Geology of solar terrestrial planets

The geology of solar terrestrial planet mainly deals with the geological aspects of four planets of the Solar system namely, Mercury, Venus, Earth and Mars and one terrestrial dwarf planet, Ceres. Objects like Pluto are similar to terrestrial planets in the fact that they do have a solid surface, but are composed of more icy materials (see Ice dwarf). During the formation of the solar system, there were probably many more (planetesimals), but they have all merged with or been destroyed by the four remaining worlds in the solar nebula. Only one terrestrial planet, Earth, is known to have an active hydrosphere.

Terrestrial planets are substantially different from gas giants, which might not have solid surfaces and are composed mostly of some combination of hydrogen, helium, and water existing in various physical states. These planets have a compact, rocky surfaces, with the last three also having an atmosphere. Their size, radius, and density are all similar.

Terrestrial planets all have roughly the same structure- a central metallic core, mostly iron, with a surrounding silicate mantle. The Moon is similar, but lacks an iron core. Three of the four solar terrestrial 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).

Formation of solar planets

The Solar System is believed to have formed according to the nebular hypothesis, first proposed in 1755 by Immanuel Kant and independently formulated by Pierre-Simon Laplace.cite journal | last = See | first = T. J. J. | date = April 23 1909 | title = The Past History of the Earth as Inferred from the Mode of Formation of the Solar System | journal = Proceedings of the American Philosophical Society | volume = 48 | issue = 191 | pages = 119–128 | url = http://links.jstor.org/sici?sici=0003-049X%28190901%2F04%2948%3A191%3C119%3ATPHOTE%3E2.0.CO%3B2-U&size=LARGE|accessdate=2006-07-23] This theory holds that 4.6 billion years ago the Solar System formed from the gravitational collapse of a giant molecular cloud. This initial cloud was likely several light-years across and probably birthed several stars.cite web|title=Lecture 13: The Nebular Theory of the origin of the Solar System|url=http://atropos.as.arizona.edu/aiz/teaching/nats102/mario/solar_system.html|work=University of Arizona|accessdate=2006-12-27]

The first solid particles were microscopic in size. These particles orbited the Sun in nearly circular orbits right next to each other, as the gas from which they condensed. Gradually the gentle collisions allowed the flakes to stick together and make larger particles which, in turn, attracted more solid particles towards them. This process is known as accretion.The objects formed by accretion are called planetesimals- they act as seeds for planet formation. Initially, planetesimals were closely packed. They coalesced into larger objects, forming clumps of up to a few kilometers across in a few million years, a small time with comparison to the age of the solar system.After the planetesimals grew bigger in sizes, collisions became highly destructive, making further growth more difficult. Only the biggest planetesimals survived the fragmentation process and continued to slowly grow into protoplanets by accretion of planetesimals of similar composition.After the protoplanet formed, accumulation of heat from radioactive decay of short-lived elements melted planet, allowing materials to differentiate i.e.to separate according to their density.

Terrestrial planets

In the warmer inner solar system, planetesimals formed from rocks and metals were cooked billions of years ago in cores of massive stars.These elements comprised of only 0.6% of the material in the solar nebula. That's why the terrestrial planets could not grew very large and could not exert large pull on hydrogen and helium gas.Also the faster collisions among particles close to the Sun were more destructive on average. Even if the terrestrial planets had hydrogen and helium, the Sun would have heated the gases and cause them to escape.Hence, solar terrestrial planets i.e. Mercury, Venus, Earth, and Mars are dense small worlds composed mostly from 2% of heavier elements contained in the solar nebula.

Surface geology of inner solar planets

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 solid crusts and semi-liquid mantles, and metals such as iron and nickel, which form their cores.

Mercury

The Mariner 10 mission (1974) mapped about half the surface of Mercury. On the basis of that data, we have a first-order understanding of the geology and history of the planet.Mariner 10 Special Issue (1975) JGR 80.] Vilas F. et al., eds. (1988) Mercury. Univ. Arizona Press, 794 pp.] Mercury's surface shows intercrater plains, basins, smooth plains, craters, and tectonic features.

