Aphids: There are five species of aphids commonly found on apples: apple grain aphid, rosy apple aphid, apple aphid, spirea aphid and the woolly apple aphid. The aphid species can be identified by their colour, the time of year when they are present and by differences in the cornicles, which are small paired projections from the rear of aphids.^[29] Aphids feed on foliage using needlelike mouthparts to suck out plant juices. When present in high numbers, certain species may reduce tree growth and vigor.^[30]
Apple scab: Symptoms of Scab are olive-green or brown blotches on the leaves.^[31] The blotches turn more brown as time progresses. Then brown scabs on the fruit (see apple picture on the left).^[29] The diseased leaves will fall early and the fruit will become increasingly covered in scabs - eventually the fruit skin will crack. Although there are chemicals to treat Scab, their use might not be encouraged as they are quite often systematic, which means they are absorbed by the tree, and spread throughout the fruit.^[31]

Among the most serious disease problems are fireblight, a bacterial disease; and Gymnosporangium rust, and black spot, two fungal diseases.^[30]

Young apple trees are also prone to mammal pests like mice and deer, which feed on the soft bark of the trees, especially in winter.

Trees in mythology

2008-10-29T08:11:00.000-07:00

Yggdrasil, the World Ash (Norse)

Trees have played an important role in many of the world's mythologies and religions, and have been given deep and sacred meanings throughout the ages. The most ancient cross-cultural symbolic representation of the universe's construction is the world tree. Other examples of trees featured in mythology are Yggdrasil and the modern tradition of the Christmas Tree in Germanic mythology, the Tree of Knowledge of Judaism and Christianity, and the Bodhi tree in Buddhism. In folk religion and folklore, trees are often said to be the homes of tree spirits. Historical Druidism as well as Germanic paganism appear to have involved cultic practice in sacred groves. The term druid itself possibly derives from the Celtic word for oak. Ficus religiosa plays an important role in Indian mythology.

Trees are a necessary attribute of the archetypical locus amoenus in all cultures. Already the Egyptian Book of the Dead mentions sycomores as part of the scenery where the soul of the deceased finds blissful repose (Gollwitzer p. 13).

Various forms of trees of life also appear in folklore, culture and fiction, often relating to immortality or fertility. These often hold cultural and religious significance to the peoples for whom they appear. For them, it may also strongly be connected with motif of the world tree.

The tree, with its branches reaching up into the sky, and roots deep into the earth, can be seen to dwell in three worlds - a link between heaven, the earth, and the underworld, uniting above and below. It is also both a feminine symbol, bearing sustenance; and a masculine, phallic symbol - another union.

In literature, a mythology was notably developed by J. R. R. Tolkien, his Two Trees of Valinor playing a central role in his mythopoeic cosmogony. Tolkien's 1964 Tree and Leaf combines the allegorical tale Leaf by Niggle and his essay On Fairy-Stories. William Butler Yeats describes a "holy tree" in his poem The Two Trees (1893).

Ultraviolet

2008-10-24T12:45:00.000-07:00

"UV" redirects here. For other uses of UV, see UV (disambiguation).

False-color image of the solar corona as seen in deep ultraviolet light at 17.1 nm by the Extreme ultraviolet Imaging Telescope instrument aboard the SOHO spacecraft

An ultraviolet photograph of the Earth taken from the Moon by Apollo 16 astronauts.

Ultraviolet (UV) light is electromagnetic radiation with a wavelength shorter than that of visible light, but longer than X-rays. It is so named because the spectrum consists of electromagnetic waves with frequencies higher than those that humans identify as the color violet.

UV light is typically found as part of the radiation received by the Earth from the Sun. Most humans are aware of the effects of UV through the painful condition of sunburn. The UV spectrum has many other effects, including both beneficial and damaging changes to human health.

[hide]

[edit] Discovery

The discovery of UV radiation was intimately associated with the observation that silver salts darken when exposed to sunlight. In 1801 the German physicist Johann Wilhelm Ritter made the hallmark observation that invisible rays just beyond the violet end of the visible spectrum were especially effective at darkening silver chloride-soaked paper. He called them "de-oxidizing rays" to emphasize their chemical reactivity and to distinguish them from "heat rays" at the other end of the visible spectrum. The simpler term "chemical rays" was adopted shortly thereafter, and it remained popular throughout the 19th century. The terms chemical and heat rays were eventually dropped in favor of ultraviolet and infrared radiation, respectively.^[1]

[edit] Origin of term

The name means "beyond violet" (from Latin ultra, "beyond"), violet being the color of the shortest wavelengths of visible light. UV light has a shorter wavelength than that of violet light.

[edit] Subtypes

The electromagnetic spectrum of ultraviolet light can be subdivided in a number of ways. The draft ISO standard on determining solar irradiances (ISO-DIS-21348)^[2] describes the following ranges:

Name	Abbreviation	Wavelength range in nanometers	Energy per photon
Ultraviolet A, long wave, or black light	UVA	400 nm – 315 nm	3.10 – 3.94 eV
Near	NUV	400 nm – 300 nm	3.10 – 4.13 eV
Ultraviolet B or medium wave	UVB	315 nm – 280 nm	3.94 – 4.43 eV
Middle	MUV	300 nm – 200 nm	4.13 – 6.20 eV
Ultraviolet C, short wave, or germicidal	UVC	280 nm – 100 nm	4.43 – 12.4 eV
Far	FUV	200 nm – 122 nm	6.20 – 10.2 eV
Vacuum	VUV	200 nm – 10 nm	6.20 – 124 eV
Extreme	EUV	121 nm – 10 nm	10.2 – 124 eV

In photolithography and laser technology, the term deep ultraviolet or DUV refers to wavelengths below 300 nm. "Vacuum UV" is so named because it is absorbed strongly by air and is therefore used in a vacuum. In the long-wave limit of this region, roughly 150–200 nm, the principal absorber is the oxygen in air. Work in this region can be performed in an oxygen free atmosphere, pure nitrogen being commonly used, which avoids the need for a vacuum chamber.

See 1 E-7 m for a list of objects of comparable sizes.

[edit] Black light

Main article: Black light

A black light, or Wood's light, is a lamp that emits long wave UV radiation and very little visible light. Commonly these are referred to as simply a "UV light". Fluorescent black lights are typically made in the same fashion as normal fluorescent lights except that only one phosphor is used and the normally clear glass envelope of the bulb may be replaced by a deep-bluish-purple glass called Wood's glass, a nickel-oxide–doped glass, which blocks almost all visible light above 400 nanometers. The color of such lamps is often referred to in the trade as "blacklight blue" or "BLB." This is to distinguish these lamps from "bug zapper" blacklight ("BL") lamps that don't have the blue Wood's glass. The phosphor typically used for a near 368 to 371 nanometer emission peak is either europium-doped strontium fluoroborate (SrB4O7F:Eu2+) or europium-doped strontium borate (SrB4O7:Eu2+) while the phosphor used to produce a peak around 350 to 353 nanometers is lead-doped barium silicate (BaSi2O5:Pb+). "Blacklight Blue" lamps peak at 365 nm.

While "black lights" do produce light in the UV range, their spectrum is confined to the longwave UVA region. Unlike UVB and UVC, which are responsible for the direct DNA damage that leads to skin cancer, black light is limited to lower energy, longer waves and does not cause sunburn. However, UVA is capable of causing damage to collagen fibers and destroying vitamin A in skin.

A black light may also be formed by simply using Wood's glass instead of clear glass as the envelope for a common incandescent bulb. This was the method used to create the very first black light sources. Though it remains a cheaper alternative to the fluorescent method, it is exceptionally inefficient at producing UV light (less than 0.1% of the input power) owing to the black body nature of the incandescent light source. Incandescent UV bulbs, due to their inefficiency, may also become dangerously hot during use. More rarely still, high power (hundreds of watts) mercury vapor black lights can be found which use a UV emitting phosphor and an envelope of Wood's glass. These lamps are used mainly for theatrical and concert displays and also become very hot during normal use.

Some UV fluorescent bulbs specifically designed to attract insects for use in bug zappers use the same near-UV emitting phosphor as normal blacklights, but use plain glass instead of the more expensive Wood's glass. Plain glass blocks less of the visible mercury emission spectrum, making them appear light blue to the naked eye. These lamps are referred to as "blacklight" or "BL" in most lighting catalogs.

Ultraviolet light can also be generated by some light-emitting diodes.

[edit] Natural sources of UV

The Sun emits ultraviolet radiation in the UVA, UVB, and UVC bands, but because of absorption in the atmosphere's ozone layer, 98.7% of the ultraviolet radiation that reaches the Earth's surface is UVA. (Some of the UVB and UVC radiation is responsible for the generation of the ozone layer.)

Ordinary glass is partially transparent to UVA but is opaque to shorter wavelengths while Silica or quartz glass, depending on quality, can be transparent even to vacuum UV wavelengths. Ordinary window glass passes about 90% of the light above 350 nm, but blocks over 90% of the light below 300 nm.^[3]^[4]^[5]

The onset of vacuum UV, 200 nm, is defined by the fact that ordinary air is opaque below this wavelength. This opacity is due to the strong absorption of light of these wavelengths by oxygen in the air. Pure nitrogen (less than about 10 ppm oxygen) is transparent to wavelengths in the range of about 150–200 nm. This has wide practical significance now that semiconductor manufacturing processes are using wavelengths shorter than 200 nm. By working in oxygen-free gas, the equipment does not have to be built to withstand the pressure differences required to work in a vacuum. Some other scientific instruments, such as circular dichroism spectrometers, are also commonly nitrogen purged and operate in this spectral region.

Extreme UV is characterized by a transition in the physics of interaction with matter: wavelengths longer than about 30 nm interact mainly with the chemical valence electrons of matter, while wavelengths shorter than that interact mainly with inner shell electrons and nuclei. The long end of the EUV/XUV spectrum is set by a prominent He⁺ spectral line at 30.4nm. XUV is strongly absorbed by most known materials, but it is possible to synthesize multilayer optics that reflect up to about 50% of XUV radiation at normal incidence. This technology has been used to make telescopes for solar imaging; it was pioneered by the NIXT and MSSTA sounding rockets in the 1990s; (current examples are SOHO/EIT and TRACE) and for nanolithography (printing of traces and devices on microchips).

[edit] Human health-related effects of UV radiation

[edit] Beneficial effects

The Earth's atmosphere blocks UV radiation from penetrating through the atmosphere by 98.7%. A positive effect of UVB exposure is that it induces the production of vitamin D in the skin. It has been estimated that tens of thousands of premature deaths occur in the United States annually from a range of cancers due to vitamin D deficiency.^[6] Another effect of vitamin D deficiency is osteomalacia (the adult equivalent of rickets), which can result in bone pain, difficulty in weight bearing and sometimes fractures. Other studies show most people get adequate Vitamin D through food and incidental exposure.^[7]

Many countries have fortified certain foods with Vitamin D to prevent deficiency. Eating fortified foods or taking a dietary supplement pill is usually preferred to UVB exposure, due to the increased risk of skin cancer from UV radiation.^[7]

Too little UVB radiation leads to a lack of Vitamin D. Too much UVB radiation leads to direct DNA damages and sunburn. An appropriate amount of UVB (What is appropriate depends on your skin colour) leads to a limited amount of direct DNA damage. This is recognized and repaired by the body. Then the melanin production is increased which leads to a long lasting tan. This tan occurs with a 2 day lag phase after irradiation, but it is much less harmful and long lasting than the one obtained from UVA.

Ultraviolet radiation has other medical applications, in the treatment of skin conditions such as psoriasis and vitiligo. UVA radiation can be used in conjunction with psoralens (PUVA treatment). UVB radiation is rarely used in conjunction with psoralens. In cases of psoriasis and vitiligo, UV light with wavelength of 311 nm is most effective.^{[citation needed]}

[edit] Harmful effects

An overexposure to UVB radiation can cause sunburn and some forms of skin cancer. In humans, prolonged exposure to solar UV radiation may result in acute and chronic health effects on the skin, eye, and immune system.^[8] However the most deadly form - malignant melanoma - is mostly caused by the indirect DNA damage (free radicals and oxidative stress). This can be seen from the absence of a UV-signature mutation in 92% of all melanoma.^[9]

UVC rays are the highest energy, most dangerous type of ultraviolet light. Little attention has been given to UVC rays in the past since they are filtered out by the atmosphere. However, their use in equipment such as pond sterilization units may pose an exposure risk, if the lamp is switched on outside of its enclosed pond sterilization unit.

Ultraviolet photons harm the DNA molecules of living organisms in different ways. In one common damage event, adjacent Thymine bases bond with each other, instead of across the "ladder". This makes a bulge, and the distorted DNA molecule does not function properly.

[edit] Skin

“	Ultraviolet (UV) irradiation present in sunlight is an environmental human carcinogen. The toxic effects of UV from natural sunlight and therapeutic artificial lamps are a major concern for human health. The major acute effects of UV irradiation on normal human skin comprise sunburn inflammation erythema, tanning, and local or systemic immunosuppression.	”
	— Matsumura and Ananthaswamy , (2004)^[10]

UVA, UVB and UVC can all damage collagen fibers and thereby accelerate aging of the skin. Both UVA and UVB destroy vitamin A in skin which may cause further damage.^[11] In the past UVA was considered less harmful, but today it is known that it can contribute to skin cancer via the indirect DNA damage (free radicals and reactive oxygen species). It penetrates deeply but it does not cause sunburn. UVA does not damage DNA directly like UVB and UVC, but it can generate highly reactive chemical intermediates, such as hydroxyl and oxygen radicals, which in turn can damage DNA. Because it does not cause reddening of the skin (erythema) it cannot be measured in the SPF testing. There is no good clinical measurement of the blocking of UVA radiation, but it is important that sunscreen block both UVA and UVB. Some scientists blame the absence of UVA filters in sunscreens for the higher melanoma-risk that was found for sunscreen users. ^[12]

The reddening of the skin due to the action of sunlight depends both on the amount of sunlight as well as the sensitivity of the skin ("erythemal action spectrum") over the UV spectrum.

UVB light can cause direct DNA damage. The radiation excites DNA molecules in skin cells, causing aberrant covalent bonds to form between adjacent cytosine bases, producing a dimer. When DNA polymerase comes along to replicate this strand of DNA, it reads the dimer as "AA" and not the original "CC". This causes the DNA replication mechanism to add a "TT" on the growing strand. This is a mutation, which can result in cancerous growths and is known as a "classical C-T mutation". The mutations that are caused by the direct DNA damage carry a UV signature mutation that is commonly seen in skin cancers. The mutagenicity of UV radiation can be easily observed in bacteria cultures. This cancer connection is one reason for concern about ozone depletion and the ozone hole. UVB causes some damage to collagen but at a very much slower rate than UVA.

As a defense against UV radiation, the amount of the brown pigment melanin in the skin increases when exposed to moderate (depending on skin type) levels of radiation; this is commonly known as a sun tan. The purpose of melanin is to absorb UV radiation and dissipate the energy as harmless heat, blocking the UV from damaging skin tissue. UVA gives a quick tan that lasts for days by oxidizing melanin that was already present and triggers the release of the melanin from melanocytes. UVB yields a tan that takes roughly 2 days to develop because it stimulates the body to produce more melanin. The photochemical properties of melanin make it an excellent photoprotectant. However, sunscreen chemicals can not dissipate the energy of the excited state as efficiently as melanin and therefore the penetration of sunscreen ingredients into the lower layers of the skin is increasing the amount of free radicals and ROS.^[13]

Sunscreen prevents the direct DNA damage which causes sunburn. Most of these products contain an SPF rating to show how well they block UVB rays. The SPF rating, however, offers no data about UVA protection. In the US, the FDA is considering adding a star rating system to show UVA protection. A similar system is already used in some European countries.

