![5 Terrestrial planets size comparison](https://upload.wikimedia.org/wikipedia/commons/3/3a/5_Terrestrial_planets_size_comparison.png)
The geology of solar terrestrial planets mainly deals with the
geological
Geology () is a branch of natural science concerned with Earth and other astronomical objects, the features or rocks of which it is composed, and the processes by which they change over time. Modern geology significantly overlaps all other E ...
aspects of the four
terrestrial planet
A terrestrial planet, telluric planet, or rocky planet, is a planet that is composed primarily of silicate rocks or metals. Within the Solar System, the terrestrial planets accepted by the IAU are the inner planets closest to the Sun: Mercury, ...
s of the
Solar System
The Solar System Capitalization of the name varies. The International Astronomical Union, the authoritative body regarding astronomical nomenclature, specifies capitalizing the names of all individual astronomical objects but uses mixed "Solar ...
–
Mercury,
Venus
Venus is the second planet from the Sun. It is sometimes called Earth's "sister" or "twin" planet as it is almost as large and has a similar composition. As an interior planet to Earth, Venus (like Mercury) appears in Earth's sky never f ...
,
Earth
Earth is the third planet from the Sun and the only astronomical object known to harbor life. While large volumes of water can be found throughout the Solar System, only Earth sustains liquid surface water. About 71% of Earth's surfa ...
, and
Mars
Mars is the fourth planet from the Sun and the second-smallest planet in the Solar System, only being larger than Mercury. In the English language, Mars is named for the Roman god of war. Mars is a terrestrial planet with a thin at ...
– and one terrestrial
dwarf planet
A dwarf planet is a small planetary-mass object that is in direct orbit of the Sun, smaller than any of the eight classical planets but still a world in its own right. The prototypical dwarf planet is Pluto. The interest of dwarf planets to ...
:
Ceres
Ceres most commonly refers to:
* Ceres (dwarf planet), the largest asteroid
* Ceres (mythology), the Roman goddess of agriculture
Ceres may also refer to:
Places
Brazil
* Ceres, Goiás, Brazil
* Ceres Microregion, in north-central Goiás ...
. Earth is the only terrestrial planet known to have an active
hydrosphere
The hydrosphere () is the combined mass of water found on, under, and above the surface of a planet, minor planet, or natural satellite. Although Earth's hydrosphere has been around for about 4 billion years, it continues to change in shape. This ...
.
Terrestrial planets are substantially different from the
giant planet
The giant planets constitute a diverse type of planet much larger than Earth. They are usually primarily composed of low-boiling-point materials (volatiles), rather than rock or other solid matter, but massive solid planets can also exist. The ...
s, which might not have
solid surfaces and are composed mostly of some combination of
hydrogen
Hydrogen is the chemical element with the symbol H and atomic number 1. Hydrogen is the lightest element. At standard conditions hydrogen is a gas of diatomic molecules having the formula . It is colorless, odorless, tasteless, non-toxic ...
,
helium
Helium (from el, ἥλιος, helios, lit=sun) is a chemical element with the symbol He and atomic number 2. It is a colorless, odorless, tasteless, non-toxic, inert, monatomic gas and the first in the noble gas group in the periodic table. ...
, and
water
Water (chemical formula ) is an Inorganic compound, inorganic, transparent, tasteless, odorless, and Color of water, nearly colorless chemical substance, which is the main constituent of Earth's hydrosphere and the fluids of all known living ...
existing in various
physical state
In physics, a state of matter is one of the distinct forms in which matter can exist. Four states of matter are observable in everyday life: solid, liquid, gas, and plasma. Many intermediate states are known to exist, such as liquid crystal, ...
s. Terrestrial planets have a compact, rocky surfaces, and Venus, Earth, and Mars each also has an
atmosphere. Their size, radius, and density are all similar.
Terrestrial planets have numerous similarities to
dwarf planets
A dwarf planet is a small planetary-mass object that is in direct orbit of the Sun, smaller than any of the eight classical planets but still a world in its own right. The prototypical dwarf planet is Pluto. The interest of dwarf planets to p ...
(objects like
Pluto
Pluto (minor-planet designation: 134340 Pluto) is a dwarf planet in the Kuiper belt, a ring of bodies beyond the orbit of Neptune. It is the ninth-largest and tenth-most-massive known object to directly orbit the Sun. It is the largest ...
), which also have a solid surface, but are primarily composed of icy materials. During the formation of the Solar System, there were probably many more (
planetesimal
Planetesimals are solid objects thought to exist in protoplanetary disks and debris disks. Per the Chamberlin–Moulton planetesimal hypothesis, they are believed to form out of cosmic dust grains. Believed to have formed in the Solar System a ...
s), but they have all merged with or been destroyed by the four remaining worlds in the
solar nebula.
The terrestrial planets all have roughly the same structure: a central metallic core, mostly
iron
Iron () is a chemical element with Symbol (chemistry), symbol Fe (from la, Wikt:ferrum, ferrum) and atomic number 26. It is a metal that belongs to the first transition series and group 8 element, group 8 of the periodic table. It is, Abundanc ...