Mercury's oldest surface is its intercrater plains,Gault D. E. et al (1975) JGR 80, 2444.] which are present (but much lessextensive) on the Moon. The intercrater plains are level to gently rolling terrain that occur between and around large craters. The plains predate the heavily cratered terrain, and have obliterated many of the early craters and basins of Mercury;Spudis P.D. and Guest J.E. (1988) in Mercury, 118-164.] they probably formed by widespread volcanism early in mercurian history.

Mercurian craters have the morphological elements of lunar craters- the smaller craters are bowl-shaped,and with increasing size, they develop scalloped rims, central peaks, and terraces on the inner walls. The ejectasheets have a hilly, lineated texture and swarms of secondary impact craters. Fresh craters of all sizes have dark orbright halos and well developed ray systems. Although mercurian and lunar craters are superficially similar, theyshow subtle differences, especially in deposit extent. The continuous ejecta and fields of secondary craters onMercury are far less extensive (by a factor of about 0.65) for a given rim diameter than those of comparable lunarcraters. This difference results from the 2.5 times higher gravitational field on Mercury compared with the Moon. As on the Moon, impact craters on Mercury are progressively degraded by subsequent impacts. Thefreshest craters have ray systems and a crisp morphology. With further degradation, the craters lose their crisp morphology and rays and features on the continuous ejecta become more blurred until only the raised rim near thecrater remains recognizable. Because craters become progressively degraded with time, the degree of degradationgives a rough indication of the crater's relative age. On the assumption that craters of similar size andmorphology are roughly the same age, it is possible to place constraints on the ages of other underlying or overlyingunits and thus to globally map the relative age of craters.

At least 15 ancient basins have been identified on Mercury. Tolstoj is a true multi-ring basin, displayingat least two, and possibly as many as four, concentric rings.Schaber G.G. et al. (1977) PEPI 15, 189.] It has a well-preserved ejecta blanket extendingoutward as much as convert|500|km|mi|0 from its rim. The basin interior is flooded with plains that clearly postdate the ejectadeposits. Beethoven has only one, subdued massif-like rim convert|625|km|mi|0 in diameter, but displays an impressive, well lineatedejecta blanket that extends as far as convert|500|km|mi|0. As at Tolstoj, Beethoven ejecta is asymmetric. The Calorisbasin is defined by a ring of mountains convert|1300|km|mi|0 in diameter.McCauley J.F. (1977) PEPI 15, 220.] McCauley J.F. et al.(1981) Icarus 47, 184] Individual massifs are typically convert|30|km|mi|0 to convert|50|km|mi|0long; the inner edge of the unit is marked by basin-facing scarps. Lineated terrain extends for about convert|1000|km|mi|0 outfrom the foot of a weak discontinuous scarp on the outer edge of the Caloris mountains; this terrain is similar to the"sculpture" surrounding the Imbrium basin on the Moon. Hummocky material forms a broad annulus aboutconvert|800|km|mi|0 from the Caloris mountains. It consists of low, closely spaced to scattered hills about 0.3 to convert|1|km|mi|0 across andfrom tens of meters to a few hundred meters high. The outer boundary of this unit is gradational with the (younger)smooth plains that occur in the same region. A hilly and furrowed terrain is found antipodal to the Caloris basin,probably created by antipodal convergence of intense seismic waves generated by the Caloris impact.Schultz, P.H. and Gault, D.E. (1975) The Moon 12, 159-177.]

The floor of the Caloris basin is deformed by sinuous ridges and fractures, giving the basin fill agrossly polygonal pattern. These plains may be volcanic, formed by the release of magma as part of the impactevent, or a thick sheet of impact melt. Widespread areas of Mercury are covered by relatively flat, sparsely crateredplains materials.Strom, R.G. et al. (1975)JGR 80, 2478.] They fill depressions that range in size from regional troughs to crater floors. The smoothplains are similar to the maria of the Moon, an obvious difference being that the smooth plains have the same albedoas the intercrater plains. Smooth plains are most strikingly exposed in a broad annulus around the Caloris basin. No unequivocal volcanic features, such as flow lobes, leveed channels, domes, or cones are visible. Craterdensities indicate that the smooth plains are significantly younger than ejecta from the Caloris basin. In addition,distinct color units, some of lobate shape, are observed in newly processed color data.Robinson M.R. and Lucey P.G. (1997) Science 275, 197-200.] Such relations stronglysupport a volcanic origin for the mercurian smooth plains, even in the absence of diagnostic landforms.