Some sunscreen lotions now include compounds such as titanium dioxide which helps protect against UVA rays. Other UVA blocking compounds found in sunscreen include zinc oxide and avobenzone. Cantaloupe extract, rich in the compound superoxide dismutase (SOD), can be bound with gliadin to form glisodin, an orally-effective protectant against UVB radiation. There are also naturally occurring compounds found in rainforest plants that have been known to protect the skin from UV radiation damage, such as the fern Phlebodium aureum.

[edit] Sunscreen safety debate

Main article: Sunscreen controversy

Medical organizations recommend that patients protect themselves from UV radiation using sunscreen. Five sunscreen ingredients have been shown to protect mice against skin tumors (see sunscreen).

However, some sunscreen chemicals produce potentially harmful substances if they are illuminated while in contact with living cells.^[14]^[15]^[16] The amount of sunscreen which penetrates through the stratum corneum may or may not be large enough to cause damage. In one study of sunscreens, the authors write:^[17]

The question whether UV filters acts on or in the skin has so far not been fully answered. Despite the fact that an answer would be a key to improve formulations of sun protection products, many publications carefully avoid addressing this question.

In an experiment that was published in 2006 by Hanson et al, the amount of harmful reactive oxygen species (ROS) had been measured in untreated and in sunscreen treated skin. In the first 20 minutes the film of sunscreen had a protective effect and the number of ROS species was smaller. After 60 minutes however the amount of absorbed sunscreen was so high, that the amount of ROS was higher in the sunscreen treated skin than in the untreated skin.^[13]

[edit] Eye

High intensities of UVB light are hazardous to the eyes, and exposure can cause welder's flash (photokeratitis or arc eye) and may lead to cataracts, pterygium,^[18]^[19] and pinguecula formation.

Protective eyewear is beneficial to those who are working with or those who might be exposed to ultraviolet radiation, particularly short wave UV. Given that light may reach the eye from the sides, full coverage eye protection is usually warranted if there is an increased risk of exposure, as in high altitude mountaineering. Mountaineers are exposed to higher than ordinary levels of UV radiation, both because there is less atmospheric filtering and because of reflection from snow and ice.

Ordinary, untreated eyeglasses give some protection. Most plastic lenses give more protection than glass lenses, because, as noted above, glass is transparent to UVA and the common acrylic plastic used for lenses is less so. Some plastic lens materials, such as polycarbonate, inherently block most UV. There are protective treatments available for eyeglass lenses that need it which will give better protection. But even a treatment that completely blocks UV will not protect the eye from light that arrives around the lens.

[edit] Degradation of polymers, pigments and dyes

Many polymers used in consumer products are degraded by UV light, and need addition of UV absorbers to inhibit attack, especially if the products are used externally and so exposed to sunlight. The problem appears as discoloration or fading, cracking and sometimes, total product disintegration if cracking has proceeded far enough. The rate of attack increases with exposure time and sunlight intensity.

It is known as UV degradation, and is one form of polymer degradation. Sensitive polymers include thermoplastics, such as polypropylene and polyethylene as well as speciality fibres like aramids. UV absorption leads to chain degradation and loss of strength at sensitive points in the chain structure. They include tertiary carbon atoms, which in polypropylene occur in every repeat unit.

In addition, many pigments and dyes absorb UV and change colour, so paintings and textiles may need extra protection both from sunlight and fluorescent lamps, two common sources of UV radiation. Old and antique paintings such as watercolour paintings for example, usually need to be placed away from direct sunlight. Common window glass provides some protection by absorbing some of the harmful UV, but valuable artifacts need shielding.

[edit] Blockers and absorbers

Ultraviolet Light Absorbers (UVAs) are molecules used in organic materials (polymers, paints, etc.) to absorb UV light in order to reduce the UV degradation (photo-oxidation) of a material. A number of different UVAs exist with different absorption properties. UVAs can disappear over time, so monitoring of UVA levels in weathered materials is necessary.

In sunscreen, ingredients which absorb UVA/UVB rays, such as avobenzone and octyl methoxycinnamate, are known as absorbers. They are contrasted with physical "blockers" of UV radiation such as titanium dioxide and zinc oxide. (See sunscreen for a more complete list.)

[edit] Applications of UV

[edit] Security

A bird appears on many Visa credit cards when held under a UV light source.

To help thwart counterfeiters, sensitive documents (e.g. credit cards, driver's licenses, passports) may also include a UV watermark that can only be seen when viewed under a UV-emitting light. Passports issued by most countries usually contain UV sensitive inks and security threads. Visa stamps and stickers on passports of visitors contain large and detailed seals invisible to the naked eye under normal lights, but strongly visible under UV illumination. Passports issued by many nations have UV sensitive watermarks on all pages of the passport. Currencies of various countries' banknotes have an image, as well as many multicolored fibers, that are visible only under ultraviolet light.

Some brands of pepper spray will leave an invisible chemical (UV Dye) that is not easily washed off on a pepper sprayed attacker, which would help police identify them later. ^[20]

[edit] Fluorescent lamps

Fluorescent lamps produce UV radiation by ionising low-pressure mercury vapour. A phosphorescent coating on the inside of the tubes absorbs the UV and converts it to visible light.

The main mercury emission wavelength is in the UVC range. Unshielded exposure of the skin or eyes to mercury arc lamps that do not have a conversion phosphor is quite dangerous.

The light from a mercury lamp is predominantly at discrete wavelengths. Other practical UV sources with more continuous emission spectra include xenon arc lamps (commonly used as sunlight simulators), deuterium arc lamps, mercury-xenon arc lamps, metal-halide arc lamps, and tungsten-halogen incandescent lamps.

[edit] Astronomy

Aurora at Jupiter's north pole as seen in ultraviolet light by the Hubble Space Telescope.

In astronomy, very hot objects preferentially emit UV radiation (see Wien's law). Because the ozone layer blocks many UV frequencies from reaching telescopes on the surface of the Earth, most UV observations are made from space. (See UV astronomy, space observatory.)

[edit] Biological surveys and pest control

Some animals, including birds, reptiles, and insects such as bees, can see into the near ultraviolet. Many fruits, flowers, and seeds stand out more strongly from the background in ultraviolet wavelengths as compared to human color vision. Scorpions glow or take on a yellow to green color under UV illumination. Many birds have patterns in their plumage that are invisible at usual wavelengths but observable in ultraviolet, and the urine and other secretions of some animals, including dogs, cats, and human beings, is much easier to spot with ultraviolet.

Many insects use the ultraviolet wavelength emissions from celestial objects as references for flight navigation. A local ultraviolet emissor will normally disrupt the navigation process and will eventually attract the flying insect.

Entomologist using a UV light for collecting beetles in the Paraguayan Chaco.

Ultraviolet traps called bug zappers are used to eliminate various small flying insects. They are attracted to the UV light, and are killed using an electric shock, or trapped once they come into contact with the device. Different designs of ultraviolet light traps are also used by entomologists for collecting nocturnal insects during faunistic survey studies.

[edit] Spectrophotometry

UV/VIS spectroscopy is widely used as a technique in chemistry, to analyze chemical structure, most notably conjugated systems. UV radiation is often used in visible spectrophotometry to determine the existence of fluorescence in a given sample.

[edit] Analyzing minerals

A collection of mineral samples brilliantly fluorescing at various wavelengths as seen while being irradiated by UV light.

Ultraviolet lamps are also used in analyzing minerals, gems, and in other detective work including authentication of various collectibles. Materials may look the same under visible light, but fluoresce to different degrees under ultraviolet light; or may fluoresce differently under short wave ultraviolet versus long wave ultraviolet.

[edit] Chemical markers

UV fluorescent dyes are used in many applications (for example, biochemistry and forensics). The Green Fluorescent Protein (GFP) is often used in genetics as a marker. Many substances, such as proteins, have significant light absorption bands in the ultraviolet that are of use and interest in biochemistry and related fields. UV-capable spectrophotometers are common in such laboratories.

[edit] Photochemotherapy

Exposure to UVA light while the skin is hyper-photosensitive by taking psoralens is an effective treatment for psoriasis called PUVA. Due to psoralens potentially causing damage to the liver, PUVA may only be used a limited number of times over a patient's lifetime

[edit] Phototherapy

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Exposure to UVB light, particularly the 310 nm narrowband UVB range, is an effective long-term treatment for many skin conditions like psoriasis, vitiligo, eczema, and others^[21]. UVB phototherapy does not require additional medications or topical preparations for the therapeutic benefit; only the light exposure is needed. However, phototherapy can be effective when used in conjunction with certain topical treatments such as anthralin, coal tar, and Vitamin A and D derivatives, or systemic treatments such as methotrexate and soriatane.^[22]

Typical treatment regimes involve short exposure to UVB rays 3 to 5 times a week at a hospital or clinic, and for the best results, up to 30 or more sessions may be required.

Side effects may include itching and redness of the skin due to UVB exposure, and possibly sunburn, if patients do not minimize exposure to natural UV rays during treatment days.

[edit] Photolithography

Ultraviolet radiation is used for very fine resolution photolithography, a procedure where a chemical known as a photoresist is exposed to UV radiation which has passed through a mask. The light allows chemical reactions to take place in the photoresist, and after development (a step that either removes the exposed or unexposed photoresist), a geometric pattern which is determined by the mask remains on the sample. Further steps may then be taken to "etch" away parts of the sample with no photoresist remaining.

UV radiation is used extensively in the electronics industry because photolithography is used in the manufacture of semiconductors, integrated circuit components^[23] and printed circuit boards.

[edit] Checking electrical insulation

A new application of UV is to detect corona discharge (often simply called "corona") on electrical apparatus. Degradation of insulation of electrical apparatus or pollution causes corona, wherein a strong electric field ionizes the air and excites nitrogen molecules, causing the emission of ultraviolet radiation. The corona degrades the insulation level of the apparatus. Corona produces ozone and to a lesser extent nitrogen oxide which may subsequently react with water in the air to form nitrous acid and nitric acid vapour in the surrounding air.^[24]

[edit] Sterilization

Main article: Ultraviolet germicidal irradiation

A low pressure mercury vapor discharge tube floods the inside of a hood with shortwave UV light when not in use, sterilizing microbiological contaminants from irradiated surfaces.

Ultraviolet lamps are used to sterilize workspaces and tools used in biology laboratories and medical facilities. Commercially-available low pressure mercury-vapor lamps emit about 86% of their light at 254 nanometers (nm) which coincides very well with one of the two peaks of the germicidal effectiveness curve (i.e., effectiveness for UV absorption by DNA). One of these peaks is at about 265 nm and the other is at about 185 nm. Although 185 nm is better absorbed by DNA, the quartz glass used in commercially-available lamps, as well as environmental media such as water, are more opaque to 185 nm than 254 nm (C. von Sonntag et al., 1992). UV light at these germicidal wavelengths causes adjacent thymine molecules on DNA to dimerize, if enough of these defects accumulate on a microorganism's DNA its replication is inhibited, thereby rendering it harmless (even though the organism may not be killed outright). However, since microorganisms can be shielded from ultraviolet light in small cracks and other shaded areas, these lamps are used only as a supplement to other sterilization techniques.

[edit] Disinfecting drinking water

UV radiation can be an effective viricide and bactericide. Disinfection using UV radiation is commonly used in wastewater treatment applications and is finding an increased usage in drinking water treatment. Many bottlers of spring water use UV disinfection equipment to sterilize their water. Solar water disinfection is the process of using PET bottles and sunlight to disinfect water.

New York City has approved the construction of a 2 billion gallon per day ultraviolet drinking water disinfection facility^[25]. There are also several facilities under construction and several in operation that treat waste water with several stages of filters, hydrogen peroxide and UV light to bring the water up to drinking standards. One such facility exists in Orange County California. ^[26] ^[27]

It used to be thought that UV disinfection was more effective for bacteria and viruses, which have more exposed genetic material, than for larger pathogens which have outer coatings or that form cyst states (e.g., Giardia) that shield their DNA from the UV light. However, it was recently discovered that ultraviolet radiation can be somewhat effective for treating the microorganism Cryptosporidium. The findings resulted in two US patents and the use of UV radiation as a viable method to treat drinking water. Giardia in turn has been shown to be very susceptible to UV-C when the tests were based on infectivity rather than excystation.^[28] It has been found that protists are able to survive high UV-C doses but are sterilized at low doses.

Solar water disinfection [1] (SODIS) has been extensively researched in Switzerland and has proven ideal to treat small quantities of water cheaply using natural sunlight. Contaminated water is poured into transparent plastic bottles and exposed to full sunlight for six hours. The sunlight treats the contaminated water through two synergetic mechanisms: UV-A irradiation and increased water temperature. If the water temperatures rises above 50 °C, the disinfection process is three times faster.

[edit] Food processing

As consumer demand for fresh and "fresh-like" food products increases, the demand for nonthermal methods of food processing is likewise on the rise. In addition, public awareness regarding the dangers of food poisoning is also raising demand for improved food processing methods. Ultraviolet radiation is used in several food processes to kill unwanted microorganisms. UV light can be used to pasteurize fruit juices by flowing the juice over a high intensity ultraviolet light source. The effectiveness of such a process depends on the UV absorbance of the juice (see Beer's law).

[edit] Fire detection

Ultraviolet detectors generally use either a solid-state device, such as one based on silicon carbide or aluminium nitride, or a gas-filled tube as the sensing element. UV detectors which are sensitive to UV light in any part of the spectrum respond to irradiation by sunlight and artificial light. A burning hydrogen flame, for instance, radiates strongly in the 185 to 260 nanometer range and only very weakly in the IR region, while a coal fire emits very weakly in the UV band yet very strongly at IR wavelengths; thus a fire detector which operates using both UV and IR detectors is more reliable than one with a UV detector alone. Virtually all fires emit some radiation in the UVC band, while the Sun's radiation at this band is absorbed by the Earth's atmosphere. The result is that the UV detector is "solar blind", meaning it will not cause an alarm in response to radiation from the Sun, so it can easily be used both indoors and outdoors.

UV detectors are sensitive to most fires, including hydrocarbons, metals, sulfur, hydrogen, hydrazine, and ammonia. Arc welding, electrical arcs, lightning, X-rays used in nondestructive metal testing equipment (though this is highly unlikely), and radioactive materials can produce levels that will activate a UV detection system. The presence of UV-absorbing gases and vapors will attenuate the UV radiation from a fire, adversely affecting the ability of the detector to detect flames. Likewise, the presence of an oil mist in the air or an oil film on the detector window will have the same effect.

[edit] Herpetology

Most reptile keepers are aware that reptiles need long wave UV light to metabolize calcium for bone and egg production. Thus, in a typical reptile enclosure, a fluorescent UV lamp is used at one end of the enclosure for calcium absorption and a common incandescent bulb is used at the other end for heat (basking).

[edit] Curing of inks, adhesives, varnishes and coatings

Certain inks, coatings and adhesives are formulated with photoinitiators and resins. When exposed to the correct energy and irradiance in the required band of UV light, polymerization occurs, and so the adhesives harden or cure. Usually, this reaction is very quick, a matter of a few seconds. Applications include glass and plastic bonding, optical fiber coatings, the coating of flooring, UV Coating and paper finishes in offset printing, and dental fillings.