, with a surrounding silicate
mantle. The
Moon
The Moon is Earth's only natural satellite. It is the fifth largest satellite in the Solar System and the largest and most massive relative to its parent planet, with a diameter about one-quarter that of Earth (comparable to the width of ...
is similar, but lacks a substantial iron core. Three of the four solar terrestrial planets (Venus, Earth, and Mars) have substantial
atmospheres; all have
impact crater
An impact crater is a circular depression in the surface of a solid astronomical object formed by the hypervelocity impact of a smaller object. In contrast to volcanic craters, which result from explosion or internal collapse, impact crater ...
s and
tectonic
Tectonics (; ) are the processes that control the structure and properties of the Earth's crust and its evolution through time. These include the processes of mountain building, the growth and behavior of the strong, old cores of continents ...
surface features such as
rift valley
A rift valley is a linear shaped lowland between several highlands or mountain ranges created by the action of a geologic rift. Rifts are formed as a result of the pulling apart of the lithosphere due to extensional tectonics. The linear d ...
s and
volcano
A volcano is a rupture in the Crust (geology), crust of a Planet#Planetary-mass objects, planetary-mass object, such as Earth, that allows hot lava, volcanic ash, and volcanic gas, gases to escape from a magma chamber below the surface.
On Ear ...
es.
The term ''inner planet'' should not be confused with ''
inferior planet
In the Solar System, a planet is said to be inferior or interior with respect to another planet if its orbit lies inside the other planet's orbit around the Sun. In this situation, the latter planet is said to be superior to the former. In the ref ...
'', which refers to any planet that is closer to the Sun than the observer's planet is, but usually refers to Mercury and Venus.
Formation of solar planets
![The_Mysterious_Case_of_the_Disappearing_Dust](https://upload.wikimedia.org/wikipedia/commons/2/2d/The_Mysterious_Case_of_the_Disappearing_Dust.jpg)
The Solar System is believed to have formed according to the
nebular hypothesis
The nebular hypothesis is the most widely accepted model in the field of cosmogony to explain the formation and evolution of the Solar System (as well as other planetary systems). It suggests the Solar System is formed from gas and dust orbiting t ...
, first proposed in 1755 by
Immanuel Kant
Immanuel Kant (, , ; 22 April 1724 – 12 February 1804) was a German philosopher and one of the central Enlightenment thinkers. Born in Königsberg, Kant's comprehensive and systematic works in epistemology, metaphysics, ethics, and ...
and independently formulated by
Pierre-Simon Laplace
Pierre-Simon, marquis de Laplace (; ; 23 March 1749 – 5 March 1827) was a French scholar and polymath whose work was important to the development of engineering, mathematics, statistics, physics, astronomy, and philosophy. He summarized ...
.
This theory holds that 4.6 billion years ago the Solar System formed from the gravitational collapse of a giant
molecular cloud
A molecular cloud, sometimes called a stellar nursery (if star formation is occurring within), is a type of interstellar cloud, the density and size of which permit absorption nebulae, the formation of molecules (most commonly molecular hydroge ...
. This initial cloud was likely several light-years across and probably birthed several stars.
The first solid particles were microscopic in size. These particles orbited the
Sun
The Sun is the star at the center of the Solar System. It is a nearly perfect ball of hot plasma, heated to incandescence by nuclear fusion reactions in its core. The Sun radiates this energy mainly as light, ultraviolet, and infrared radi ...
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
Planetesimals are solid objects thought to exist in protoplanetary disks and debris disks. Per the Chamberlin–Moulton planetesimal hypothesis, they are believed to form out of cosmic dust grains. Believed to have formed in the Solar System a ...
—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
protoplanet
A protoplanet is a large planetary embryo that originated within a protoplanetary disc and has undergone internal melting to produce a differentiated interior. Protoplanets are thought to form out of kilometer-sized planetesimals that gravitation ...
s by accretion of planetesimals of similar composition.
After the protoplanet formed, accumulation of heat from radioactive decay of short-lived elements melted the planet, allowing materials to differentiate (i.e. to separate according to their
density
Density (volumetric mass density or specific mass) is the substance's mass per unit of volume. The symbol most often used for density is ''ρ'' (the lower case Greek letter rho), although the Latin letter ''D'' can also be used. Mathematical ...
).
Terrestrial planets
In the warmer inner Solar System, planetesimals formed from
rock
Rock most often refers to:
* Rock (geology), a naturally occurring solid aggregate of minerals or mineraloids
* Rock music, a genre of popular music
Rock or Rocks may also refer to:
Places United Kingdom
* Rock, Caerphilly, a location in Wales ...
s and
metal
A metal (from Greek μέταλλον ''métallon'', "mine, quarry, metal") is a material that, when freshly prepared, polished, or fractured, shows a lustrous appearance, and conducts electricity and heat relatively well. Metals are typicall ...
s cooked billions of years ago in the cores of massive
stars.
These elements constituted only 0.6% of the material in the
solar nebula. That is why the terrestrial planets could not grow 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 had
hydrogen
Hydrogen is the chemical element with the symbol H and atomic number 1. Hydrogen is the lightest element. At standard conditions hydrogen is a gas of diatomic molecules having the formula . It is colorless, odorless, tasteless, non-toxic ...
and
helium
Helium (from el, ἥλιος, helios, lit=sun) is a chemical element with the symbol He and atomic number 2. It is a colorless, odorless, tasteless, non-toxic, inert, monatomic gas and the first in the noble gas group in the periodic table. ...
, the Sun would have heated the gases and caused them to escape.
Hence, solar terrestrial planets such as
Mercury,
Venus
Venus is the second planet from the Sun. It is sometimes called Earth's "sister" or "twin" planet as it is almost as large and has a similar composition. As an interior planet to Earth, Venus (like Mercury) appears in Earth's sky never f ...
,
Earth
Earth is the third planet from the Sun and the only astronomical object known to harbor life. While large volumes of water can be found throughout the Solar System, only Earth sustains liquid surface water. About 71% of Earth's surfa ...