Lobate scarps are widely distributed over MercuryMelosh H.J. and McKinnonW.B. (1988) In Mercury, 374-400.] and consist of sinuous to arcuatescarps that transect preexisting plains and craters. They are most convincingly interpreted as thrust faults, indicatinga period of global compression. The lobate scarps typically transect smooth plains materials (early Calorianage) on the floors of craters, but post-Caloris craters are superposed on them. These observations suggest thatlobate-scarp formation was confined to a relatively narrow interval of time, beginning in the late pre-Tolstojanperiod and ending in the middle to late Calorian Period. In addition to scarps, wrinkle ridges occur in the smoothplains materials. These ridges probably were formed by local to regional surface compression caused by lithosphericloading by dense stacks of volcanic lavas, as suggested for those of the lunar maria.

Venus

The surface of Venus is comparatively very flat. When 93% of the topography was mapped by "Pioneer Venus", scientists found that the total distance from the lowest point to the highest point on the entire surface was about 13 kilometres (8 mi), while on the Earth the distance from the basins to the Himalayas is about 20 kilometres (12.4 mi).According to the data of the altimeters of the "Pioneer", nearly 51% of the surface is found located within 500 metres (1640 ft) of the median radius of 6,052 km (3760 mi); only 2% of the surface is located at greater elevations than convert|2|km|mi|0 from the median radius.

Venus shows no evidence of active plate tectonics. There is debatable evidence of active tectonics in the planet's distant past; however, events taking place since then (such as the plausible and generally accepted hypothesis that the Venusian lithosphere has thickened greatly over the course of several hundred million years) has made constraining the course of its geologic record difficult. However, the numerous well-preserved impact craters has been utilized as a dating method to approximately date the Venusian surface (since there are thus far no known samples of Venusian rock to be dated by more reliable methods). Dates derived are the dominantly in the range ~500 Mya - 750Mya, although ages of up to ~1.2 Gya have been calculated. This research has led to the fairly well accepted hypothesis that Venus has undergone an essentially complete volcanic resurfacing at least once in its distant past, with the last event taking place approximately within the range of estimated surface ages. While the mechanism of such an impressionable thermal event remains a debated issue in Venusian geosciences, some scientists are advocates of processes involving plate motion to some extent.

There are almost 1,000 impact craters on Venus, more or less evenly distributed across its surface.Earth-based radar surveys made it possible to identify some topographic patterns related to craters, and the "Venera 15" and "Venera 16" probes identified almost 150 such features of probable impact origin. Global coverage from "Magellan" subsequently made it possible to identify nearly 900 impact craters. Crater counts give an important estimate for the age of the surface of a planet. Over time, bodies in the solar system are randomly impacted, so the more craters a surface has, the older it is. Compared to Mercury, the Moon and other such bodies, Venus has very few craters. In part, this is because Venus's dense atmosphere burns up smaller meteorites before they hit the surface. The "Venera" and "Magellan" data agree: there are very few impact craters with a diameter less than convert|30|km|mi|0, and data from "Magellan" show an absence of any craters less than convert|2|km|mi|0 in diameter. However, there are also fewer of the large craters, and those appear relatively young; they are rarely filled with lava, showing that they happened after volcanic activity in the area, and radar shows that they are rough and have not had time to be eroded down.