An industry has developed around the manufacture of UV lamps sourced for UV curing applictions. Fast processes such as flexo or offset printing require high intensity light focused via reflectors onto a moving substrate and medium and high pressure Hg (mercury) or Fe (iron) based bulbs are used which can be energised with electric arc or microwaves. Lower power fluorescent lamps can be used for static applications and in some cases, small high pressure lamps can have light focused and transmitted to the work area via liquid filled or fibre optic light guides.

Radtech is a trade association dedicated to the promotion of this technology.

[edit] Deterring substance abuse in public places

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UV lights have been installed in some parts of the world in public restrooms, and on public transport, for the purpose of deterring substance abuse. The blue color of these lights, combined with the fluorescence of the skin, make it harder for intravenous drug users to find a vein.^[29] The efficacy of these lights for that purpose has been questioned, with some suggesting that drug users simply find a vein outside the public restroom and mark the spot with a marker for accessibility when inside the restroom. There is currently no published evidence supporting the idea of a deterrent effect.

[edit] Sun tanning

Sun tanning describes a darkening of the skin in a natural physiological response stimulated by exposure to ultraviolet radiation from sunshine (or a sunbed). With excess exposure to the sun, a suntanned area can also develop sunburn. The increased production of melanin is triggered by the direct DNA damage.^[30] This kind of damage is recognized by the body and as a defense against UV radiation the skin produces more melanin. Melanin dissipates the UV energy as harmless heat, and therefore it is an excellent photoprotectant. Melanin protects against the direct DNA damage and against the indirect DNA damage. Sunscreen protects only against the direct DNA damage, but increases the indirect DNA damage^[14]^[15]^[16] - this causes the higher amount of melanoma that had been found repeatedly in sunscreen users compared to non-users.^[31]^[32]^[12]^[33]^[34]

[edit] Erasing EPROM modules

Some EPROM (electronically programmable read-only memory) modules are erased by exposure to UV radiation. These modules often have a transparent glass (quartz) window on the top of the chip that allows the UV radiation in. These have been largely superseded by EEPROM and flash memory chips in most devices.

[edit] Preparing low surface energy polymers

UV radiation is useful in preparing low surface energy polymers for adhesives. Polymers exposed to UV light will oxidize thus raising the surface energy of the polymer. Once the surface energy of the polymer has been raised, the bond between the adhesive and the polymer will not be smaller.

[edit] Reading otherwise illegible papyruses

Using multi-spectral imaging it is possible to read illegible papyruses, such as the burned papyruses of the Villa of the Papyri or of Oxyrhynchus. The technique involves taking pictures of the illegible papyruses using different filters in the infrared or ultraviolet range, finely tuned to capture certain wavelengths of light. Thus, the optimum spectral portion can be found for distinguishing ink from paper on the papyrus surface.

[edit] Lasers

Ultraviolet lasers have applications in industry (laser engraving), medicine (dermatology and keratectomy), free air secure communications and computing (optical storage). They can be made by applying frequency conversion to lower-frequency lasers, or from Ce:LiSAF crystals (cerium doped with lithium strontium aluminum fluoride), a process developed in the 1990s at Lawrence Livermore National Laboratory.^[35]

[edit] Evolutionary significance

Evolution of early reproductive proteins and enzymes is attributed in modern models of evolutionary theory to ultraviolet light. UVB light causes thymine base pairs next to each other in genetic sequences to bond together into thymine dimers, a disruption in the strand which reproductive enzymes cannot copy (see picture above). This leads to frameshifting during genetic replication and protein synthesis, usually killing the organism. As early prokaryotes began to approach the surface of the ancient oceans, before the protective ozone layer had formed, blocking out most wavelengths of UV light, they almost invariably died out. The few that survived had developed enzymes which verified the genetic material and broke up thymine dimer bonds, known as excision repair enzymes. Many enzymes and proteins involved in modern mitosis and meiosis are extremely similar to excision repair enzymes, and are believed to be evolved modifications of the enzymes originally used to overcome UV light.^[36]

Geology

2008-10-24T12:35:00.000-07:00

65 Ma Continental Crust Shield Platform Orogen Basin Large igneous province Extended crust">65 Ma Continental Crust Shield Platform Orogen Basin Large igneous province Extended crust" src="http://upload.wikimedia.org/wikipedia/commons/thumb/a/a9/World_geologic_provinces.jpg/300px-World_geologic_provinces.jpg" class="thumbimage" border="0" height="159" width="300">

World geologic provinces
Oceanic crust 0-20 Ma 20-65 Ma >65 Ma Continental Crust Shield Platform Orogen Basin Large igneous province Extended crust

At Wikiversity you can learn more and teach others about Geology at:

The School of Geology

Geology (from Greek: γη, gê, "earth"; and λόγος, logos, "speech" lit. to talk about the earth) is the science and study of the solid and liquid matter that constitute the Earth. The field of geology encompasses the study of the composition, structure, physical properties, dynamics, and history of Earth materials, and the processes by which they are formed, moved, and changed. The field is important in academics, industry (due to mineral and hydrocarbon extraction), and for social issues such as geotechnical engineering, the mitigation of natural hazards, and knowledge about past climate and climate change.

[edit] Etymology

The word "geology" was first used by Jean-André Deluc in the year 1778 and introduced as a fixed term by Horace-Bénédict de Saussure in the year 1779. The science was not included in Encyclopædia Britannica's third edition completed in 1797, but had a lengthy entry in the fourth edition completed by 1809.^[1] An older meaning of the word was first used by Richard de Bury to distinguish between earthly and theological jurisprudence.

[edit] History

Main article: History of geology

A mosquito and a fly in this Baltic amber necklace are between 40 and 60 million years old

The work Peri Lithon (On Stones) by Theophrastus (372-287 BC), a student of Aristotle, remained authoritative for millennia. Peri Lithon was translated into Latin and some other foreign languages. Its interpretation of fossils was the most dominant theory in classical Antiquity and the early Middle Ages, until it was replaced by Avicenna's theory of petrifying fluids (succus lapidificatus) in the late Middle Ages.^[2]^[3] In the Roman period, Pliny the Elder produced a very extensive discussion of many more minerals and metals then widely used for practical ends. He is among the first to correctly identify the origin of amber as a fossilized resin from pine trees by the observation of insects trapped within some pieces. He also laid the basis of crystallography by recognising the octahedral habit of diamond.

Some modern scholars, such as Fielding H. Garrison, are of the opinion that modern geology began in the medieval Islamic world.^[4] Abu al-Rayhan al-Biruni (973-1048 AD) was one of the earliest Muslim geologists, whose works included the earliest writings on the geology of India, hypothesizing that the Indian subcontinent was once a sea.^[5] Ibn Sina (Avicenna, 981-1037), in particular, made significant contributions to geology and the natural sciences (which he called Attabieyat) along with other natural philosophers such as Ikhwan AI-Safa and many others. He wrote an encyclopaedic work entitled “Kitab al-Shifa” (the Book of Cure, Healing or Remedy from ignorance), in which Part 2, Section 5, contains his essay on Mineralogy and Meteorology, in six chapters: Formation of mountains, The advantages of mountains in the formation of clouds; Sources of water; Origin of earthquakes; Formation of minerals; The diversity of earth’s terrain. These principles were later known in the Renaissance of Europe as the law of superposition of strata, the concept of catastrophism, and the doctrine of uniformitarianism. These concepts were also embodied in the Theory of the Earth by James Hutton in the Eighteenth century C.E. Academics such as Toulmin and Goodfield (1965), commented on Avicenna's contribution: "Around A.D. 1000, Avicenna was already suggesting a hypothesis about the origin of mountain ranges, which in the Christian world, would still have been considered quite radical eight hundred years later".^[6] Avicenna's scientific methodology of field observation was also original in the Earth sciences, and remains an essential part of modern geological investigations.^[3]

In China, the polymath Shen Kua (1031-1095) formulated a hypothesis for the process of land formation: based on his observation of fossil animal shells in a geological stratum in a mountain hundreds of miles from the ocean, he inferred that the land was formed by erosion of the mountains and by deposition of silt.

Georg Agricola (1494-1555), a physician, wrote the first systematic treatise about mining and smelting works, De re metallica libri XII, with an appendix Buch von den Lebewesen unter Tage (Book of the Creatures Beneath the Earth). He covered subjects like wind energy, hydrodynamic power, melting cookers, transport of ores, extraction of soda, sulfur and alum, and administrative issues. The book was published in 1556. Nicolas Steno (1638-1686) is credited with the law of superposition, the principle of original horizontality, and the principle of lateral continuity: three defining principles of stratigraphy. Previous attempts at such statements met accusations of heresy from the Church.^{[citation needed]}

By the 1700s Jean-Étienne Guettard and Nicolas Desmarest hiked central France and recorded their observations on geological maps; Guettard recorded the first observation of the volcanic origins of this part of France.

William Smith's geologic map of England, Wales, and southern Scotland. completed in 1815, it was the first national-scale geologic map, and by far the most accurate of its time.

William Smith (1769-1839) drew some of the first geological maps and began the process of ordering rock strata (layers) by examining the fossils contained in them.

James Hutton is often viewed as the first modern geologist. In 1785 he presented a paper entitled Theory of the Earth to the Royal Society of Edinburgh. In his paper, he explained his theory that the Earth must be much older than had previously been supposed in order to allow enough time for mountains to be eroded and for sediments to form new rocks at the bottom of the sea, which in turn were raised up to become dry land. Hutton published a two-volume version of his ideas in 1795 (Vol. 1, Vol. 2).

The geologist, 19th century painting by Carl Spitzweg.

Followers of Hutton were known as Plutonists because they believed that some rocks were formed by vulcanism which is the deposition of lava from volcanoes, as opposed to the Neptunists, who believed that all rocks had settled out of a large ocean whose level gradually dropped over time.

In 1811 Georges Cuvier and Alexandre Brongniart published their explanation of the antiquity of the Earth, inspired by Cuvier's discovery of fossil elephant bones in Paris. To prove this, they formulated the principle of stratigraphic succession of the layers of the earth. They were independently anticipated by William Smith's stratigraphic studies on England and Scotland.

Sir Charles Lyell first published his famous book, Principles of Geology, in 1830. Lyell continued to publish new revisions until he died in 1875. The book, which influenced the thought of Charles Darwin, successfully promoted the doctrine of uniformitarianism. This theory states that slow geological processes have occurred throughout the Earth's history and are still occurring today. In contrast, catastrophism is the theory that Earth's features formed in single, catastrophic events and remained unchanged thereafter. Though Hutton believed in uniformitarianism, the idea was not widely accepted at the time.

Plate tectonics - seafloor spreading and continental drift illustrated on relief globe of the Field Museum

19th century geology revolved around the question of the Earth's exact age. Estimates varied from a few 100,000 to billions of years. The most significant advance in 20th century geology has been the development of the theory of plate tectonics in the 1960s. Plate tectonic theory arose out of two separate geological observations: seafloor spreading and continental drift. The theory revolutionized the Earth sciences.

The theory of continental drift was proposed by Frank Bursley Taylor in 1908, expanded by Alfred Wegener in 1912 and by Arthur Holmes, but wasn't broadly accepted until the late 1960s when the theory of plate tectonics was developed.

[edit] Important principles in the Development of Geology

There are a number of important principles that were developed near the beginning of geology as a formal science. Many of these involve the ability to provide the relative ages of strata or the manner in which they were formed. These principles are still often used today as a means to provide information about geologic history and the timing of geologic events.

The principle of intrusive relationships concerns crosscutting intrusions. In geology, when an igneous intrusion cuts across a formation of sedimentary rock, it can be determined that the igneous intrusion is younger than the sedimentary rock. There are a number of different types of intrusions, including stocks, laccoliths, batholiths, sills and dikes.

The principle of cross-cutting relationships pertains to the formation of faults and the age of the sequences through which they cut. Faults are younger than the rocks they cut; accordingly, if a fault is found that penetrates some formations but not those on top of it, then the formations that were cut are older than the fault, and the ones that are not cut must be younger than the fault. Finding the key bed in these situations may help determine whether the fault is a normal fault or a thrust fault.

The principle of inclusions and components states that, with sedimentary rocks, if inclusions (or clasts) are found in a formation, then the inclusions must be older than the formation that contains them. For example, in sedimentary rocks, it is common for gravel from an older formation to be ripped up and included in a newer layer. A similar situation with igneous rocks occurs when xenoliths are found. These foreign bodies are picked up as magma or lava flows, and are incorporated, later to cool in the matrix. As a result, xenoliths are older than the rock which contains them.

The principle of uniformitarianism states that the geologic processes observed in operation that modify the Earth's crust at present have worked in much the same way over geologic time. A fundamental principle of geology advanced by the 18th century Scottish physician and geologist James Hutton, is that "the present is the key to the past." In Hutton's words: "the past history of our globe must be explained by what can be seen to be happening now."

The principle of original horizontality states that the deposition of sediments occurs as essentially horizontal beds. Observation of modern marine and non-marine sediments in a wide variety of environments supports this generalization (although cross-bedding is inclined, the overall orientation of cross-bedded units is horizontal).

The principle of superposition states that a sedimentary rock layer in a tectonically undisturbed sequence is younger than the one beneath it and older than the one above it. Logically a younger layer cannot slip beneath a layer previously deposited. This principle allows sedimentary layers to be viewed as a form of vertical time line, a partial or complete record of the time elapsed from deposition of the lowest layer to deposition of the highest bed.

The principle of faunal succession is based on the appearance of fossils in sedimentary rocks. As organisms exist at the same time period throughout the world, their presence or (sometimes) absence may be used to provide a relative age of the formations in which they are found. Based on principles laid out by William Smith almost a hundred years before the publication of Charles Darwin's theory of evolution, the principles of succession were developed independently of evolutionary thought. The principle becomes quite complex, however, given the uncertainties of fossilization, the localization of fossil types due to lateral changes in habitat (facies change in sedimentary strata), and that not all fossils may be found globally at the same time.

[edit] Modern Geology

[edit] Radioactive decay and the Age of the Earth

Main article: Radiometric dating

A large advance in geology in the advent of the 20th century was the ability to use ratios of radioactive isotopes to find the amount of time that has passed since a rock passed through a particular temperature.

Main article: Age of the Earth

Geologists have established the age of the Earth at about 4.54 billion (4.5x10⁹) years, and the age of the oldest planetary material (Carbonaceous Chondrite meteorites) at 4.567 billion years through the use of Uranium-lead dating.

Oceanic-continental convergence resulting in subduction and volcanic arcs illustrates one effect of plate tectonics.

[edit] Plate Tectonics

Main article: plate tectonics

Geologists have determined that the that the Earth's lithosphere, which includes the crust and rigid uppermost portion of the upper mantle, is separated into a number of tectonic plates. These tectonic plates move across the plastically-deforming, solid, upper mantle, which is called the asthenosphere. There is an intimate coupling between the movement of the plates on the surface and the convection of the mantle: plate motions and mantle convection currents always move in the same direction. This coupling between rigid plates moving on the surface of the Earth and the convecting mantle is called plate tectonics.

[edit] Earth Structure

Main article: Structure of the Earth

Earth layered structure. (1) inner core; (2) outer core; (3) lower mantle; (4) uppper mantle; (5) lithosphere; (6) crust

Advances in seismology, computer modelling, and mineralogy and crystallography at high temperatures and pressures give insights into the internal composition and structure of the Earth.

Earth layered structure. Typical wave paths from earthquakes like these gave early seismologists insights into the layered structure of the Earth

Seismologists can use the arrival times of seismic waves in reverse to image the interior of the Earth. Early advances in this field showed the existence of a liquid outer core (where shear waves were not able to propagate) and a dense solid inner core. These advances led to the development of a layered model of the Earth, with a crust and lithosphere on top, the mantle below (separated within itself by seismic discontinuities at 410 and 660 kilometers), and the outer core and inner core below that. More recently, seismologists have been able to create detailed images of wave speeds inside the earth in the same way a doctor images a body in a CT scan. These images have led to a much more detailed view of the interior of the Earth, and have replaced the simplified layered model with a much more dynamic model.