, and
Mars
Mars is the fourth planet from the Sun and the second-smallest planet in the Solar System, only being larger than Mercury. In the English language, Mars is named for the Roman god of war. Mars is a terrestrial planet with a thin at ...
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 planet
A terrestrial planet, telluric planet, or rocky planet, is a planet that is composed primarily of silicate rocks or metals. Within the Solar System, the terrestrial planets accepted by the IAU are the inner planets closest to the Sun: Mercury, ...
s have dense,
rocky
''Rocky'' is a 1976 American sports drama film directed by John G. Avildsen and written by and starring Sylvester Stallone. It is the first installment in the ''Rocky'' franchise and stars Talia Shire, Burt Young, Carl Weathers, and Burge ...
compositions, few or no
moons
A natural satellite is, in the most common usage, an astronomical body that orbits a planet, dwarf planet, or small Solar System body (or sometimes another natural satellite). Natural satellites are often colloquially referred to as ''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
Iron () is a chemical element with Symbol (chemistry), symbol Fe (from la, Wikt:ferrum, ferrum) and atomic number 26. It is a metal that belongs to the first transition series and group 8 element, group 8 of the periodic table. It is, Abundanc ...
and
nickel
Nickel is a chemical element with symbol Ni and atomic number 28. It is a silvery-white lustrous metal with a slight golden tinge. Nickel is a hard and ductile transition metal. Pure nickel is chemically reactive but large pieces are slow ...
, which form their
cores.
Mercury
The Mariner 10 mission (1974) mapped about half the surface of Mercury. On the basis of that data, scientists 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
Basin may refer to:
Geography and geology
* Depression (geology)
** Back-arc basin, a submarine feature associated with island arcs and subduction zones
** Debris basin, designed to prevent damage from debris flow
** Drainage basin (hydrology), a ...
, smooth
plain
In geography, a plain is a flat expanse of land that generally does not change much in elevation, and is primarily treeless. Plains occur as lowlands along valleys or at the base of mountains, as coastal plains, and as plateaus or uplands ...
s,
craters, and
tectonic
Tectonics (; ) are the processes that control the structure and properties of the Earth's crust and its evolution through time. These include the processes of mountain building, the growth and behavior of the strong, old cores of continents ...
features.
Mercury's oldest surface is its intercrater plains,
[Gault D. E. et al. (1975) JGR 80, 2444.] which are present (but much less extensive) on the
Moon
The Moon is Earth's only natural satellite. It is the fifth largest satellite in the Solar System and the largest and most massive relative to its parent planet, with a diameter about one-quarter that of Earth (comparable to the width of ...
. The intercrater plains are level to gently rolling
terrain
Terrain or relief (also topographical relief) involves the vertical and horizontal dimensions of land surface. The term bathymetry is used to describe underwater relief, while hypsometry studies terrain relative to sea level. The Latin wo ...
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 ejecta sheets have a hilly, lineated texture and swarms of secondary impact craters. Fresh craters of all sizes have dark or bright halos and well-developed ray systems. Although mercurian and lunar craters are superficially similar, they show subtle differences, especially in deposit extent. The continuous ejecta and fields of secondary craters on Mercury are far less extensive (by a factor of about 0.65) for a given rim diameter than those of comparable lunar craters. 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.
The freshest 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 the crater remains recognizable. Because craters become progressively degraded with time, the degree of degradation gives a rough indication of the crater's relative age.
On the assumption that craters of similar size and morphology are roughly the same age, it is possible to place constraints on the ages of other underlying or overlying units and thus to globally map the relative age of craters.
![Caloris basin labeled](https://upload.wikimedia.org/wikipedia/commons/7/75/Caloris_basin_labeled.png)
At least 15 ancient basins have been identified on Mercury.
Tolstoj is a true
multi-ring basin
A multi-ringed basin (also a multi-ring impact basin) is not a simple bowl-shaped crater, or a Peak ring (crater), peak ring crater, but one containing multiple concentric Topography, topographic rings; a multi-ringed basin could be described as a ...
, displaying at 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 extending outward as much as from its rim. The basin interior is flooded with plains that clearly postdate the ejecta deposits.
Beethoven
Ludwig van Beethoven (baptised 17 December 177026 March 1827) was a German composer and pianist. Beethoven remains one of the most admired composers in the history of Western music; his works rank amongst the most performed of the classic ...
has only one, subdued massif-like rim in diameter, but displays an impressive, well lineated ejecta blanket that extends as far as . As at Tolstoj, Beethoven ejecta is asymmetric. The Caloris basin is defined by a ring of mountains in diameter.
[McCauley J.F. (1977) PEPI 15, 220.][McCauley J.F. et al.
(1981) Icarus 47, 184] Individual massifs are typically to long; the inner edge of the unit is marked by basin-facing scarps.
Lineated terrain extends for about out from 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 about from the
Caloris mountains. It consists of low, closely spaced to scattered hills about 0.3 to across and from 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.]
![Mercury's 'Weird Terrain'](https://upload.wikimedia.org/wikipedia/commons/7/74/Mercury%27s_%27Weird_Terrain%27.jpg)
The floor of the Caloris basin is deformed by sinuous ridges and fractures, giving the basin fill a grossly polygonal pattern. These plains may be volcanic, formed by the release of magma as part of the impact event, or a thick sheet of impact melt. Widespread areas of Mercury are covered by relatively flat, sparsely cratered plains materials.