[
pancake domes in Venus's Alpha Regio] Much of Venus' surface appears to have been shaped by volcanic activity. Overall, Venus has several times as many volcanoes as Earth, and it possesses some 167 giant volcanoes that are over convert|100|km|mi|0 across. The only volcanic complex of this size on Earth is the Big Island of Hawaii. However, this is not because Venus is more volcanically active than Earth, but because its crust is older. Earth's crust is continually recycled by subduction at the boundaries of tectonic plates, and has an average age of about 100 million years, while Venus' surface is estimated to be about 500 million years old.Frankel C. (1996), "Volcanoes of the solar system", Cambridge University Press, Cambridge, New York] Venusian craters range from convert|3|km|mi|0 to convert|280|km|mi|0 in diameter. There are no craters smaller than 3 km, because of the effects of the dense atmosphere on incoming objects. Objects with less than a certain kinetic energy are slowed down so much by the atmosphere that they do not create an impact crater. [Herrick R.R., Phillips R.J. (1993), "Effects of the Venusian atmosphere on incoming meteoroids and the impact crater population", Icarus, v. 112, p. 253–281]

Earth

The Earth's terrain varies greatly from place to place. About 70.8%cite web
last = Pidwirny
first = Michael
year = 2006
url = http://www.physicalgeography.net/fundamentals/7h.html
title = Fundamentals of Physical Geography
edition = 2nd Edition
publisher = PhysicalGeography.net
accessdate = 2007-03-19 ] of the surface is covered by water, with much of the continental shelf below sea level. The submerged surface has mountainous features, including a globe-spanning mid-ocean ridge system, as well as undersea volcanoes,cite web
author=Sandwell, D. T.; Smith, W. H. F.
date = Jul7 26, 2006
url =http://www.ngdc.noaa.gov/mgg/bathymetry/predicted/explore.HTML
title =Exploring the Ocean Basins with Satellite Altimeter Data
publisher = NOAA/NGDC
accessdate = 2007-04-21] oceanic trenches, submarine canyons, oceanic plateaus and abyssal plains. The remaining 29.2% not covered by water consists of mountains, deserts, plains, plateaus, and other geomorphologies.

The planetary surface undergoes reshaping over geological time periods due to the effects of tectonics and erosion. The surface features built up or deformed through plate tectonics are subject to steady weathering from precipitation, thermal cycles, and chemical effects. Glaciation, coastal erosion, the build-up of coral reefs, and large meteorite impacts [cite web | last = Kring | first = David A. | url = http://www.lpl.arizona.edu/SIC/impact_cratering/intro/ | title = Terrestrial Impact Cratering and Its Environmental Effects | publisher = Lunar and Planetary Laboratory | accessdate = 2007-03-22 ] also act to reshape the landscape.

As the continental plates migrate across the planet, the ocean floor is subducted under the leading edges. At the same time, upwellings of mantle material create a divergent boundary along mid-ocean ridges. The combination of these processes continually recycles the ocean plate material. Most of the ocean floor is less than 100 million years in age. The oldest ocean plate is located in the Western Pacific, and has an estimated age of about 200 million years. By comparison, the oldest fossils found on land have an age of about 3 billion years. [cite web | last = Duennebier | first = Fred | date = August 12, 1999 | url = http://www.soest.hawaii.edu/GG/ASK/plate-tectonics2.html | title = Pacific Plate Motion | publisher = University of Hawaii | accessdate = 2007-03-14 ] [cite web | author=Mueller, R.D.; Roest, W.R.; Royer, J.-Y.; Gahagan, L.M.; Sclater, J.G. | date = March 7, 2007 | url = http://www.ngdc.noaa.gov/mgg/fliers/96mgg04.html | title = Age of the Ocean Floor Poster | publisher = NOAA | accessdate = 2007-03-14 ]