The seismically-imaged Farallon Plate subducting beneath North America. The only remnants of this plate on the Surface are the Juan de Fuca Plate and Explorer plate in the Northwestern USA / Southwestern Canada, and the Cocos Plate on the west coast of Mexico.

Mineralogists have been able to use the pressure and temperature data from the seismic and modelling studies alongside knowledge of the elemental composition of the Earth at depth to reproduce these conditions in experimental settings and measure changes in crystal structure. These studies explain the chemical changes associated with the major seismic discontinuities in the mantle, and show the crystallographic structures expected in the inner core of the Earth.

[edit] Planetary Geology

Surface of Mars as photographed by the Viking 2 lander December 9, 1977.

Main article: Planetary geology

Main article: Geology of solar terrestrial planets

With the advent of space exploration in the twentieth century, geologists have begun to look at other planetary bodies in the same way as the Earth. This has led to the oxymoron term, commonly used in the professional literature, of planetary geology.

Planetary geology (sometimes known as Astrogeology) refers to the application of geologic principles to other bodies of the solar system. Specialised terms such as selenology (studies of the moon), areology (of Mars), etc., are also in use. Colloquially, geology is most often used with another noun when indicating extra-Earth bodies (e.g. "the geology of Mars").

[edit] Societal Applications of Geology

[edit] Economic Geology

Main article: Economic Geology

Geologists help locate and manage the Earth's natural resources, such as petroleum and coal, as well as metals such as iron, copper, and uranium. Additional economic interests include gemstones and many minerals such as asbestos, perlite, mica, phosphates, zeolites, clay, pumice, quartz, and silica, as well as elements such as sulfur, chlorine, and helium.

[edit] Soil Mechanics and Geotechnical Engineering

Main article: soil mechanics

Main article: geotechnical engineering

In the field of civil engineering, geological principles and analyses are used in order to ascertain the mechanical principles of the material on which structures are built.

[edit] Hydrology and Environmental Issues

Geology and geologic principles can be applied to various environmental problems, such as stream restoration, the restoration of brownfields, and the understanding of the interactions between natural habitat and the geologic environment. Groundwater hydrology, or hydrogeology, is used to provide water in arid regions and to monitor the spread of contaminants in groundwater wells.

Geologists also obtain data through stratigraphy, boreholes, and core samples, including ice cores, which tell geologists about past and present climate and ecosystems. These data are our primary source of information on global climate change outside of instrumental data.

[edit] Natural Hazards

Main article: Natural hazard

Geologists and geophysicists study natural hazards in order to enact safe building codes and warning systems that are used to prevent loss of property and life.

[edit] Fields or related disciplines

An illustrated depiction of a syncline and anticline commonly studied in Structural geology and Geomorphology.

[edit] Regional geology

[edit] By nations

Geology of the United States of America
- US geology by state:

- US Geology by region or feature:
  - Geology of the Appalachians
  - Geology of the Pacific Northwest
  - Geology of the Bryce Canyon area(Utah)
  - Geology of the Canyonlands area (Utah)
  - Geology of the Capitol Reef area (Utah)
  - Geology of the Death Valley area (California)
  - Geology of the Grand Canyon area (Arizona)
  - Geology of the Grand Teton area (Wyoming)
  - Geology of the Lassen area (California)
  - Geology of Mount Adams (Washington)
  - Geology of Mount Shasta (California)
  - Geology of the Yosemite area (California)
  - Geology of the Zion and Kolob canyons area (Utah)
  - Glacial geology of the Genesee River (New York, Pennsylvania)

Planetary Geology

History of Earth

2008-10-22T12:25:00.000-07:00

For the history of modern humans, see History of the world.

Geological time put in a diagram called a geological clock, showing the relative lengths of the eons of the Earth's history.

The history of Earth covers approximately 4.6 billion years (4,567,000,000 years), from Earth’s formation out of the solar nebula to the present. This article presents a broad overview, summarizing the leading, most current scientific theories.

[hide]

[edit] Origin

An artist's impression of protoplanetary disk.

Main article: Formation and evolution of the Solar System

The Earth formed as part of the birth of the Solar System: what eventually became the solar system initially existed as a large, rotating cloud of dust, rocks, and gas. It was composed of hydrogen and helium produced in the Big Bang, as well as heavier elements ejected by supernovas. Then, as one theory suggests, about 4.6 billion years ago a nearby star was destroyed in a supernova and the explosion sent a shock wave through the solar nebula, causing it to gain angular momentum. As the cloud began to accelerate its rotation, gravity and inertia flattened it into a protoplanetary disk oriented perpendicularly to its axis of rotation. Most of the mass concentrated in the middle and began to heat up, but small perturbations due to collisions and the angular momentum of other large debris created the means by which protoplanets began to form. The infall of material, increase in rotational speed and the crush of gravity created an enormous amount of kinetic heat at the center. Its inability to transfer that energy away through any other process at a rate capable of relieving the build-up resulted in the disk's center heating up. Ultimately, nuclear fusion of hydrogen into helium began, and eventually, after contraction, a T Tauri star ignited to create the Sun. Meanwhile, as gravity caused matter to condense around the previously perturbed objects outside of the new sun's gravity grasp, dust particles and the rest of the protoplanetary disk began separating into rings. Successively larger fragments collided with one another and became larger objects, ultimately destined to become protoplanets.^[1] These included one collection approximately 150 million kilometers from the center: Earth. The solar wind of the newly formed T Tauri star cleared out most of the material in the disk that had not already condensed into larger bodies.

[edit] Moon

Animation (not to scale) of Theia forming in Earth’s L₅ point and then, perturbed by gravity, colliding to help form the moon. The animation progresses in one-year steps making Earth appear not to move. The view is of the south pole.

Main articles: Origin and geologic evolution and Giant impact hypothesis

The origin of the Moon is still uncertain, although much evidence exists for the giant impact hypothesis. Earth may not have been the only planet forming 150 million kilometers from the Sun. It is hypothesized that another collection occurred 150 million kilometers from both the Sun and the Earth, at their fourth or fifth Lagrangian point. This planet, named Theia, is thought to have been smaller than the current Earth, probably about the size and mass of Mars. Its orbit may at first have been stable, but destabilized as Earth increased its mass by the accretion of more and more material. Theia swung back and forth relative to Earth until, finally, an estimated 4.533 billion years ago,^[2] it collided at a low, oblique angle. The low speed and angle were not enough to destroy Earth, but a large portion of its crust was ejected into space. Heavier elements from Theia sank to Earth’s core, while the remaining material and ejecta condensed into a single body within a couple of weeks. Under the influence of its own gravity, this became a more spherical body: the Moon.^[3] The impact is also thought to have changed Earth’s axis to produce the large 23.5° axial tilt that is responsible for Earth’s seasons. (A simple, ideal model of the planets’ origins would have axial tilts of 0° with no recognizable seasons.) It may also have sped up Earth’s rotation and initiated the planet’s plate tectonics.

[edit] The Hadean eon

Main article: Hadean

Volcanic eruptions would have been common in Earth's early days.

The early Earth, during the very early Hadean eon, was very different from the world known today. There were no oceans and no oxygen in the atmosphere. It was bombarded by planetoids and other material left over from the formation of the solar system. This bombardment, combined with heat from radioactive breakdown, residual heat, and heat from the pressure of contraction, caused the planet at this stage to be fully molten. During the iron catastrophe heavier elements sank to the center while lighter ones rose to the surface producing the layered structure of the Earth and also setting up the formation of Earth's magnetic field. Earth's early atmosphere would have comprised surrounding material from the solar nebula, especially light gases such as hydrogen and helium, but the solar wind and Earth's own heat would have driven off this atmosphere.

This changed when Earth was about 40% its present radius, and gravitational attraction allowed the retention of an atmosphere which included water. Temperatures plummeted and the crust of the planet was accumulated on a solid surface, with areas melted by large impacts on the scale of decades to hundreds of years between impacts. Large impacts would have caused localized melting and partial differentiation, with some lighter elements on the surface or released to the moist atmosphere.^[4]

The surface cooled quickly, forming the solid crust within 150 million years;^[5] although new research^[6] suggests that the actual number is 100 million years based on the level of hafnium found in the geology at Jack Hills in Western Australia. From 4 to 3.8 billion years ago, Earth underwent a period of heavy asteroidal bombardment.^[7] Steam escaped from the crust while more gases were released by volcanoes, completing the second atmosphere. Additional water was imported by bolide collisions, probably from asteroids ejected from the outer asteroid belt under the influence of Jupiter's gravity. The planet cooled. Clouds formed. Rain gave rise to the oceans within 750 million years (3.8 billion years ago), but probably earlier. Recent evidence suggests the oceans may have begun forming by 4.2 billion years ago^[8].^[9] The new atmosphere probably contained ammonia, methane, water vapor, carbon dioxide, and nitrogen, as well as smaller amounts of other gases. Any free oxygen would have been bound by hydrogen or minerals on the surface. Volcanic activity was intense and, without an ozone layer to hinder its entry, ultraviolet radiation flooded the surface.

[edit] Life

The replicator in virtually all known life is deoxyribonucleic acid. DNA is far more complex than the original replicator and its replication systems are highly elaborate.

Main article: Origin of life

The details of the origin of life are unknown, though the broad principles have been established. Two schools of thought regarding the origin of life have been proposed. The first suggests that organic components may have arrived on Earth from space (see “Panspermia”), while the other argues for terrestrial origins. The mechanisms by which life would initially arise are nevertheless held to be similar.^[10] If life arose on Earth, the timing of this event is highly speculative—perhaps it arose around 4 billion years ago.^[11] In the energetic chemistry of early Earth, a molecule (or even something else) gained the ability to make copies of itself–the replicator. The nature of this molecule is unknown, its function having long since been superseded by life’s current replicator, DNA. In making copies of itself, the replicator did not always perform accurately: some copies contained an “error.” If the change destroyed the copying ability of the molecule, there could be no more copies, and the line would “die out.” On the other hand, a few rare changes might make the molecule replicate faster or better: those “strains” would become more numerous and “successful.” As choice raw materials (“food”) became depleted, strains which could exploit different materials, or perhaps halt the progress of other strains and steal their resources, became more numerous.^[12]

Several different models have been proposed explaining how a replicator might have developed. Different replicators have been posited, including organic chemicals such as modern proteins, nucleic acids, phospholipids, crystals,^[13] or even quantum systems.^[14] There is currently no method of determining which of these models, if any, closely fits the origin of life on Earth. One of the older theories, and one which has been worked out in some detail, will serve as an example of how this might occur. The high energy from volcanoes, lightning, and ultraviolet radiation could help drive chemical reactions producing more complex molecules from simple compounds such as methane and ammonia.^[15] Among these were many of the relatively simple organic compounds that are the building blocks of life. As the amount of this “organic soup” increased, different molecules reacted with one another. Sometimes more complex molecules would result—perhaps clay provided a framework to collect and concentrate organic material.^[16] The presence of certain molecules could speed up a chemical reaction. All this continued for a very long time, with reactions occurring more or less at random, until by chance there arose a new molecule: the replicator. This had the bizarre property of promoting the chemical reactions which produced a copy of itself, and evolution began properly. Other theories posit a different replicator. In any case, DNA took over the function of the replicator at some point; all known life (with the exception of some viruses and prions) use DNA as their replicator, in an almost identical manner (see genetic code).

[edit] Cells

A small section of a cell membrane. This modern cell membrane is far more sophisticated than the original simple phospholipid bilayer (the small blue spheres with two tails). Proteins and carbohydrates serve various functions in regulating the passage of material through the membrane and in reacting to the environment.

Modern life has its replicating material packaged neatly inside a cellular membrane. It is easier to understand the origin of the cell membrane than the origin of the replicator, since the phospholipid molecules that make up a cell membrane will often form a bilayer spontaneously when placed in water. Under certain conditions, many such spheres can be formed (see “The bubble theory”).^[17] It is not known whether this process preceded or succeeded the origin of the replicator (or perhaps it was the replicator). The prevailing theory is that the replicator, perhaps RNA by this point (the RNA world hypothesis), along with its replicating apparatus and maybe other biomolecules, had already evolved. Initial protocells may have simply burst when they grew too large; the scattered contents may then have recolonized other “bubbles.” Proteins that stabilized the membrane, or that later assisted in an orderly division, would have promoted the proliferation of those cell lines. RNA is a likely candidate for an early replicator since it can both store genetic information and catalyze reactions. At some point DNA took over the genetic storage role from RNA, and proteins known as enzymes took over the catalysis role, leaving RNA to transfer information and modulate the process. There is increasing belief that these early cells may have evolved in association with underwater volcanic vents known as “black smokers”.^[18] or even hot, deep rocks.^[19] However, it is believed that out of this multiplicity of cells, or protocells, only one survived. Current evidence suggests that the last universal common ancestor lived during the early Archean eon, perhaps roughly 3.5 billion years ago or earlier.^[20]^,^[21] This “LUCA” cell is the ancestor of all cells and hence all life on Earth. It was probably a prokaryote, possessing a cell membrane and probably ribosomes, but lacking a nucleus or membrane-bound organelles such as mitochondria or chloroplasts. Like all modern cells, it used DNA as its genetic code, RNA for information transfer and protein synthesis, and enzymes to catalyze reactions. Some scientists believe that instead of a single organism being the last universal common ancestor, there were populations of organisms exchanging genes in lateral gene transfer.^[20]

[edit] Photosynthesis and oxygen

The harnessing of the sun’s energy led to several major changes in life on Earth.

It is likely that the initial cells were all heterotrophs, using surrounding organic molecules (including those from other cells) as raw material and an energy source.^[22] As the food supply diminished, a new strategy evolved in some cells. Instead of relying on the diminishing amounts of free-existing organic molecules, these cells adopted sunlight as an energy source. Estimates vary, but by about 3 billion years ago^[23], something similar to modern photosynthesis had probably developed. This made the sun’s energy available not only to autotrophs but also to the heterotrophs that consumed them. Photosynthesis used the plentiful carbon dioxide and water as raw materials and, with the energy of sunlight, produced energy-rich organic molecules (carbohydrates).

Moreover, oxygen was produced as a waste product of photosynthesis. At first it became bound up with limestone, iron, and other minerals. There is substantial proof of this in iron-oxide rich layers in geological strata that correspond with this time period. The oceans would have turned to a green color while oxygen was reacting with minerals. When the reactions stopped, oxygen could finally enter the atmosphere. Though each cell only produced a minute amount of oxygen, the combined metabolism of many cells over a vast period of time transformed Earth’s atmosphere to its current state.^[24] Among the oldest examples of oxygen-producing lifeforms are fossil Stromatolites.

This, then, is Earth’s third atmosphere. Some of the oxygen was stimulated by incoming ultraviolet radiation to form ozone, which collected in a layer near the upper part of the atmosphere. The ozone layer absorbed, and still absorbs, a significant amount of the ultraviolet radiation that once had passed through the atmosphere. It allowed cells to colonize the surface of the ocean and ultimately the land:^[25] without the ozone layer, ultraviolet radiation bombarding the surface would have caused unsustainable levels of mutation in exposed cells. Besides making large amounts of energy available to life-forms and blocking ultraviolet radiation, the effects of photosynthesis had a third, major, and world-changing impact. Oxygen was toxic; probably much life on Earth died out as its levels rose (the “Oxygen Catastrophe”).^[25] Resistant forms survived and thrived, and some developed the ability to use oxygen to enhance their metabolism and derive more energy from the same food.