[Strom, R.G. et al. (1975)
JGR 80, 2478.] They fill depressions that range in size from regional troughs to crater floors. The smooth plains are similar to the maria of the Moon, an obvious difference being that the smooth plains have the same albedo as 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. Crater densities 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 strongly support a volcanic origin for the mercurian smooth plains, even in the absence of diagnostic landforms.
Lobate scarps are widely distributed over Mercury
[Melosh H.J. and McKinnon
W.B. (1988) In Mercury, 374-400.] and consist of sinuous to arcuate scarps that transect preexisting plains and craters. They are most convincingly interpreted as
thrust faults
A thrust fault is a break in the Earth's crust, across which older rocks are pushed above younger rocks.
Thrust geometry and nomenclature
Reverse faults
A thrust fault is a type of reverse fault that has a dip of 45 degrees or less.
If ...
, indicating a period of global compression.
The lobate scarps typically transect smooth plains materials (early Calorian age) on the floors of craters, but post-Caloris craters are superposed on them. These observations suggest that lobate-scarp formation was confined to a relatively narrow interval of time, beginning in the late pre-Tolstojan period and ending in the middle to late Calorian Period. In addition to scarps, wrinkle ridges occur in the smooth plains materials. These ridges probably were formed by local to regional surface compression caused by lithospheric loading 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
Topography is the study of the forms and features of land surfaces. The topography of an area may refer to the land forms and features themselves, or a description or depiction in maps.
Topography is a field of geoscience and planetary sc ...
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
Basin may refer to:
Geography and geology
* Depression (geology)
** Back-arc basin, a submarine feature associated with island arcs and subduction zones
** Debris basin, designed to prevent damage from debris flow
** Drainage basin (hydrology), a ...
to the
Himalaya
The Himalayas, or Himalaya (; ; ), is a mountain range in Asia, separating the plains of the Indian subcontinent from the Tibetan Plateau. The range has some of the planet's highest peaks, including the very highest, Mount Everest. Over 100 ...
s is about 20 kilometres (12.4 mi).
According to the data of the
altimeter
An altimeter or an altitude meter is an instrument used to measure the altitude of an object above a fixed level. The measurement of altitude is called altimetry, which is related to the term bathymetry, the measurement of depth under water. The m ...
s of the ''Pioneer'', nearly 51% of the surface is found located within 500 metres (1,640 ft) of the median radius of 6,052 km (3760 mi); only 2% of the surface is located at greater elevations than from the median radius.
![PIA00103 Venus - 3-D Perspective View of Lavinia Planitia](https://upload.wikimedia.org/wikipedia/commons/6/60/PIA00103_Venus_-_3-D_Perspective_View_of_Lavinia_Planitia.jpg)
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 crater
An impact crater is a circular depression in the surface of a solid astronomical object formed by the hypervelocity impact of a smaller object. In contrast to volcanic craters, which result from explosion or internal collapse, impact crater ...
s 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
Venera 15 (russian: Венера-15 meaning ''Venus 15'') was a spacecraft sent to Venus by the Soviet Union. This uncrewed orbiter was to map the surface of Venus using high resolution imaging systems. The spacecraft was identical to Venera 16 ...
'' and ''
Venera 16
Venera 16 (russian: Венера-16 meaning ''Venus 16'') was a spacecraft sent to Venus by the Soviet Union. This uncrewed orbiter was to map the surface of Venus using high resolution imaging systems. The spacecraft was identical to Venera 1 ...
'' probes identified almost 150 such features of probable impact origin. Global coverage from ''Magellan'' subsequently made it possible to identify nearly 900 impact craters.
![Venus Danilova Aglaonice Saskia](https://upload.wikimedia.org/wikipedia/commons/d/d1/Venus_Danilova_Aglaonice_Saskia.png)
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
The Moon is Earth's only natural satellite. It is the fifth largest satellite in the Solar System and the largest and most massive relative to its parent planet, with a diameter about one-quarter that of Earth (comparable to the width of ...
and other such bodies, Venus has very few craters. In part, this is because Venus's dense atmosphere burns up smaller
meteor
A meteoroid () is a small rocky or metallic body in outer space.
Meteoroids are defined as objects significantly smaller than asteroids, ranging in size from grains to objects up to a meter wide. Objects smaller than this are classified as mi ...
ites before they hit the surface. The ''Venera'' and ''Magellan'' data agree: there are very few impact craters with a diameter less than , and data from ''Magellan'' show an absence of any craters less than 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.
![Venus dome 3D](https://upload.wikimedia.org/wikipedia/commons/4/44/Venus_dome_3D.jpg)
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 across. The only volcanic complex of this size on Earth is the
Big Island of
Hawaii
Hawaii ( ; haw, Hawaii or ) is a state in the Western United States, located in the Pacific Ocean about from the U.S. mainland. It is the only U.S. state outside North America, the only state that is an archipelago, and the only state ...
. 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 plate
Plate tectonics (from the la, label=Late Latin, tectonicus, from the grc, τεκτονικός, lit=pertaining to building) is the generally accepted scientific theory that considers the Earth's lithosphere to comprise a number of large te ...
s, 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 to 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
In physics, the kinetic energy of an object is the energy that it possesses due to its motion.
It is defined as the work needed to accelerate a body of a given mass from rest to its stated velocity. Having gained this energy during its acc ...
are slowed down so much by the atmosphere that they do not create an impact crater.