The continental plates consist of lower density material such as the igneous rocks granite and andesite. Less common is basalt, a denser volcanic rock that is the primary constituent of the ocean floors. [cite web | author=Staff | url = http://volcano.und.edu/vwdocs/vwlessons/plate_tectonics/part1.html | title = Layers of the Earth | publisher = Volcano World | accessdate = 2007-03-11 ] Sedimentary rockis formed from the accumulation of sediment that becomes compacted together. Nearly 75% of the continental surfaces are covered by sedimentary rocks, although they form only about 5% of the crust. [cite web | last=Jessey | first=David | url = http://geology.csupomona.edu/drjessey/class/Gsc101/Weathering.html | title = Weathering and Sedimentary Rocks | publisher = Cal Poly Pomona | accessdate = 2007-03-20 ] The third form of rock material found on Earth is metamorphic rock, which is created from the transformation of pre-existing rock types through high pressures, high temperatures, or both. The most abundant silicate minerals on the Earth's surface include quartz, the feldspars, amphibole, mica, pyroxene and olivine. [cite web | author=Staff | url = http://natural-history.uoregon.edu/Pages/web/mineral.htm | title = Minerals | publisher = Museum of Natural History, Oregon | accessdate = 2007-03-20 ] Common carbonate minerals include calcite (found in limestone), aragonite and dolomite. [cite web
last=Cox
first=Ronadh
year=2003
url=http://madmonster.williams.edu/geos.302/L.08.html
title=Carbonate sediments
publisher=Williams College
accessdate=2007-04-21] The pedosphere is the outermost layer of the Earth that is composed of soil and subject to soil formation processes. It exists at the interface of the lithosphere, atmosphere, hydrosphere and biosphere. Currently the total arable land is 13.31% of the land surface, with only 4.71% supporting permanent crops.cite web | author=Staff | date = February 8, 2007 | url = https://www.cia.gov/library/publications/the-world-factbook/geos/xx.html | title = The World Factbook | publisher = U.S. C.I.A. | accessdate = 2007-02-25 ] Close to 40% of the Earth's land surface is presently used for cropland and pasture, or an estimated 1.3e|9 hectares (3.3e|9 acres) of cropland and 3.4e|9 hectares (8.4e|9 acres) of pastureland. [cite book
author=FAO Staff
year=1995
title=FAO Production Yearbook 1994
edition=Volume 48
publisher=Food and Agriculture Organization of the United Nations
location=Rome, Italy
id=ISBN 9250038445 ]

The physical features of land are remarkably varied. The largest mountain ranges- the Himalayas in Asia and the Andes in South America- extend for thousands of kilometres. The longest rivers are the river Nile in Africa (convert|6695|km|mi|0|disp=/) and the Amazon river in South America (convert|6437|km|mi|0|disp=/). Deserts cover about 20% of the total land area. The largest is the Sahara, which covers nearly one-third of Africa.

The elevation of the land surface of the Earth varies from the low point of −418 m (−1,371 ft) at the Dead Sea, to a 2005-estimated maximum altitude of 8,848 m (29,028 ft) at the top of Mount Everest. The mean height of land above sea level is 686 m (2,250 ft).cite journal | last = Mill | first = Hugh Robert | title=The Permanence of Ocean Basins | journal=The Geographical Journal | year=1893 | volume=1 | issue=3 | pages=230–234 | url=http://www.wku.edu/~smithch/wallace/S453.htm | accessdate=2007-02-25 | doi=10.2307/1773821 ]

The geological history of Earth can be broadly classified into two periods namely:
* Precambrian: includes approximately 90% of geologic time. It extends from 4.6 billion years ago to the beginning of the Cambrian Period (about 570 Ma). It is generally believed that small proto-continents existed prior to 3000 Ma, and that most of the Earth's landmasses collected into a single supercontinent around 1000 Ma.
* Phanerozoic: is the current eon in the geologic timescale. It covers roughly 545 million years. During the period covered, continents drifted about, eventually collected into a single landmass known as Pangea and then split up into the current continental landmasses.

Mars

The surface of Mars is thought to be primarily composed of basalt, based upon the observed lava flows from volcanos, the Martian meteorite collection, and data from landers and orbital observations. The lava flows from Martian volcanos show that that lava has a very low viscosity, typical of basalt.cite web
url = http://erc.arc.nasa.gov/MarsVolc/basalt.htm
title = NASA Mars Page|work=Volcanology of Mars
accessdate = June 13
accessyear = 2006] Analysis of the soil samples collected by the Viking landers in 1976 indicate iron-rich clays consistent with weathering of basaltic rocks.lala] There is some evidence that some portion of the Martian surface might be more silica-rich than typical basalt, perhaps similar to andesitic rocks on Earth, though these observations may also be explained by silica glass, phyllosilicates, or opal. Much of the surface is deeply covered by dust as fine as talcum powder. The red/orange appearance of Mars' surface is caused by iron(III) oxide (rust).Peplow, Mark, [http://www.nature.com/news/2004/040503/full/040503-6.html;jsessionid=2E40E6D0BDA9D25A3BAF1DFC53F9FA40 "How Mars got its rust"] - 6 May 2004 article from [http://www.nature.com Nature.com] . URL accessed 18 April 2006.] [cite web
last = Peplow
first = Mark
url = http://www.bioedonline.org/news/news.cfm?art=953
title = How Mars got its rust
accessdate = March 3
accessyear = 2007] Mars has twice as much iron oxide in its outer layer as Earth does, despite their supposed similar origin. It is thought that Earth, being hotter, transported much of the iron downwards in the convert|1800|km|mi|0 deep, convert|3200|°C|°F|0|lk=on, lava seas of the early planet, while Mars, with a lower lava temperature of convert|2200|°C|°F|0 was too cool for this to happen.lala]