[edit] Endosymbiosis and the three domains of life

Main article: Endosymbiotic theory

Some of the pathways by which the various endosymbionts might have arisen.

Modern taxonomy classifies life into three domains. The time of the origin of these domains are speculative. The Bacteria domain probably first split off from the other forms of life (sometimes called Neomura), but this supposition is controversial. Soon after this, by 2 billion years ago,^[26] the Neomura split into the Archaea and the Eukarya. Eukaryotic cells (Eukarya) are larger and more complex than prokaryotic cells (Bacteria and Archaea), and the origin of that complexity is only now coming to light. Around this time period a bacterial cell related to today’s Rickettsia^[27] entered a larger prokaryotic cell. Perhaps the large cell attempted to ingest the smaller one but failed (maybe due to the evolution of prey defenses). Perhaps the smaller cell attempted to parasitize the larger one. In any case, the smaller cell survived inside the larger cell. Using oxygen, it was able to metabolize the larger cell’s waste products and derive more energy. Some of this surplus energy was returned to the host. The smaller cell replicated inside the larger one, and soon a stable symbiotic relationship developed. Over time the host cell acquired some of the genes of the smaller cells, and the two kinds became dependent on each other: the larger cell could not survive without the energy produced by the smaller ones, and these in turn could not survive without the raw materials provided by the larger cell. Symbiosis developed between the larger cell and the population of smaller cells inside it to the extent that they are considered to have become a single organism, the smaller cells being classified as organelles called mitochondria. A similar event took place with photosynthetic cyanobacteria^[28] entering larger heterotrophic cells and becoming chloroplasts.^[29]^,^[30] Probably as a result of these changes, a line of cells capable of photosynthesis split off from the other eukaryotes some time before one billion years ago. There were probably several such inclusion events, as the figure at right suggests. Besides the well-established endosymbiotic theory of the cellular origin of mitochondria and chloroplasts, it has been suggested that cells gave rise to peroxisomes, spirochetes gave rise to cilia and flagella, and that perhaps a DNA virus gave rise to the cell nucleus,^[31]^,^[32] though none of these theories are generally accepted.^[33] During this period, the supercontinent Columbia is believed to have existed, probably from around 1.8 to 1.5 billion years ago; it is the oldest hypothesized supercontinent.^[34]

[edit] Multicellularity

Volvox aureus is believed to be similar to the first multicellular plants.

Archaeans, bacteria, and eukaryotes continued to diversify and to become more sophisticated and better adapted to their environments. Each domain repeatedly split into multiple lineages, although little is known about the history of the archaea and bacteria. Around 1.1 billion years ago, the supercontinent Rodinia was assembling.^[35] The plant, animal, and fungi lines had all split, though they still existed as solitary cells. Some of these lived in colonies, and gradually some division of labor began to take place; for instance, cells on the periphery might have started to assume different roles from those in the interior. Although the division between a colony with specialized cells and a multicellular organism is not always clear, around 1 billion years ago,^[36] the first multicellular plants emerged, probably green algae.^[37] Possibly by around 900 million years ago,^[38] true multicellularity had also evolved in animals. At first it probably somewhat resembled that of today’s sponges, where all cells were totipotent and a disrupted organism could reassemble itself.^[39] As the division of labor became more complete in all lines of multicellular organisms, cells became more specialized and more dependent on each other; isolated cells would die. Many scientists believe that a very severe ice age began around 770 million years ago, so severe that the surface of all the oceans completely froze (Snowball Earth). Eventually, after 20 million years, enough carbon dioxide escaped through volcanic outgassing that the resulting greenhouse effect raised global temperatures.^[40] By around the same time, 750 million years ago,^[41] Rodinia began to break up.

[edit] Colonization of land

For most of Earth’s history, there were no multicellular organisms on land. Parts of the surface may have vaguely resembled this view of Mars, one of Earth’s neighboring planets.^{[citation needed]}

Oxygen accumulation from photosynthesis resulted in the formation of an ozone layer that absorbed much of Sun’s ultraviolet radiation, meaning unicellular organisms that reached land were less likely to die, and prokaryotes began to multiply and become better adapted to survival out of the water. Prokaryotes had likely colonized the land as early as 2.6 billion years ago^[42] even before the origin of the eukaryotes. For a long time, the land remained barren of multicellular organisms. The supercontinent Pannotia formed around 600 million years ago and then broke apart a short 50 million years later.^[43] Fish, the earliest vertebrates, evolved in the oceans around 530 million years ago.^[44] A major extinction event occurred near the end of the Cambrian period,^[45] which ended 488 million years ago^[46].

Several hundred million years ago, plants (probably resembling algae) and fungi started growing at the edges of the water, and then out of it.^[47] The oldest fossils of land fungi and plants date to 480–460 million years ago, though molecular evidence suggests the fungi may have colonized the land as early as 1000 million years ago and the plants 700 million years ago.^[48] Initially remaining close to the water’s edge, mutations and variations resulted in further colonization of this new environment. The timing of the first animals to leave the oceans is not precisely known: the oldest clear evidence is of arthropods on land around 450 million years ago^[49], perhaps thriving and becoming better adapted due to the vast food source provided by the terrestrial plants. There is also some unconfirmed evidence that arthropods may have appeared on land as early as 530 million years ago^[50]. At the end of the Ordovician period, 440 million years ago, additional extinction events occurred, perhaps due to a concurrent ice age.^[51] Around 380 to 375 million years ago, the first tetrapods evolved from fish.^[52] It is thought that perhaps fins evolved to become limbs which allowed the first tetrapods to lift their heads out of the water to breathe air. This would let them survive in oxygen-poor water or pursue small prey in shallow water.^[52] They may have later ventured on land for brief periods. Eventually, some of them became so well adapted to terrestrial life that they spent their adult lives on land, although they hatched in the water and returned to lay their eggs. This was the origin of the amphibians. About 365 million years ago, another period of extinction occurred, perhaps as a result of global cooling.^[53] Plants evolved seeds, which dramatically accelerated their spread on land, around this time (by approximately 360 million years ago).^[54]^,^[55]

Pangaea, the most recent supercontinent, existed from 300 to 180 million years ago. The outlines of the modern continents and other land masses are indicated on this map.

Some 20 million years later (340 million years ago^[56]), the amniotic egg evolved, which could be laid on land, giving a survival advantage to tetrapod embryos. This resulted in the divergence of amniotes from amphibians. Another 30 million years (310 million years ago^[57]) saw the divergence of the synapsids (including mammals) from the sauropsids (including birds and non-avian, non-mammalian reptiles). Other groups of organisms continued to evolve and lines diverged—in fish, insects, bacteria, and so on—but less is known of the details. 300 million years ago, the most recent hypothesized supercontinent formed, called Pangaea. The most severe extinction event to date took place 250 million years ago, at the boundary of the Permian and Triassic periods; 95% of life on Earth died out,^[58] possibly due to the Siberian Traps volcanic event. The discovery of the Wilkes Land crater in Antarctica may suggest a connection with the Permian-Triassic extinction, but the age of that crater is not known.^[59] But life persevered, and around 230 million years ago ^[60], dinosaurs split off from their reptilian ancestors. An extinction event between the Triassic and Jurassic periods 200 million years ago spared many of the dinosaurs,^[61] and they soon became dominant among the vertebrates. Though some of the mammalian lines began to separate during this period, existing mammals were probably all small animals resembling shrews.^[62] By 180 million years ago, Pangaea broke up into Laurasia and Gondwana. The boundary between avian and non-avian dinosaurs is not clear, but Archaeopteryx, traditionally considered one of the first birds, lived around 150 million years ago.^[63] The earliest evidence for the angiosperms evolving flowers is during the Cretaceous period, some 20 million years later (132 million years ago)^[64] Competition with birds drove many pterosaurs to extinction, and the dinosaurs were probably already in decline for various reasons^[65] when, 65 million years ago, a 10-kilometer meteorite likely struck Earth just off the Yucatán Peninsula, ejecting vast quantities of particulate matter and vapor into the air that occluded sunlight, inhibiting photosynthesis. Most large animals, including the non-avian dinosaurs, became extinct.^[66] marking the end of the Cretaceous period and Mesozoic era. Thereafter, in the Paleocene epoch, mammals rapidly diversified, grew larger, and became the dominant vertebrates. Perhaps a couple of million years later (around 63 million years ago), the last common ancestor of primates lived.^[67] By the late Eocene epoch, 34 million years ago, some terrestrial mammals had returned to the oceans to become animals such as Basilosaurus which later gave rise to dolphins and whales.^[68]

[edit] Humanity

Australopithecus africanus, an early hominid.

Main article: Human evolution

A small African ape living around six million years ago was the last animal whose descendants would include both modern humans and their closest relatives, the bonobos, and chimpanzees.^[69] Only two branches of its family tree have surviving descendants. Very soon after the split, for reasons that are still debated, apes in one branch developed the ability to walk upright.^[70] Brain size increased rapidly, and by 2 million years ago, the very first animals classified in the genus Homo had appeared.^[71] Of course, the line between different species or even genera is rather arbitrary as organisms continuously change over generations. Around the same time, the other branch split into the ancestors of the common chimpanzee and the ancestors of the bonobo as evolution continued simultaneously in all life forms.^[69] The ability to control fire likely began in Homo erectus (or Homo ergaster), probably at least 790,000 years ago^[72] but perhaps as early as 1.5 million years ago.^[73] In addition it has sometimes suggested that the use and discovery of controlled fire may even predate Homo erectus. Fire was possibly used by the early Lower Paleolithic (Oldowan) hominid Homo habilis and/or by robust australopithecines such as Paranthropus.^[74] However it is more difficult to establish the origin of language; it is unclear whether Homo erectus could speak or if that capability had not begun until Homo sapiens.^[75] As brain size increased, babies were born sooner, before their heads grew too large to pass through the pelvis. As a result, they exhibited more plasticity, and thus possessed an increased capacity to learn and required a longer period of dependence. Social skills became more complex, language became more advanced, and tools became more elaborate. This contributed to further cooperation and brain development.^[76] Anatomically modern humans — Homo sapiens — are believed to have originated somewhere around 200,000 years ago or earlier in Africa; the oldest fossils date back to around 160,000 years ago.^[77] The first humans to show evidence of spirituality are the Neanderthals (usually classified as a separate species with no surviving descendants); they buried their dead, often apparently with food or tools.^[78] However, evidence of more sophisticated beliefs, such as the early Cro-Magnon cave paintings (probably with magical or religious significance)^[79] did not appear until some 32,000 years ago.^[80] Cro-Magnons also left behind stone figurines such as Venus of Willendorf, probably also signifying religious belief.^[79] By 11,000 years ago, Homo sapiens had reached the southern tip of South America, the last of the uninhabited continents (except for Antartica, which remained undiscovered until 1820 AD) .^[81] Tool use and language continued to improve; interpersonal relationships became more complex.

[edit] Civilization

Main article: History of the world

Vitruvian Man by Leonardo da Vinci epitomizes the advances in art and science seen during the Renaissance.

Throughout more than 90% of its history, Homo sapiens lived in small bands as nomadic hunter-gatherers.^[82] As language became more complex, the ability to remember and transmit information resulted in a new sort of replicator: the meme.^[83] Ideas could be rapidly exchanged and passed down the generations. Cultural evolution quickly outpaced biological evolution, and history proper began. Somewhere between 8500 and 7000 BC, humans in the Fertile Crescent in Middle East began the systematic husbandry of plants and animals: agriculture.^[84] This spread to neighboring regions, and also developed independently elsewhere, until most Homo sapiens lived sedentary lives in permanent settlements as farmers. Not all societies abandoned nomadism, especially those in isolated areas of the globe poor in domesticable plant species, such as Australia.^[85] However, among those civilizations that did adopt agriculture, the relative security and increased productivity provided by farming allowed the population to expand. Agriculture had a major impact; humans began to affect the environment as never before. Surplus food allowed a priestly or governing class to arise, followed by increasing division of labor. This led to Earth’s first civilization at Sumer in the Middle East, between 4000 and 3000 BC.^[86] Additional civilizations quickly arose in ancient Egypt, at the Indus River valley and in China.

Starting around 3000 BC, Hinduism, one of the oldest religions still practiced today, began to take form.^[87] Others soon followed. The invention of writing enabled complex societies to arise: record-keeping and libraries served as a storehouse of knowledge and increased the cultural transmission of information. Humans no longer had to spend all their time working for survival—curiosity and education drove the pursuit of knowledge and wisdom. Various disciplines, including science (in a primitive form), arose. New civilizations sprang up, traded with one another, and engaged in war for territory and resources: empires began to form. By around 500 BC, there were empires in the Middle East, Iran, India, China, and Greece, approximately on equal footing; at times one empire expanded, only to decline or be driven back later.^[88]

In the fourteenth century, the Renaissance began in Italy with advances in religion, art, and science.^[89] Starting around 1500, European civilization began to undergo changes leading to the scientific and industrial revolutions: that continent began to exert political and cultural dominance over human societies around the planet.^[90] From 1914 to 1918 and 1939 to 1945, nations around the world were embroiled in world wars. Established following World War I, the League of Nations was a first step in establishing international institutions to resolve disputes peacefully; after its failure to prevent World War II and the subsequent end of the conflict it was replaced by the United Nations. In 1992, several European nations joined together in the European Union. As transportation and communication improved, the economies and political affairs of nations around the world have become increasingly intertwined. This globalization has often produced discord, although increased collaboration has resulted as well.

Further information: History of Africa, History of the Americas, History of Antarctica, and History of Eurasia

[edit] Recent events

Main article: Modern era

Four and a half billion years after the planet's formation, one of Earth’s life forms broke free of the biosphere. For the first time in history, Earth was viewed first hand from the vantage of space.

Change has continued at a rapid pace from the mid-1940s to today. Technological developments include nuclear weapons, computers, genetic engineering, and nanotechnology. Economic globalization spurred by advances in communication and transportation technology has influenced everyday life in many parts of the world. Cultural and institutional forms such as democracy, capitalism, and environmentalism have increased influence. Major concerns and problems such as disease, war, poverty, violent radicalism, and more recently, global warming have risen as the world population increases.

In 1957, the Soviet Union launched the first artificial satellite into orbit and, soon afterward, Yuri Gagarin became the first human in space. Neil Armstrong, an American, was the first to set foot on another astronomical object, the Earth's Moon. Unmanned probes have been sent to all the major planets in the solar system, with some (such as Voyager) having left the solar system. The Soviet Union and the United States of America were the primary early leaders in space exploration in the 20th Century. Five space agencies, representing over fifteen countries,^[91] have worked together to build the International Space Station. Aboard it, there has been a continuous human presence in space since 2000.^[92]

See also: Modernity and Future

Solar System

2008-10-22T12:15:00.000-07:00

This article is about the Sun and its planetary system. For other systems, see Planetary system and Star system.

Planets and some dwarf planets of the Solar System. Sizes are to scale, but relative distances from the Sun are not.

The Solar System^[a] consists of the Sun and those celestial objects bound to it by gravity. These objects are the eight planets, their 166 known moons,^[1] five dwarf planets, and billions of small bodies. The small bodies include asteroids, icy Kuiper belt objects, comets, meteoroids, and interplanetary dust.

The charted regions of the Solar System are the Sun, four terrestrial inner planets, the asteroid belt, four gas giant outer planets, the Kuiper belt, the scattered disc, and the hypothetical Oort cloud.