Earth
![AYool topography 15min](https://upload.wikimedia.org/wikipedia/commons/2/24/AYool_topography_15min.png)
The Earth's
terrain
Terrain or relief (also topographical relief) involves the vertical and horizontal dimensions of land surface. The term bathymetry is used to describe underwater relief, while hypsometry studies terrain relative to sea level. The Latin wo ...
varies greatly from place to place. About 70.8%
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
A mid-ocean ridge (MOR) is a seafloor mountain system formed by plate tectonics. It typically has a depth of about and rises about above the deepest portion of an ocean basin. This feature is where seafloor spreading takes place along a diver ...
system, as well as undersea
volcano
A volcano is a rupture in the Crust (geology), crust of a Planet#Planetary-mass objects, planetary-mass object, such as Earth, that allows hot lava, volcanic ash, and volcanic gas, gases to escape from a magma chamber below the surface.
On Ear ...
es,
oceanic trench
Oceanic trenches are prominent long, narrow topographic depressions of the ocean floor. They are typically wide and below the level of the surrounding oceanic floor, but can be thousands of kilometers in length. There are about of oceanic tren ...
es,
submarine canyon
A submarine canyon is a steep-sided valley cut into the seabed of the continental slope, sometimes extending well onto the continental shelf, having nearly vertical walls, and occasionally having canyon wall heights of up to 5 km, from c ...
s,
oceanic plateaus, and
abyssal plain
An abyssal plain is an underwater plain on the deep ocean floor, usually found at depths between and . Lying generally between the foot of a continental rise and a mid-ocean ridge, abyssal plains cover more than 50% of the Earth's surface. ...
s. The remaining 29.2% not covered by water consists of
mountains,
deserts
A desert is a barren area of landscape where little precipitation occurs and, consequently, living conditions are hostile for plant and animal life. The lack of vegetation exposes the unprotected surface of the ground to denudation. About on ...
,
plain
In geography, a plain is a flat expanse of land that generally does not change much in elevation, and is primarily treeless. Plains occur as lowlands along valleys or at the base of mountains, as coastal plains, and as plateaus or uplands ...
s,
plateau
In geology and physical geography, a plateau (; ; ), also called a high plain or a tableland, is an area of a highland consisting of flat terrain that is raised sharply above the surrounding area on at least one side. Often one or more sides ...
s, and other
geomorphologies.
The planetary surface undergoes reshaping over geological time periods due to the effects of tectonics and
erosion
Erosion is the action of surface processes (such as water flow or wind) that removes soil, rock, or dissolved material from one location on the Earth's crust, and then transports it to another location where it is deposited. Erosion is dis ...
. The surface features built up or deformed through plate tectonics are subject to steady
weathering
Weathering is the deterioration of rocks, soils and minerals as well as wood and artificial materials through contact with water, atmospheric gases, and biological organisms. Weathering occurs ''in situ'' (on site, with little or no movement) ...
from
precipitation
In meteorology, precipitation is any product of the condensation of atmospheric water vapor that falls under gravitational pull from clouds. The main forms of precipitation include drizzle, rain, sleet, snow, ice pellets, graupel and hail. ...
, thermal cycles, and chemical effects.
Glaciation
A glacial period (alternatively glacial or glaciation) is an interval of time (thousands of years) within an ice age that is marked by colder temperatures and glacier advances. Interglacials, on the other hand, are periods of warmer climate be ...
,
coastal erosion
Coastal erosion is the loss or displacement of land, or the long-term removal of sediment and rocks along the coastline due to the action of waves, currents, tides, wind-driven water, waterborne ice, or other impacts of storms. The landwa ...
, the build-up of
coral reef
A coral reef is an underwater ecosystem characterized by reef-building corals. Reefs are formed of colonies of coral polyps held together by calcium carbonate. Most coral reefs are built from stony corals, whose polyps cluster in groups.
C ...
s, and large meteorite impacts also act to reshape the landscape.
As the continental plates migrate across the planet, the ocean floor is
subducted
Subduction is a geological process in which the oceanic lithosphere is recycled into the Earth's mantle at convergent boundaries. Where the oceanic lithosphere of a tectonic plate converges with the less dense lithosphere of a second plate, the ...
under the leading edges. At the same time, upwellings of mantle material create a
divergent boundary along
mid-ocean ridge
A mid-ocean ridge (MOR) is a seafloor mountain system formed by plate tectonics. It typically has a depth of about and rises about above the deepest portion of an ocean basin. This feature is where seafloor spreading takes place along a diver ...
s. 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.
The continental plates consist of lower density material such as the
igneous rock
Igneous rock (derived from the Latin word ''ignis'' meaning fire), or magmatic rock, is one of the three main rock types, the others being sedimentary and metamorphic. Igneous rock is formed through the cooling and solidification of magma o ...
s
granite
Granite () is a coarse-grained ( phaneritic) intrusive igneous rock composed mostly of quartz, alkali feldspar, and plagioclase. It forms from magma with a high content of silica and alkali metal oxides that slowly cools and solidifies under ...
and
andesite
Andesite () is a volcanic rock of intermediate composition. In a general sense, it is the intermediate type between silica-poor basalt and silica-rich rhyolite. It is fine-grained (aphanitic) to porphyritic in texture, and is composed predo ...
. Less common is
basalt
Basalt (; ) is an aphanitic (fine-grained) extrusive igneous rock formed from the rapid cooling of low-viscosity lava rich in magnesium and iron (mafic lava) exposed at or very near the surface of a rocky planet or moon. More than 90 ...
, a denser volcanic rock that is the primary constituent of the ocean floors.
Sedimentary rock
Sedimentary rocks are types of rock that are formed by the accumulation or deposition of mineral or organic particles at Earth's surface, followed by cementation. Sedimentation is the collective name for processes that cause these particles ...
is 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. The third form of rock material found on Earth is
metamorphic rock
Metamorphic rocks arise from the transformation of existing rock to new types of rock in a process called metamorphism. The original rock ( protolith) is subjected to temperatures greater than and, often, elevated pressure of or more, caus ...