The core is surrounded by a silicate mantle that formed many of the tectonic and volcanic features on the planet. The average thickness of the planet's crust is about 50 km, and it is no thicker than convert|125|km|mi|0, [ cite news |author = Dave Jacqué |url = http://www.anl.gov/Media_Center/News/2003/030926mars.htm |title = APS X-rays reveal secrets of Mars' core | publisher = Argonne National Laboratory | date = 2003-09-26 | accessdate = 2006-07-01 | language = English ] which is much thicker than Earth's crust which varies between convert|5|km|mi|0 and convert|70|km|mi|0. As a result Mars' crust does not easily deform, as was shown by the recent radar map of the south polar ice cap which does not deform the crust despite being about 3km thick.cite web
last =Dunham
first =Will
authorlink =Will Dunham
title =Immense ice deposits found at south pole of Mars
work =Yahoo! News
publisher =Yahoo!, Inc.
date =2007-03-15
url =http://news.yahoo.com/s/nm/20070315/sc_nm/mars_water_dc_2
accessdate = 2007-03-16 ]

Crater morphology provides information about the physical structure and composition of the surface. Impact craters allow us to look deep below the surface and into Mars geological past. Lobate ejecta blankets (pictured left) and central pit craters are common on Mars but uncommon on the Moon, which may indicate the presence of near-surface volatiles (ice and water) on Mars. Degraded impact structures record variations in volcanic, fluvial, and eolian activity. [cite web
title = Stones, Wind and Ice
author = Nadine Barlow
publisher = Lunar and Planetary Institute
url = http://www.lpi.usra.edu/publications/slidesets/stones/
accessdate = 2007-03-15]

The Yuty crater is an example of a Rampart crater so called because of the rampart like edge of the ejecta. In the Yuty crater the ejecta completely covers an older crater at its side, showing that the ejected material is just a thin layer. [cite web | title = Viking Orbiter Views Of Mars
author =
publisher = NASA
url = http://history.nasa.gov/SP-441/ch7.htm
accessdate = 2007-03-16]

The geological history of Mars can be broadly classified into many epochs, but the following are the three major ones:
* Noachian epoch (named after Noachis Terra): Formation of the oldest extant surfaces of Mars, 3.8 billion years ago to 3.5 billion years ago. Noachian age surfaces are scarred by many large impact craters. The Tharsis bulge volcanic upland is thought to have formed during this period, with extensive flooding by liquid water late in the epoch.
* Hesperian epoch (named after Hesperia Planum): 3.5 billion years ago to 1.8 billion years ago. The Hesperian epoch is marked by the formation of extensive lava plains.
* Amazonian epoch (named after Amazonis Planitia): 1.8 billion years ago to present. Amazonian regions have few meteorite impact craters but are otherwise quite varied. Olympus Mons formed during this period along with lava flows elsewhere on Mars.

Small Solar System bodies

Asteroids, comets and meteoroids are all debris remaining from the nebula in which the Solar system formed 4.6 billion years ago.

Asteroid belt

Asteroid belt is located between Mars and Jupiter. It is made of thousands of rocky planetesimals from convert|1000|km|mi|0 to a few meters across. These are thought to be debris of the formation of the solar system that could not form a planet due to Jupiter's gravity. When asteroids collide they produce small fragments that occasionally fall on Earth. These rocks are called meteorites and provide information about the primordial solar nebula. Most of these fragments have the size of sand grains. They burn up in the Earth's atmosphere, causing them to glow like meteors.