A flow of plasma from the Sun (the solar wind) permeates the Solar System. This creates a bubble in the interstellar medium known as the heliosphere, which extends out to the middle of the scattered disc.

In order of their distances from the Sun, the eight planets are:

As of mid-2008, five smaller objects are classified as dwarf planets. Ceres is in the asteroid belt, and four orbit the Sun beyond Neptune: Pluto (formerly classified as the ninth planet), Haumea, Makemake, and Eris.

Six of the planets and three of the dwarf planets are orbited by natural satellites, usually termed "moons" after Earth's Moon. Each of the outer planets is encircled by planetary rings of dust and other particles.

Discovery and exploration

Main article: Discovery and exploration of the Solar System

For many thousands of years, humanity, with a few notable exceptions, did not recognise the existence of the Solar System. They believed the Earth to be stationary at the centre of the universe and categorically different from the divine or ethereal objects that moved through the sky. Although the Indian mathematician-astronomer Aryabhata and the Greek philosopher Aristarchus of Samos had speculated on a heliocentric reordering of the cosmos, Nicolaus Copernicus was the first to develop a mathematically predictive heliocentric system. His 17th-century successors Galileo Galilei, Johannes Kepler, and Isaac Newton developed an understanding of physics which led to the gradual acceptance of the idea that the Earth moves round the Sun and that the planets are governed by the same physical laws that governed the Earth. In more recent times, this led to the investigation of geological phenomena such as mountains and craters and seasonal meteorological phenomena such as clouds, dust storms and ice caps on the other planets.

Structure

The relative masses of the Solar planets. Jupiter at 71% of the total and Saturn at 21% dominate the system. Mercury and Mars, which together are less than 0.1%, are not visible at this scale.

The orbits of the bodies in the Solar System to scale (clockwise from top left)

The principal component of the Solar System is the Sun, a main sequence G2 star that contains 99.86 percent of the system's known mass and dominates it gravitationally.^[2] Jupiter and Saturn, the Sun's two largest orbiting bodies, account for more than 90 percent of the system's remaining mass.^[b]

Most large objects in orbit around the Sun lie near the plane of Earth's orbit, known as the ecliptic. The planets are very close to the ecliptic while comets and Kuiper belt objects are usually at significantly greater angles to it.

All of the planets and most other objects also orbit with the Sun's rotation (counter-clockwise, as viewed from above the Sun's north pole). There are exceptions, such as Halley's Comet.

Kepler's laws of planetary motion describe the orbits of objects about the Sun. According to Kepler's laws, each object travels along an ellipse with the Sun at one focus. Objects closer to the Sun (with smaller semi-major axes) have shorter years. On an elliptical orbit, a body's distance from the Sun varies over the course of its year. A body's closest approach to the Sun is called its perihelion, while its most distant point from the Sun is called its aphelion. Each body moves fastest at its perihelion and slowest at its aphelion. The orbits of the planets are nearly circular, but many comets, asteroids and Kuiper belt objects follow highly elliptical orbits.

To cope with the vast distances involved, many representations of the Solar System show orbits the same distance apart. In reality, with a few exceptions, the farther a planet or belt is from the Sun, the larger the distance between it and the previous orbit. For example, Venus is approximately 0.33 astronomical units (AU)^[c] farther out than Mercury, while Saturn is 4.3 AU out from Jupiter, and Neptune lies 10.5 AU out from Uranus. Attempts have been made to determine a correlation between these orbital distances (see Titius-Bode law), but no such theory has been accepted.

Most of the planets in the Solar System possess secondary systems of their own. Many are in turn orbited by planetary objects called natural satellites, or moons, some of which are larger than planets. Most of the largest natural satellites are in synchronous orbit, with one face permanently turned toward their parent. The four largest planets also possess planetary rings, thin bands of tiny particles that orbit them in unison.

Terminology

Informally, the Solar System is sometimes divided into separate regions. The inner Solar System includes the four terrestrial planets and the main asteroid belt. The outer Solar System is beyond the asteroids, including the four gas giant planets.^[3] Since the discovery of the Kuiper belt, the outermost parts of the Solar System are considered a distinct region consisting of the objects beyond Neptune.^[4]

Dynamically and physically, objects orbiting the Sun are classed into three categories: planets, dwarf planets and small Solar System bodies. A planet is any body in orbit around the Sun that has enough mass to form itself into a spherical shape and has cleared its immediate neighbourhood of all smaller objects. By this definition, the Solar System has eight known planets: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune. Pluto was demoted from planetary status, as it has not cleared its orbit of surrounding Kuiper belt objects.^[5] A dwarf planet is a celestial body orbiting the Sun that is massive enough to be rounded by its own gravity but which has not cleared its neighbouring region of planetesimals and is not a satellite.^[5] By this definition, the Solar System has five known dwarf planets: Ceres, Pluto, Haumea, Makemake, and Eris.^[6] Other objects that may become classified as dwarf planets are Sedna, Orcus, and Quaoar. Dwarf planets that orbit in the trans-Neptunian region are called "plutoids."^[7] The remainder of the objects in orbit around the Sun are small Solar System bodies.^[5]

The regions (or zones) of the Solar system: the inner solar system, the asteroid belt, the giant planets (Jovians) and the Kuiper belt. Sizes and orbits not to scale.

Planetary scientists use the terms gas, ice, and rock to describe the various classes of substances found throughout the Solar System. Rock is used to describe compounds with high melting points (greater than roughly 500 K), such as silicates. Rocky substances are prevalent in the inner Solar System, forming most of the terrestrial planets and asteroids. Gases are materials with low melting points such as atomic hydrogen, helium, and noble gases; they dominate the middle region, comprising most of Jupiter and Saturn. Ices, like water, methane, ammonia, and carbon dioxide,^[8] have melting points up to a few hundred Kelvin. Icy substances comprise the majority of the satellites of the giant planets, as well as most of Uranus and Neptune (the so-called "ice giants") and the numerous small objects that lie beyond Neptune's orbit.^[9] The term volatiles refers collectively to all materials with low boiling points (less than a few hundred Kelvin), including gases and ices; depending on the temperature, volatiles can be found as ices, liquids, or gases in various places in the Solar System.

Sun

Main article: Sun

The Sun as seen in the x-ray region of the electromagnetic spectrum

The Sun is the Solar System's parent star, and far and away its chief component. Its large mass gives it an interior density high enough to sustain nuclear fusion, which releases enormous amounts of energy, mostly radiated into space as electromagnetic radiation such as visible light.

The Sun is classified as a moderately large yellow dwarf, but this name is misleading as, compared to stars in our galaxy, the Sun is rather large and bright. Stars are classified by the Hertzsprung-Russell diagram, a graph which plots the brightness of stars against their surface temperatures. Generally, hotter stars are brighter. Stars following this pattern are said to be on the main sequence; the Sun lies right in the middle of it. However, stars brighter and hotter than the Sun are rare, while stars dimmer and cooler are common.^[10]

It is believed that the Sun's position on the main sequence puts it in the "prime of life" for a star, in that it has not yet exhausted its store of hydrogen for nuclear fusion. The Sun is growing brighter; early in its history it was 75 percent as bright as it is today.^[11]

The Sun is a population I star; it was born in the later stages of the universe's evolution. It contains more elements heavier than hydrogen and helium ("metals" in astronomical parlance) than older population II stars.^[12] Elements heavier than hydrogen and helium were formed in the cores of ancient and exploding stars, so the first generation of stars had to die before the universe could be enriched with these atoms. The oldest stars contain few metals, while stars born later have more. This high metallicity is thought to have been crucial to the Sun's developing a planetary system, because planets form from accretion of metals.^[13]

The heliospheric current sheet.

Interplanetary medium

Main article: Interplanetary medium

Along with light, the Sun radiates a continuous stream of charged particles (a plasma) known as the solar wind. This stream of particles spreads outwards at roughly 1.5 million kilometres per hour,^[14] creating a tenuous atmosphere (the heliosphere) that permeates the Solar System out to at least 100 AU (see heliopause). This is known as the interplanetary medium. Geomagnetic storms on the Sun's surface, such as solar flares and coronal mass ejections, disturb the heliosphere, creating space weather.^[15] The Sun's rotating magnetic field acts on the interplanetary medium to create the heliospheric current sheet, the largest structure in the Solar System.^[16]

Aurora australis seen from orbit.

Earth's magnetic field protects its atmosphere from interacting with the solar wind. Venus and Mars do not have magnetic fields, and the solar wind causes their atmospheres to gradually bleed away into space.^[17] The interaction of the solar wind with Earth's magnetic field creates the aurorae seen near the magnetic poles.

Cosmic rays originate outside the Solar System. The heliosphere partially shields the Solar System, and planetary magnetic fields (for those planets that have them) also provide some protection. The density of cosmic rays in the interstellar medium and the strength of the Sun's magnetic field change on very long timescales, so the level of cosmic radiation in the Solar System varies, though by how much is unknown.^[18]

The interplanetary medium is home to at least two disc-like regions of cosmic dust. The first, the zodiacal dust cloud, lies in the inner Solar System and causes zodiacal light. It was likely formed by collisions within the asteroid belt brought on by interactions with the planets.^[19] The second extends from about 10 AU to about 40 AU, and was probably created by similar collisions within the Kuiper belt.^[20]^[21]

Inner Solar System

The inner Solar System is the traditional name for the region comprising the terrestrial planets and asteroids. Composed mainly of silicates and metals, the objects of the inner Solar System huddle very closely to the Sun; the radius of this entire region is shorter than the distance between Jupiter and Saturn.

Inner planets

Main article: Terrestrial planet

The inner planets. From left to right: Mercury, Venus, Earth, and Mars (sizes to scale)

The four inner or terrestrial planets have dense, rocky compositions, few or no moons, and no ring systems. They are composed largely of minerals with high melting points, such as the silicates which form their crusts and mantles, and metals such as iron and nickel, which form their cores. Three of the four inner planets (Venus, Earth and Mars) have substantial atmospheres; all have impact craters and tectonic surface features such as rift valleys and volcanoes. The term inner planet should not be confused with inferior planet, which designates those planets which are closer to the Sun than Earth is (i.e. Mercury and Venus).

Mercury

Mercury (0.4 AU) is the closest planet to the Sun and the smallest planet (0.055 Earth masses). Mercury has no natural satellites, and its only known geological features besides impact craters are lobed ridges or rupes, probably produced by a period of contraction early in its history.^[22] Mercury's almost negligible atmosphere consists of atoms blasted off its surface by the solar wind.^[23] Its relatively large iron core and thin mantle have not yet been adequately explained. Hypotheses include that its outer layers were stripped off by a giant impact, and that it was prevented from fully accreting by the young Sun's energy.^[24]^[25]

Venus

Venus (0.7 AU) is close in size to Earth, (0.815 Earth masses) and like Earth, has a thick silicate mantle around an iron core, a substantial atmosphere and evidence of internal geological activity. However, it is much drier than Earth and its atmosphere is ninety times as dense. Venus has no natural satellites. It is the hottest planet, with surface temperatures over 400 °C, most likely due to the amount of greenhouse gases in the atmosphere.^[26] No definitive evidence of current geological activity has been detected on Venus, but it has no magnetic field that would prevent depletion of its substantial atmosphere, which suggests that its atmosphere is regularly replenished by volcanic eruptions.^[27]

Earth

Earth (1 AU) is the largest and densest of the inner planets, the only one known to have current geological activity, and the only planet known to have life. Its liquid hydrosphere is unique among the terrestrial planets, and it is also the only planet where plate tectonics has been observed. Earth's atmosphere is radically different from those of the other planets, having been altered by the presence of life to contain 21% free oxygen.^[28] It has one natural satellite, the Moon (Latin: Luna), the only large satellite of a terrestrial planet in the Solar System.

Mars

Mars (1.5 AU) is smaller than Earth and Venus (0.107 Earth masses). It possesses a tenuous atmosphere of mostly carbon dioxide. Its surface, peppered with vast volcanoes such as Olympus Mons and rift valleys such as Valles Marineris, shows geological activity that may have persisted until very recently. Its red color comes from rust in its iron-rich soil.^[29] Mars has two tiny natural satellites (Deimos and Phobos) thought to be captured asteroids.^[30]

Asteroid belt

Main article: Asteroid belt

Image of the main asteroid belt and the Trojan asteroids

Asteroids are mostly small Solar System bodies composed mainly of rocky and metallic non-volatile minerals.

The main asteroid belt occupies the orbit between Mars and Jupiter, between 2.3 and 3.3 AU from the Sun. It is thought to be remnants from the Solar System's formation that failed to coalesce because of the gravitational interference of Jupiter.

Asteroids range in size from hundreds of kilometres across to microscopic. All asteroids save the largest, Ceres, are classified as small Solar System bodies, but some asteroids such as Vesta and Hygieia may be reclassed as dwarf planets if they are shown to have achieved hydrostatic equilibrium.

The asteroid belt contains tens of thousands, possibly millions, of objects over one kilometre in diameter.^[31] Despite this, the total mass of the main belt is unlikely to be more than a thousandth of that of the Earth.^[32] The main belt is very sparsely populated; spacecraft routinely pass through without incident. Asteroids with diameters between 10 and 10^-4 m are called meteoroids.^[33]

Ceres

Ceres (2.77 AU) is the largest body in the asteroid belt and is classified as a dwarf planet. It has a diameter of slightly under 1000 km, large enough for its own gravity to pull it into a spherical shape. Ceres was considered a planet when it was discovered in the 19th century, but was reclassified as an asteroid in the 1850s as further observation revealed additional asteroids.^[34] It was again reclassified in 2006 as a dwarf planet.

Asteroid groups

Asteroids in the main belt are divided into asteroid groups and families based on their orbital characteristics. Asteroid moons are asteroids that orbit larger asteroids. They are not as clearly distinguished as planetary moons, sometimes being almost as large as their partners. The asteroid belt also contains main-belt comets which may have been the source of Earth's water.^[35]

Trojan asteroids are located in either of Jupiter's L₄ or L₅ points (gravitationally stable regions leading and trailing a planet in its orbit); the term "Trojan" is also used for small bodies in any other planetary or satellite Lagrange point. Hilda asteroids are in a 2:3 resonance with Jupiter; that is, they go around the Sun three times for every two Jupiter orbits.

The inner Solar System is also dusted with rogue asteroids, many of which cross the orbits of the inner planets.

Outer Solar System

The outer region of the Solar System is home to the gas giants and their planet-sized satellites. Many short period comets, including the centaurs, also orbit in this region. The solid objects in this region are composed of a higher proportion of volatiles (such as water, ammonia, methane, often called ices in planetary science) than the rocky denizens of the inner Solar System.

Outer planets

Main article: Gas giant

From top to bottom: Neptune, Uranus, Saturn, and Jupiter (not to scale)

The four outer planets, or gas giants (sometimes called Jovian planets), collectively make up 99 percent of the mass known to orbit the Sun. Jupiter and Saturn consist overwhelmingly of hydrogen and helium; Uranus and Neptune possess a greater proportion of ices in their makeup. Some astronomers suggest they belong in their own category, “ice giants.”^[36] All four gas giants have rings, although only Saturn's ring system is easily observed from Earth. The term outer planet should not be confused with superior planet, which designates planets outside Earth's orbit (the outer planets and Mars).

Jupiter

Jupiter (5.2 AU), at 318 Earth masses, masses 2.5 times all the other planets put together. It is composed largely of hydrogen and helium. Jupiter's strong internal heat creates a number of semi-permanent features in its atmosphere, such as cloud bands and the Great Red Spot. Jupiter has sixty-three known satellites. The four largest, Ganymede, Callisto, Io, and Europa, show similarities to the terrestrial planets, such as volcanism and internal heating.^[37] Ganymede, the largest satellite in the Solar System, is larger than Mercury.