, 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
Quartz is a hard, crystalline mineral composed of silica ( silicon dioxide). The atoms are linked in a continuous framework of SiO4 silicon-oxygen tetrahedra, with each oxygen being shared between two tetrahedra, giving an overall chemical ...
, the
feldspar
Feldspars are a group of rock-forming aluminium tectosilicate minerals, also containing other cations such as sodium, calcium, potassium, or barium. The most common members of the feldspar group are the ''plagioclase'' (sodium-calcium) felds ...
s,
amphibole,
mica,
pyroxene, and
olivine
The mineral olivine () is a magnesium iron silicate with the chemical formula . It is a type of nesosilicate or orthosilicate. The primary component of the Earth's upper mantle, it is a common mineral in Earth's subsurface, but weathers quickl ...
. Common carbonate minerals include
calcite (found in
limestone
Limestone ( calcium carbonate ) is a type of carbonate sedimentary rock which is the main source of the material lime. It is composed mostly of the minerals calcite and aragonite, which are different crystal forms of . Limestone forms whe ...
),
aragonite
Aragonite is a carbonate mineral, one of the three most common naturally occurring crystal forms of calcium carbonate, (the other forms being the minerals calcite and vaterite). It is formed by biological and physical processes, including pre ...
, and
dolomite Dolomite may refer to:
*Dolomite (mineral), a carbonate mineral
*Dolomite (rock), also known as dolostone, a sedimentary carbonate rock
*Dolomite, Alabama, United States, an unincorporated community
*Dolomite, California, United States, an unincor ...
.
![Earth elevation histogram 2](https://upload.wikimedia.org/wikipedia/commons/9/9f/Earth_elevation_histogram_2.svg)
The
pedosphere
The pedosphere (from Greek ''pedon'' "ground" or "earth" and ''sphaira'' "sphere") 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, h ...
is the outermost layer of the Earth that is composed of
soil
Soil, also commonly referred to as earth or dirt
Dirt is an unclean matter, especially when in contact with a person's clothes, skin, or possessions. In such cases, they are said to become dirty.
Common types of dirt include:
* Debri ...
and subject to
soil formation processes. It exists at the interface of the
lithosphere,
atmosphere,
hydrosphere
The hydrosphere () is the combined mass of water found on, under, and above the surface of a planet, minor planet, or natural satellite. Although Earth's hydrosphere has been around for about 4 billion years, it continues to change in shape. This ...
, and
biosphere
The biosphere (from Greek βίος ''bíos'' "life" and σφαῖρα ''sphaira'' "sphere"), also known as the ecosphere (from Greek οἶκος ''oîkos'' "environment" and σφαῖρα), is the worldwide sum of all ecosystems. It can also ...
. Currently the total arable land is 13.31% of the land surface, with only 4.71% supporting permanent crops.
Close to 40% of the Earth's land surface is presently used for cropland and pasture, or an estimated of cropland and of pastureland.
The physical features of land are remarkably varied. The largest mountain ranges—the
Himalayas
The Himalayas, or Himalaya (; ; ), is a mountain range in Asia, separating the plains of the Indian subcontinent from the Tibetan Plateau. The range has some of the planet's highest peaks, including the very highest, Mount Everest. Over 10 ...
in Asia and the
Andes
The Andes, Andes Mountains or Andean Mountains (; ) are the longest continental mountain range in the world, forming a continuous highland along the western edge of South America. The range is long, wide (widest between 18°S – 20°S ...
in South America—extend for thousands of kilometres. The longest rivers are the river Nile in Africa () and the Amazon river in South America (). 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
Mount Everest (; Tibetic languages, Tibetan: ''Chomolungma'' ; ) is List of highest mountains on Earth, Earth's highest mountain above sea level, located in the Mahalangur Himal sub-range of the Himalayas. The China–Nepal border ru ...
. The mean height of land above sea level is 686 m (2,250 ft).
The
geological history of Earth
Geology () is a branch of natural science concerned with Earth and other astronomical objects, the features or rocks of which it is composed, and the processes by which they change over time. Modern geology significantly overlaps all other Ea ...
can be broadly classified into two periods namely:
*
Precambrian: extends for approximately 90% of geologic time, from 4.6 billion years ago to the beginning of the Cambrian Period (539
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
In geology, a supercontinent is the assembly of most or all of Earth's continental blocks or cratons to form a single large landmass. However, some geologists use a different definition, "a grouping of formerly dispersed continents", which leav ...
around 1000 Ma.
*
Phanerozoic: the current eon in the geologic timescale. It covers 539 million years. During this time, continents drifted about, eventually collected into a single landmass known as
Pangea
Pangaea or Pangea () was a supercontinent that existed during the late Paleozoic and early Mesozoic eras. It assembled from the earlier continental units of Gondwana, Euramerica and Siberia during the Carboniferous approximately 335 million y ...
and then split up into the current continental landmasses.
Mars
![Mars rocks](https://upload.wikimedia.org/wikipedia/commons/f/fa/Mars_rocks.jpg)
The surface of
Mars
Mars is the fourth planet from the Sun and the second-smallest planet in the Solar System, only being larger than Mercury. In the English language, Mars is named for the Roman god of war. Mars is a terrestrial planet with a thin at ...
is thought to be primarily composed of
basalt
Basalt (; ) is an aphanitic (fine-grained) extrusive igneous rock formed from the rapid cooling of low-viscosity lava rich in magnesium and iron (mafic lava) exposed at or very near the surface of a rocky planet or moon. More than 90 ...