Comets

A comet is a small body in the solar system that orbits the Sun and (at least occasionally) exhibits a coma (or atmosphere) and/or a tail — both primarily from the effects of solar radiation upon the comet's nucleus, which itself is a minor body composed of rock, dust, and ice.

Kuiper belt

The Kuiper belt sometimes called the Edgeworth-Kuiper belt, is a region of the Solar System beyond the planets extending from the orbit of Neptune (at 30 AU) [One AU, or "astronomical unit", is the average distance between the Earth and the Sun, or roughly 149 597 870 691 metres. It is the standard unit of measurement for interplanetary distances.] to approximately 55 AU from the Sun. [cite web | title=Collisional Erosion in the Primordial Edgeworth-Kuiper Belt and the Generation of the 30–50 AU Kuiper Gap | author=S. ALAN STERN | work=Geophysical, Astrophysical, and Planetary Sciences, Space Science Department, Southwest Research Institute | url=http://www.journals.uchicago.edu/doi/full/10.1086/304912 | year=1997 | accessdate=2007-06-01] It is similar to the asteroid belt, although it is far larger; 20 times as wide and 20–200 times as massive.cite web|title=The Solar System Beyond The Planets|author=Audrey Delsanti and David Jewitt|work=Institute for Astronomy, University of Hawaii|url=http://www.ifa.hawaii.edu/faculty/jewitt/papers/2006/DJ06.pdf|accessdate=2007-03-09] [cite journal| authorlink= Georgij A. Krasinsky | first=G. A. | last= Krasinsky | coauthors=Pitjeva, E. V.; Vasilyev, M. V.; Yagudina, E. I. | url=http://adsabs.harvard.edu/cgi-bin/nph-bib_query?bibcode=2002Icar..158...98K&db_key=AST&data_type=HTML&format=&high=4326fb2cf906949| title=Hidden Mass in the Asteroid Belt| journal=Icarus| volume=158| issue=1| pages=98–105| month= July| year= 2002| doi=10.1006/icar.2002.6837] Like the asteroid belt, it consists mainly of small bodies (remnants from the Solar System's formation) and at least one dwarf planet – Pluto. But while the asteroid belt is composed primarily of rock and metal, the Kuiper belt is composed largely of ices, such as methane, ammonia, and water. The objects within the Kuiper belt, together with the members of the scattered disc and any potential Hills cloud or Oort cloud objects, are collectively referred to as trans-Neptunian objects (TNOs). [cite web|title= DESCRIPTION OF THE SYSTEM OF ASTEROIDS AS OF MAY 20, 2004|author= Gérard FAURE|url=http://www.astrosurf.com/aude/map/us/AstFamilies2004-05-20.htm|year=2004|accessdate=2007-06-01]

References

External links

* [http://www.iau.org International Astronomical Union]
* [http://www.fourmilab.ch/cgi-bin/uncgi/Solar/ Solar System Live] (an interactive orrery)
* [http://janus.astro.umd.edu/javadir/orbits/ssv.html Solar System Viewer] (animation)
* [http://www.sky-pics.net/ Pictures of the Solar System]
* [http://gw.marketingden.com/planets/sun.html Renderings of the planets]
* [http://planetquest.jpl.nasa.gov/ NASA Planet Quest]
* [http://www.co-intelligence.org/newsletter/comparisons.html Illustration comparing the sizes of the planets with each other, the sun, and other stars]
* [http://www.space.com/scienceastronomy/060816_planet_qanda.html Q&A: The IAU's Proposed Planet Definition]
* [http://news.bbc.co.uk/1/hi/sci/tech/4798205.stm Q&A New planets proposal]
* [http://space.about.com/cs/solarsystem/ Solar system] — About Space
* [http://history.nasa.gov/SP-423/sp423.htm Atlas of Mercury — NASA]
* [http://www.nineplanets.org/ Nine Planets Information]
* [http://nssdc.gsfc.nasa.gov/planetary/factsheet/ NASA’s fact sheet]
* [http://www.psrd.hawaii.edu/ Planetary Science Research Discoveries]