Saturn

Saturn (9.5 AU), distinguished by its extensive ring system, has similarities to Jupiter, such as its atmospheric composition. Saturn is far less massive, being only 95 Earth masses. Saturn has sixty known satellites (and three unconfirmed); two of which, Titan and Enceladus, show signs of geological activity, though they are largely made of ice.^[38] Titan is larger than Mercury and the only satellite in the Solar System with a substantial atmosphere.

Uranus

Uranus (19.6 AU), at 14 Earth masses, is the lightest of the outer planets. Uniquely among the planets, it orbits the Sun on its side; its axial tilt is over ninety degrees to the ecliptic. It has a much colder core than the other gas giants, and radiates very little heat into space.^[39] Uranus has twenty-seven known satellites, the largest ones being Titania, Oberon, Umbriel, Ariel and Miranda.

Neptune

Neptune (30 AU), though slightly smaller than Uranus, is more massive (equivalent to 17 Earths) and therefore more dense. It radiates more internal heat, but not as much as Jupiter or Saturn.^[40] Neptune has thirteen known satellites. The largest, Triton, is geologically active, with geysers of liquid nitrogen.^[41] Triton is the only large satellite with a retrograde orbit. Neptune is accompanied in its orbit by a number of minor planets, termed Neptune Trojans, that are in 1:1 resonance with it.

Comets

Main article: Comet

Comet Hale-Bopp

Comets are small Solar System bodies, usually only a few kilometres across, composed largely of volatile ices. They have highly eccentric orbits, generally a perihelion within the orbits of the inner planets and an aphelion far beyond Pluto. When a comet enters the inner Solar System, its proximity to the Sun causes its icy surface to sublimate and ionise, creating a coma: a long tail of gas and dust often visible to the naked eye.

Short-period comets have orbits lasting less than two hundred years. Long-period comets have orbits lasting thousands of years. Short-period comets are believed to originate in the Kuiper belt, while long-period comets, such as Hale-Bopp, are believed to originate in the Oort cloud. Many comet groups, such as the Kreutz Sungrazers, formed from the breakup of a single parent.^[42] Some comets with hyperbolic orbits may originate outside the Solar System, but determining their precise orbits is difficult.^[43] Old comets that have had most of their volatiles driven out by solar warming are often categorised as asteroids.^[44]

Centaurs

The centaurs are icy comet-like bodies with a semi-major axis greater than Jupiter (5.5 AU) and less than Neptune (30 AU). The largest known centaur, 10199 Chariklo, has a diameter of about 250 km.^[45] The first centaur discovered, 2060 Chiron, has also been classified as comet (95P) since it develops a coma just as comets do when they approach the Sun.^[46] Some astronomers classify centaurs as inward-scattered Kuiper belt objects along with the outward-scattered residents of the scattered disc.^[47]

Trans-Neptunian region

The area beyond Neptune, or the "trans-Neptunian region", is still largely unexplored. It appears to consist overwhelmingly of small worlds (the largest having a diameter only a fifth that of the Earth and a mass far smaller than that of the Moon) composed mainly of rock and ice. This region is sometimes known as the "outer Solar System", though others use that term to mean the region beyond the asteroid belt.

Kuiper belt

Main article: Kuiper belt

Plot of all known Kuiper belt objects, set against the four outer planets

The Kuiper belt, the region's first formation, is a great ring of debris similar to the asteroid belt, but composed mainly of ice. It extends between 30 and 50 AU from the Sun. It is composed mainly of small Solar System bodies, but many of the largest Kuiper belt objects, such as Quaoar, Varuna, and Orcus, may be reclassified as dwarf planets. There are estimated to be over 100,000 Kuiper belt objects with a diameter greater than 50 km, but the total mass of the Kuiper belt is thought to be only a tenth or even a hundredth the mass of the Earth.^[48] Many Kuiper belt objects have multiple satellites, and most have orbits that take them outside the plane of the ecliptic.

Diagram showing the resonant and classical Kuiper belt divisions

The Kuiper belt can be roughly divided into the "classical" belt and the resonances. Resonances are orbits linked to that of Neptune (e.g. twice for every three Neptune orbits, or once for every two). The first resonance actually begins within the orbit of Neptune itself. The classical belt consists of objects having no resonance with Neptune, and extends from roughly 39.4 AU to 47.7 AU.^[49] Members of the classical Kuiper belt are classified as cubewanos, after the first of their kind to be discovered, (15760) 1992 QB₁.^[50]

Pluto and Charon

Pluto (39 AU average), a dwarf planet, is the largest known object in the Kuiper belt. When discovered in 1930, it was considered to be the ninth planet; this changed in 2006 with the adoption of a formal definition of planet. Pluto has a relatively eccentric orbit inclined 17 degrees to the ecliptic plane and ranging from 29.7 AU from the Sun at perihelion (within the orbit of Neptune) to 49.5 AU at aphelion.

Pluto and its three known moons

It is unclear whether Charon, Pluto's largest moon, will continue to be classified as such or as a dwarf planet itself. Both Pluto and Charon orbit a barycenter of gravity above their surfaces, making Pluto-Charon a binary system. Two much smaller moons, Nix and Hydra, orbit Pluto and Charon.

Pluto lies in the resonant belt and has a 3:2 resonance with Neptune, meaning that Pluto orbits twice round the Sun for every three Neptunian orbits. Kuiper belt objects whose orbits share this resonance are called plutinos.^[51]

Haumea and Makemake

Haumea (43.34 AU average), and Makemake (45.79 AU average) are the largest known objects in the classical Kuiper belt. Haumea is an egg-shaped object with two moons. Makemake is the brightest object in the Kuiper belt after Pluto. Originally designated 2003 EL₆₁ and 2005 FY₉ respectively, they were granted names (and the status of dwarf planet) in 2008.^[6] Their orbits are far more inclined than Pluto's (28° and 29°)^[52] and unlike Pluto are not affected by Neptune, being part of the classical KBO population.

Scattered disc

Main article: Scattered disc

Black: scattered; blue: classical; green: resonant

The scattered disc overlaps the Kuiper belt but extends much further outwards. This region is thought to be the source of short-period comets. Scattered disc objects are believed to have been ejected into erratic orbits by the gravitational influence of Neptune's early outward migration. Most scattered disc objects (SDOs) have perihelia within the Kuiper belt but aphelia as far as 150 AU from the Sun. SDOs' orbits are also highly inclined to the ecliptic plane, and are often almost perpendicular to it. Some astronomers consider the scattered disc to be merely another region of the Kuiper belt, and describe scattered disc objects as "scattered Kuiper belt objects."^[53]

Eris and its moon Dysnomia

Eris

Eris (68 AU average) is the largest known scattered disc object, and caused a debate about what constitutes a planet, since it is at least 5% larger than Pluto with an estimated diameter of 2400 km (1500 mi). It is the largest of the known dwarf planets.^[54] It has one moon, Dysnomia. Like Pluto, its orbit is highly eccentric, with a perihelion of 38.2 AU (roughly Pluto's distance from the Sun) and an aphelion of 97.6 AU, and steeply inclined to the ecliptic plane.

Farthest regions

The point at which the Solar System ends and interstellar space begins is not precisely defined, since its outer boundaries are shaped by two separate forces: the solar wind and the Sun's gravity. The outer limit of the solar wind's influence is roughly four times Pluto's distance from the Sun; this heliopause is considered the beginning of the interstellar medium.^[55] However, the Sun's Roche sphere, the effective range of its gravitational influence, is believed to extend up to a thousand times farther.

Heliopause

The Voyagers entering the heliosheath.

The heliosphere is divided into two separate regions. The solar wind travels at roughly 40,000 km/s until it collides with flows of plasma in the interstellar medium. The collision occurs at the termination shock, which is roughly 80–100 AU from the Sun in the upwind direction and roughly 200 AU from the Sun downwind.^[56] Here the wind slows dramatically, condenses and becomes more turbulent,^[56] forming a great oval structure known as the heliosheath that looks and behaves very much like a comet's tail, extending outward for a further 40 AU on the upwind side but tailing many times that distance in the opposite direction. Both Voyager 1 and Voyager 2 are reported to have passed the termination shock and entered the heliosheath, at 94 and 84 AU from the Sun, respectively.^[57]^[58] The outer boundary of the heliosphere, the heliopause, is the point at which the solar wind finally terminates and is the beginning of interstellar space.^[55]

The shape and form of the outer edge of the heliosphere is likely affected by the fluid dynamics of interactions with the interstellar medium^[56] as well as solar magnetic fields prevailing to the south, e.g. it is bluntly shaped with the northern hemisphere extending 9 AU (roughly 900 million miles) farther than the southern hemisphere. Beyond the heliopause, at around 230 AU, lies the bow shock, a plasma "wake" left by the Sun as it travels through the Milky Way.^[59]

No spacecraft have yet passed beyond the heliopause, so it is impossible to know for certain the conditions in local interstellar space. It is expected that NASA's Voyager spacecraft will pass the heliopause some time in the next decade and transmit valuable data on radiation levels and solar wind back to the Earth.^[60] How well the heliosphere shields the Solar System from cosmic rays is poorly understood. A NASA-funded team has developed a concept of a "Vision Mission" dedicated to sending a probe to the heliosphere.^[61]^[62]

Oort cloud

Main article: Oort cloud

Artist's rendering of the Kuiper Belt and hypothetical Oort cloud.

The hypothetical Oort cloud is a great mass of up to a trillion icy objects that is believed to be the source for all long-period comets and to surround the Solar System at roughly 50,000 AU (around 1 light-year (LY)), and possibly to as far as 100,000 AU (1.87 LY). It is believed to be composed of comets which were ejected from the inner Solar System by gravitational interactions with the outer planets. Oort cloud objects move very slowly, and can be perturbed by infrequent events such as collisions, the gravitational effects of a passing star, or the galactic tide, the tidal force exerted by the Milky Way.^[63]^[64]

Telescopic image of Sedna

Sedna

90377 Sedna (525.86 AU average) is a large, reddish Pluto-like object with a gigantic, highly elliptical orbit that takes it from about 76 AU at perihelion to 928 AU at aphelion and takes 12,050 years to complete. Mike Brown, who discovered the object in 2003, asserts that it cannot be part of the scattered disc or the Kuiper belt as its perihelion is too distant to have been affected by Neptune's migration. He and other astronomers consider it to be the first in an entirely new population, which also may include the object 2000 CR₁₀₅, which has a perihelion of 45 AU, an aphelion of 415 AU, and an orbital period of 3420 years.^[65] Brown terms this population the "Inner Oort cloud," as it may have formed through a similar process, although it is far closer to the Sun.^[66] Sedna is very likely a dwarf planet, though its shape has yet to be determined with certainty.

Boundaries

See also: Vulcanoid asteroid, Planets beyond Neptune, and Nemesis (star)

Much of our Solar System is still unknown. The Sun's gravitational field is estimated to dominate the gravitational forces of surrounding stars out to about two light years (125,000 AU). The outer extent of the Oort cloud, by contrast, may not extend farther than 50,000 AU.^[67] Despite discoveries such as Sedna, the region between the Kuiper belt and the Oort cloud, an area tens of thousands of AU in radius, is still virtually unmapped. There are also ongoing studies of the region between Mercury and the Sun.^[68] Objects may yet be discovered in the Solar System's uncharted regions.

Galactic context

Location of the Solar System within our galaxy

The Solar System is located in the Milky Way galaxy, a barred spiral galaxy with a diameter of about 100,000 light-years containing about 200 billion stars.^[69] Our Sun resides in one of the Milky Way's outer spiral arms, known as the Orion Arm or Local Spur.^[70] The Sun lies between 25,000 and 28,000 light years from the Galactic Centre, and its speed within the galaxy is about 220 kilometres per second, so that it completes one revolution every 225–250 million years. This revolution is known as the Solar System's galactic year.^[71]

The Solar System's location in the galaxy is very likely a factor in the evolution of life on Earth. Its orbit is close to being circular and is at roughly the same speed as that of the spiral arms, which means it passes through them only rarely. Since spiral arms are home to a far larger concentration of potentially dangerous supernovae, this has given Earth long periods of interstellar stability for life to evolve.^[72] The Solar System also lies well outside the star-crowded environs of the galactic centre. Near the centre, gravitational tugs from nearby stars could perturb bodies in the Oort Cloud and send many comets into the inner Solar System, producing collisions with potentially catastrophic implications for life on Earth. The intense radiation of the galactic centre could also interfere with the development of complex life.^[72] Even at the Solar System's current location, some scientists have hypothesised that recent supernovae may have adversely affected life in the last 35,000 years by flinging pieces of expelled stellar core towards the Sun in the form of radioactive dust grains and larger, comet-like bodies.^[73]

Neighbourhood

Artist's conception of the Local Bubble

The immediate galactic neighbourhood of the Solar System is known as the Local Interstellar Cloud or Local Fluff, an area of dense cloud in an otherwise sparse region known as the Local Bubble, an hourglass-shaped cavity in the interstellar medium roughly 300 light years across. The bubble is suffused with high-temperature plasma that suggests it is the product of several recent supernovae.^[74]

The solar apex, the direction of the Sun's path through interstellar space, is near the constellation of Hercules in the direction of the current location of the bright star Vega.^[75]

There are relatively few stars within ten light years (95 trillion km) of the Sun. The closest is the triple star system Alpha Centauri, which is about 4.4 light years away. Alpha Centauri A and B are a closely tied pair of Sun-like stars, while the small red dwarf Alpha Centauri C (also known as Proxima Centauri) orbits the pair at a distance of 0.2 light years. The stars next closest to the Sun are the red dwarfs Barnard's Star (at 5.9 light years), Wolf 359 (7.8 light years) and Lalande 21185 (8.3 light years). The largest star within ten light years is Sirius, a bright main sequence star roughly twice the Sun's mass and orbited by a white dwarf called Sirius B. It lies 8.6 light years away. The remaining systems within ten light years are the binary red dwarf system Luyten 726-8 (8.7 light years) and the solitary red dwarf Ross 154 (9.7 light years).^[76] Our closest solitary sunlike star is Tau Ceti, which lies 11.9 light years away. It has roughly 80 percent the Sun's mass, but only 60 percent its luminosity.^[77] The closest known extrasolar planet to the Sun lies around the star Epsilon Eridani, a star slightly dimmer and redder than the Sun, which lies 10.5 light years away. Its one confirmed planet, Epsilon Eridani b, is roughly 1.5 times Jupiter's mass and orbits its star every 6.9 years.^[78]

Formation and evolution

Solar System's Most
Abundant Isotopes^[79]
Isotope	Nuclei per Million
Hydrogen-1	705,700
Helium-4	275,200
Oxygen-16	5,920
Carbon-12	3,032
Neon-20	1,548
Iron-56	1,169
Nitrogen-14	1,105
Silicon-28	653
Magnesium-24	513
Sulfur-32	396
Neon-22	208
Magnesium-26	79
Argon-36	77
Iron-54	72
Magnesium-25	69
Calcium-40	60
Aluminum-27	58
Nickel-58	49
Carbon-13	37
Helium-3	35
Silicon-29	34
Sodium-23	33
Iron-57	28
Hydrogen-2	23
Silicon-30	23

Main article: Formation and evolution of the Solar System

Hubble image of protoplanetary disks in the Orion Nebula, a light-years-wide "stellar nursery" likely very similar to the primordial nebula from which our Sun formed.