, based upon the observed lava flows from volcanos, the
Martian meteorite
A Martian meteorite is a rock that formed on Mars, was ejected from the planet by an impact event, and traversed interplanetary space before landing on Earth as a meteorite. , 277 meteorites had been classified as Martian, less than half a percen ...
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.
Analysis of the soil samples collected by the Viking landers in 1976 indicate iron-rich
clay
Clay is a type of fine-grained natural soil material containing clay minerals (hydrous aluminium phyllosilicates, e.g. kaolin, Al2 Si2 O5( OH)4).
Clays develop plasticity when wet, due to a molecular film of water surrounding the clay par ...
s consistent with weathering of basaltic rocks.
There is some evidence that some portion of the Martian surface might be more silica-rich than typical
basalt
Basalt (; ) is an aphanitic (fine-grained) extrusive igneous rock formed from the rapid cooling of low-viscosity lava rich in magnesium and iron (mafic lava) exposed at or very near the surface of a rocky planet or moon. More than 90 ...
, perhaps similar to
andesitic
Andesite () is a volcanic rock of intermediate composition. In a general sense, it is the intermediate type between silica-poor basalt and silica-rich rhyolite. It is fine-grained (aphanitic) to porphyritic in texture, and is composed predomin ...
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]
"How Mars got its rust"
– 6 May 2004 article fro
Nature.com
URL accessed 18 April 2006. 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 deep, , lava seas of the early planet, while Mars, with a lower lava temperature of was too cool for this to happen.
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 , which is much thicker than Earth's crust which varies between and . 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 3 km thick.
![Mars rampart crater](https://upload.wikimedia.org/wikipedia/commons/d/df/Mars_rampart_crater.jpg)
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 crater
A pit crater (also called a subsidence crater or collapse crater) is a depression formed by a sinking or collapse of the surface lying above a void or empty chamber, rather than by the eruption of a volcano or lava vent. Pit craters are found ...
s are common on Mars but uncommon on the
Moon
The Moon is Earth's only natural satellite. It is the fifth largest satellite in the Solar System and the largest and most massive relative to its parent planet, with a diameter about one-quarter that of Earth (comparable to the width of ...
, which may indicate the presence of near-surface
volatiles
Volatiles are the group of chemical elements and chemical compounds that can be readily vaporized. In contrast with volatiles, elements and compounds that are not readily vaporized are known as refractory substances.
On planet Earth, the term ...
(ice and water) on Mars. Degraded impact structures record variations in
volcanic
A volcano is a rupture in the crust of a planetary-mass object, such as Earth, that allows hot lava, volcanic ash, and gases to escape from a magma chamber below the surface.
On Earth, volcanoes are most often found where tectonic plates a ...
,
fluvial, and
aeolian activity.
The
Yuty crater is an example of a
Rampart crater
Rampart craters are a specific type of impact crater which are accompanied by distinctive fluidized ejecta features found mainly on Mars. Only one example is known on Earth, the Nördlinger Ries impact structure in Germany. A rampart crater displ ...
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.
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, the largest volcano in the known Universe, formed during this period along with lava flows elsewhere on Mars.
Ceres
The geology of the dwarf planet, Ceres, was largely unknown until Dawn spacecraft explored it in early 2015. However, certain surface features such as "Piazzi", named after the dwarf planets' discoverer, had been resolved.
Ceres's oblateness is consistent with a differentiated body, a rocky core overlain with an icy mantle. This 100-kilometer-thick mantle (23%–28% of Ceres by mass; 50% by volume) contains 200 million cubic kilometers of water, which is more than the amount of fresh water on Earth. This result is supported by the observations made by the Keck telescope in 2002 and by evolutionary modeling. Also, some characteristics of its surface and history (such as its distance from the Sun, which weakened solar radiation enough to allow some fairly low-freezing-point components to be incorporated during its formation), point to the presence of volatile materials in the interior of Ceres. It has been suggested that a remnant layer of liquid water may have survived to the present under a layer of ice.
The surface composition of Ceres is broadly similar to that of C-type asteroids. Some differences do exist. The ubiquitous features of the Cererian IR spectra are those of hydrated materials, which indicate the presence of significant amounts of water in the interior. Other possible surface constituents include iron-rich clay minerals (cronstedtite) and carbonate minerals (dolomite and siderite), which are common minerals in carbonaceous chondrite meteorites. The spectral features of carbonates and clay minerals are usually absent in the spectra of other C-type asteroids. Sometimes Ceres is classified as a G-type asteroid.
The Cererian surface is relatively warm. The maximum temperature with the Sun overhead was estimated from measurements to be 235 K (about −38 °C, −36 °F) on 5 May 1991.
Prior to the Dawn mission, only a few Cererian surface features had been unambiguously detected. High-resolution ultraviolet Hubble Space Telescope images taken in 1995 showed a dark spot on its surface, which was nicknamed "Piazzi" in honor of the discoverer of Ceres. This was thought to be a crater. Later near-infrared images with a higher resolution taken over a whole rotation with the Keck telescope using adaptive optics showed several bright and dark features moving with Ceres's rotation. Two dark features had circular shapes and are presumably craters; one of them was observed to have a bright central region, whereas another was identified as the "Piazzi" feature. More recent visible-light Hubble Space Telescope images of a full rotation taken in 2003 and 2004 showed 11 recognizable surface features, the natures of which are currently unknown. One of these features corresponds to the "Piazzi" feature observed earlier.