The Solar System formed from the gravitational collapse of a giant molecular cloud 4.6 billion years ago. This initial cloud was likely several light-years across and probably birthed several stars.^[80]

As the region that would become the Solar System, known as the pre-solar nebula,^[81] collapsed, conservation of angular momentum made it rotate faster. The centre, where most of the mass collected, became increasingly hotter than the surrounding disc.^[80] As the contracting nebula rotated, it began to flatten into a spinning protoplanetary disc with a diameter of roughly 200 AU^[80] and a hot, dense protostar at the centre.^[82]^[83] At this point in its evolution, the Sun is believed to have been a T Tauri star. Studies of T Tauri stars show that they are often accompanied by discs of pre-planetary matter with masses of 0.001–0.1 solar masses, with the vast majority of the mass of the nebula in the star itself.^[84] The planets formed by accretion from this disk.^[85]

Within 50 million years, the pressure and density of hydrogen in the centre of the protostar became great enough for it to begin thermonuclear fusion.^[86] The temperature, reaction rate, pressure, and density increased until hydrostatic equilibrium was achieved, with the thermal energy countering the force of gravitational contraction. At this point the Sun became a full-fledged main sequence star.^[87]

Artist's conception of the future evolution of our Sun. Left: main sequence; middle: red giant; right: white dwarf

The Solar System as we know it today will last until the Sun begins its evolution off of the main sequence of the Hertzsprung-Russell diagram. As the Sun burns through its supply of hydrogen fuel, the energy output supporting the core tends to decrease, causing it to collapse in on itself. This increase in pressure heats the core, so it burns even faster. As a result, the Sun is growing brighter at a rate of roughly ten percent every 1.1 billion years.^[88]

Around 5.4 billion years from now, the hydrogen in the core of the Sun will have been entirely converted to helium, ending the main sequence phase. At this time, the outer layers of the Sun will expand to roughly up to 260 times its current diameter; the Sun will become a red giant. Because of its vastly increased surface area, the surface of the Sun will be considerably cooler than it is on the main sequence (2600 K at the coolest).^[89]

Eventually, the Sun's outer layers will fall away, leaving a white dwarf, an extraordinarily dense object, half the original mass of the Sun but only the size of the Earth.^[90] The ejected outer layers will form what is known as a planetary nebula, returning some of the material that formed the Sun to the interstellar medium.

Todays Space Online ............

2008-10-09T23:38:00.000-07:00

The Peoples Republic of China launched its first satellite -- known as China 1 or Mao 1 -- to Earth orbit on its own "Long March" space rocket on April 24, 1970. The 390-lb. electronic ball floated around the Earth blaring the patriotic song The East Is Red. The launch made China the fifth nation with a space rocket. Before that first successful launch, the Chinese may have sustained a launch failure in 1969. They may have suffered three failures in 1974 and another in 1979. China has made scores of successful satellite launches since 1970. By the end of 2001, China had launched nearly 50 satellites with a 90 percent success rate. The spacecraft have included remote sensing, communications and weather satellites for both civilian and military use. China started selling commercial space launches to foreign satellite owners in 1986 during a time when U.S. shuttles and European rockets were grounded. Numerous satellites have been launched for paying foreign owners. China's commercial space launch firm is the Great Wall Industrial Corp. Pakistan's Badr-A. China launched Pakistan's first satellite to a 375-mi.-high circular orbit on July 16, 1990. The satellite, Badr-A, was launched aboard the maiden flight of the Long March 2E rocket from Xichang Launch Center in China. After 146 days in space, Badr-A fell into the atmosphere and burned. China and AsiaSat. Western Union's Westar 6 satellite and the Indonesian satellite Palapa B2 were carried to orbit in 1984 by shuttle Challenger. Palapa and Westar were dropped off in orbits lower than planned so both satellites failed. Later that year, the pair were recaptured by astronauts spacewalking from shuttle Discovery. They were returned to Earth and refurbished on the ground. The retrieved Westar 6 was renamed AsiaSat and launched by China using a Long March rocket, the first American satellite sent to orbit by a non-Western country. Homing satellite. In November 1975, the first Long March 2 rocket carried China's first "homing satellite" to orbit. That made China the third nation capable of retrieving a satellite. Since then, the PRC has sent numerous satellites to orbit with packages to be retrieved from space. Multiple Launches. The pace of China's space industry picked up in the 1980s and 1990s. In September 1981, the PRC successfully launched three satellites to orbit with one rocket. Manned Capsules. In 1999, China launched and recovered an unmanned capsule designed to carry men and women into orbit in the 21st century. The successful launch was Nov. 20 and the controlled landing was Nov. 21. The flight was part of preparations to send the PRC's first persons into orbit in the 21st century. China wants to become the third nation on Earth to put a human in space. Only the United States and Russia have done so using their own rockets. The dome-shaped capsule was named "Shenzhou," meaning "Divine Vessel" or "Vessel of the Gods." Shenzhou is similar to Russia's Soyuz capsule, which carries cosmonauts to and from Russia's Mir space station. The unmanned craft was launched atop a new model of China's Long March rocket from the Jiuquan Satellite Launch Center in northwest China. About 10 minutes after liftoff, Shenzhou separated from its launch vehicle and went into orbit, circling Earth 14 times over 21 hours before controllers brought it down safely in Inner Mongolia.

2008-10-09T23:35:00.000-07:00

State of the Art. China has come a long way in space. In 2000, Beijing orbited its first high-resolution electro-optical imaging satellite, which relays its state-of-the-art digital pictures by radio to ground stations. In the past, Chinese satellites snapped pictures on photographic film which then was dropped down to Earth in canisters. The resolution of the digital-imaging satellite is less than the capability of the sharpest U.S. military reconnaissance satellites, but comparable to the sharp images produced by U.S. and European commercial satellites, which produce pictures with a resolution of about nine feet. That means the Chinese satellite, named Ziyuan-2 (ZY-2), could produce photographs showing objects ranging in size down to nine feet across -- a resolution more than three times the capability of China's earlier earth sensing satellite, Ziyuan-1 (ZY-1). ZY-2 is lower in orbit than ZY-1, which also means the satellite could offer higher resolution. Remote sensing. When the satellite was launched Sept. 1, 2000, from the Taiyuan Satellite Launching Center in the northern Shanxi Province, the official Xinhua news agency had called it Ziyuan-2 (ZY-2) and described it as a civilian "remote sensing" spacecraft. Ziyuan means "resource." Earth sensing satellites monitor environmental changes and explore for natural resources on the ground. Xinhua said the satellite would be employed mostly for territorial surveying, city planning, crop yield assessment, disaster monitoring and space science experimentation. More remote sensors. China successfully put a second ZY-2 in orbit on Oct. 27 2002. Then, on Nov. 6, 2004, China launched a third ZY-2 to orbit with a Long March 4-B rocket from the Taiyuan Satellite Launch Center in northern Shanxi Province. The ZY-2 remote sensing satellites are used mainly for land resource surveying, environmental supervision and protection, city planning, crop yield assessment, disaster monitoring and other science experiments. The first and second ZY-2 satellites are still in orbit. The third has improved performance and technology in comparison with the first two resource satellites. Ground control for the satellites is at the Xi'an Satellite Monitor and Control Center in northwest China. Upgraded Long March. The Long March 4-B booster rocket is an upgraded version of the Long March 4-A. The Nov. 5, 2004, launch was the 82nd time that a Long March rocket had been used and the 40th continuous success since China launched the first Long March 4 rocket in October 1996.
The big secret. There have been unsubstantiated reports that, in reality, Chinese military forces have merely disguised all or part of the ZY-2 satellites as civilian devices, while actually using them to spy on U.S. and other forces in Asia. That is according to a report in the Washington Times newspaper. U.S. intelligence officers reportedly told the newspaper the spysats are orbiting with false identities as civilian Earth-monitoring systems. The reports held that publicly, the satellites are named Ziyuan-2 (ZY-2), but secretly they are designated Jianbing-3. If the reports were accurate, such photo-reconnaissance satellites could be used for planning combat missions, targeting missiles at U.S. forces in Japan, or preparing aircraft strikes on Taiwan, an island nation that Beijing claims as a province of China. The ZY-2/Jianbing-3 satellites complete elliptical orbits around Earth every 94.3 minutes at an altitude ranging from 294 to 305 miles. Built by the Chinese Academy of Space Technology, each of the spacecraft is expected to work for two years in orbit.
Military satellites. China launched its first military communications satellite in January 2000 as part of a People's Liberation Army command-and-control network linking forces for combat. China will launch more high-technology space platforms, including even-higher-resolution imagery satellites, electronic signals intelligence (SIGINT) satellites and military communications satellites. Today, however, Chinese satellite technology not only serves military purposes, but it serves many areas of the national economy. Future satellites will be especially useful in developing the remote western areas of China. Five year plan. China is planning to launch at least 35 different science and application satellites during the years 2002-2006, according to Xinhua News Agency. The satellites would be used for communications and direct-to-home broadcasting, meteorological and oceanographic observations, navigation and positioning, disaster mitigation, and seed breeding. They also plan to launch manned spacecraft. CASC. China Aerospace Science and Technology Corporation (CASC) is a large state-owned enterprise that builds five different series of satellites. They include:
Dongfanghong communications satellites
Fengyun weather satellites
Shijian science exploration satellites
Ziyuan remote sensing Earth resource satellites
Beidou navigation satellites
retrievable satellites
and other types of satellites CAST. Chinese Academy of Space Technology (CAST) said some of the satellites -- such as a polar-orbiting Sun-synchronous weather satellite FY-1D and the oceanorgaphic satellite Haiyang-1 -- are being constructed, while others are in planning. a direct-broadcasting satellite (DBS) is being prepared for launch in 2004. That satellite would provide television broadcasts, and educational and information transmissions, as well as other services to the vast expanse of western China.

Todays Space Online ............

2008-10-09T23:34:00.001-07:00

Weather satellites. China's National Satellite Meteorological Center (NSMC)said the nation plans to launch six more Fengyun (FY) meteorological satellites from 2002-2007 before the Olympiad in 2008, according to the Beijing Evening Post. Fengyun means "Wind and Cloud." The first of the six would be the polar-orbiting Sun-synchronous Fengyun-1D (FY-1D) to be launched in 2002 on a Changzheng-4 (Long March 4) rocket. Then, a geostationary weather satellite, FY-2C, would be launched in 2003. The FY-3 series would be the next generation of polar-orbiting Sun-synchronous weather satellites. FY-3A would be launched in 2004 with FY-3B and FY-2D in 2006, and FY-3C in 2008. These satellites would be designed to work two to three years in space. NSMC is a scientific research and operational facility affiliated with the China Meteorological Administration (CMA). It receives, processes and distributes satellite weather data to users. The new satellites would forecast conditions and monitor bad weather around the clock, particularly convective rainstorms, thunderstorms and hailstorms. They also would monitor developing sandstorms as well as air quality and provide early warnings. The satellites launched in 2006 and 2008 would help forecasters predict weather for the Olympics. Meteorological satellites are important not only in meteorology, but als in oceanography, agriculture, forestry, hydrology, aviation, navigation, environmental protection and national defense. They contribute to a national economy and to preventing and mitigating disasters. Communications satellites. China refers to its communications satellites as Dongfanghong (DFH). Dongfanghong means "East Is Red." China's next generation of large communications satellites will carry C-, Ku-, Ka- and L-band transponders. That increased capacity will help the nation meet a growing demand for educational and commercial television broadcasts, stationary and mobile telecommunications, and data, voice and video transmissions for businesses.

Todays Space Online ............

2008-10-09T23:31:00.001-07:00

Oceanography satellites. China's Haiyang (HY-1 and HY-2) oceanographic microsatellites will carry radar altimeters, microwave scatterometers, ocean color scanners, and multichannel microwave radiometers for realtime views of oceans and coastal zones for biological resources, pollution monitoring and prevention, and monitoring of estuaries, bays and navigation routes. Haiyang means "Ocean." The two satellites are to be launched on Changzheng-4 (Long March 4) rockets to 500-mile-high circular Sun-synchronous orbits, crossing the equator near noon local time, and passing over places on earth every 2 to 3 days. Seed breeding satellites. Chinese scientists claim that seeds exposed to cosmic radiation yield superior quality produce. They would like to cultivate seedlings in space, then grow them in the climate of western China to help develop agriculture there. China's first satellite dedicated to seed breeding may fly in 2003. The satellite would house a variety of seeds and expose them to radiation before returning them to Earth. Remote sensing satellites. China calls its remote sensing Earth resource satellites Ziyuan (ZY). Ziyuan means "Resource." First in the series was the China-Brazil Earth Resources Satellite (CBERS-1 or ZY-1). Later models will be able to take higher resolution photos and work longer in space. Scientists plan to use the ZY satellites to survey national resources, monitor crop growth and yields, watch for disasters and environment pollution, and evaluate project sites. They are used for city planning, surveying and cartography. Microgravity satellites. Retrievable satellites are used to conduct experiments in space life science, space environment, and space materials and new technologies. China refers to its science exploration satellites as Shijian (SJ). Shijian means "Practice."
Chinese Double Starartist conceptDouble Star Satellite. China, in a project coordinated with the European Space Agency, will launch in 2003 a pair of Double Star Project (DSP) satellites to study the effects of the Sun on Earth's environment. Ten European instruments will be inside each of the two Chinese Double Star spacecraft, which will complement ESA's four Cluster spacecraft already in space. An additional eight science experiments will be provided by Chinese institutes.

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Apple

Apple

Greek mythology

Apple

Apple cultivars

Apple

Apple production

Apple breeding

Pollination

Apple

Trees in mythology

Ultraviolet

Contents

[edit] Discovery

[edit] Origin of term

[edit] Subtypes

[edit] Black light

[edit] Natural sources of UV

[edit] Human health-related effects of UV radiation

[edit] Beneficial effects

[edit] Harmful effects

[edit] Skin

[edit] Sunscreen safety debate

[edit] Eye

[edit] Degradation of polymers, pigments and dyes

[edit] Blockers and absorbers

[edit] Applications of UV

[edit] Security

[edit] Fluorescent lamps

[edit] Astronomy

[edit] Biological surveys and pest control

[edit] Spectrophotometry

[edit] Analyzing minerals

[edit] Chemical markers

[edit] Photochemotherapy

[edit] Phototherapy

[edit] Photolithography

[edit] Checking electrical insulation

[edit] Sterilization

[edit] Disinfecting drinking water

[edit] Food processing

[edit] Fire detection

[edit] Herpetology

[edit] Curing of inks, adhesives, varnishes and coatings

[edit] Deterring substance abuse in public places

[edit] Sun tanning

[edit] Erasing EPROM modules

[edit] Preparing low surface energy polymers

[edit] Reading otherwise illegible papyruses

[edit] Lasers

[edit] Evolutionary significance

Geology

Contents

[edit] Etymology

[edit] History

[edit] Important principles in the Development of Geology

[edit] Modern Geology

[edit] Radioactive decay and the Age of the Earth

[edit] Plate Tectonics

[edit] Earth Structure

[edit] Planetary Geology

[edit] Societal Applications of Geology

[edit] Economic Geology

[edit] Soil Mechanics and Geotechnical Engineering

[edit] Hydrology and Environmental Issues

[edit] Natural Hazards

[edit] Fields or related disciplines

[edit] Regional geology

[edit] By nations

History of Earth

Contents

[edit] Origin

[edit] Moon

[edit] The Hadean eon

[edit] Life

[edit] Cells

[edit] Photosynthesis and oxygen

[edit] Endosymbiosis and the three domains of life

[edit] Multicellularity

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