These last observations also determined that the north pole of Ceres points in the direction of right ascension 19 h 24 min (291°), declination +59°, in the constellation Draco. This means that Ceres's axial tilt is very small—about 3°.
Atmosphere
There are indications that Ceres may have a tenuous atmosphere and water frost on the surface. Surface water ice is unstable at distances less than 5 AU from the Sun, so it is expected to sublime if it is exposed directly to solar radiation. Water ice can migrate from the deep layers of Ceres to the surface, but escapes in a very short time. As a result, it is difficult to detect water vaporization. Water escaping from polar regions of Ceres was possibly observed in the early 1990s but this has not been unambiguously demonstrated. It may be possible to detect escaping water from the surroundings of a fresh impact crater or from cracks in the subsurface layers of Ceres. Ultraviolet observations by the IUE spacecraft detected statistically significant amounts of hydroxide ions near the Cererean north pole, which is a product of water-vapor dissociation by ultraviolet solar radiation.
In early 2014, using data from the Herschel Space Observatory, it was discovered that there are several localized (not more than 60 km in diameter) mid-latitude sources of water vapor on Ceres, which each give off about 10
26 molecules (or 3 kg) of water per second. Two potential source regions, designated Piazzi (123°E, 21°N) and Region A (231°E, 23°N), have been visualized in the near infrared as dark areas (Region A also has a bright center) by the W. M. Keck Observatory. Possible mechanisms for the vapor release are sublimation from about 0.6 km2 of exposed surface ice, or cryovolcanic eruptions resulting from radiogenic internal heat or from pressurization of a subsurface ocean due to growth of an overlying layer of ice. Surface sublimation would be expected to decline as Ceres recedes from the Sun in its eccentric orbit, whereas internally powered emissions should not be affected by orbital position. The limited data available are more consistent with cometary-style sublimation. The spacecraft Dawn is approaching Ceres at aphelion, which may constrain Dawn's ability to observe this phenomenon.
Note: This info was taken directly from the main article, sources for the material are included there.
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
![InnerSolarSystem-en](https://upload.wikimedia.org/wikipedia/commons/f/f3/InnerSolarSystem-en.png)
The asteroid belt is located between
Mars
Mars is the fourth planet from the Sun and the second-smallest planet in the Solar System, only being larger than Mercury. In the English language, Mars is named for the Roman god of war. Mars is a terrestrial planet with a thin at ...
and
Jupiter
Jupiter is the fifth planet from the Sun and the largest in the Solar System. It is a gas giant with a mass more than two and a half times that of all the other planets in the Solar System combined, but slightly less than one-thousandth t ...
. It is made of thousands of rocky planetesimals from 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
A meteoroid () is a small rocky or metallic body in outer space.
Meteoroids are defined as objects significantly smaller than asteroids, ranging in size from grains to objects up to a meter wide. Objects smaller than this are classified as mi ...
.
Comets
A comet is a
small Solar System body
A small Solar System body (SSSB) is an object in the Solar System that is neither a planet, a dwarf planet, nor a natural satellite. The term was first defined in 2006 by the International Astronomical Union (IAU) as follows: "All other objects, ...
that orbits the
Sun
The Sun is the star at the center of the Solar System. It is a nearly perfect ball of hot plasma, heated to incandescence by nuclear fusion reactions in its core. The Sun radiates this energy mainly as light, ultraviolet, and infrared radi ...
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
Nucleus ( : nuclei) is a Latin word for the seed inside a fruit. It most often refers to:
*Atomic nucleus, the very dense central region of an atom
* Cell nucleus, a central organelle of a eukaryotic cell, containing most of the cell's DNA
Nucl ...
, 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
The Solar System Capitalization of the name varies. The International Astronomical Union, the authoritative body regarding astronomical nomenclature, specifies capitalizing the names of all individual astronomical objects but uses mixed "Solar ...
beyond the planets extending from the
orbit
In celestial mechanics, an orbit is the curved trajectory of an object such as the trajectory of a planet around a star, or of a natural satellite around a planet, or of an artificial satellite around an object or position in space such as ...
of
Neptune (at 30
AU) to approximately 55 AU from the
Sun
The Sun is the star at the center of the Solar System. It is a nearly perfect ball of hot plasma, heated to incandescence by nuclear fusion reactions in its core. The Sun radiates this energy mainly as light, ultraviolet, and infrared radi ...
. It is similar to the
asteroid belt
The asteroid belt is a torus-shaped region in the Solar System, located roughly between the orbits of the planets Jupiter and Mars. It contains a great many solid, irregularly shaped bodies, of many sizes, but much smaller than planets, c ...
, although it is far larger; 20 times as wide and 20–200 times as massive.
Like the asteroid belt, it consists mainly of
small bodies (remnants from the Solar System's formation) and at least one
dwarf planet
A dwarf planet is a small planetary-mass object that is in direct orbit of the Sun, smaller than any of the eight classical planets but still a world in its own right. The prototypical dwarf planet is Pluto. The interest of dwarf planets to ...
—
Pluto
Pluto (minor-planet designation: 134340 Pluto) is a dwarf planet in the Kuiper belt, a ring of bodies beyond the orbit of Neptune. It is the ninth-largest and tenth-most-massive known object to directly orbit the Sun. It is the largest ...
, which may be geologically active. But while the asteroid belt is composed primarily of
rock
Rock most often refers to:
* Rock (geology), a naturally occurring solid aggregate of minerals or mineraloids
* Rock music, a genre of popular music
Rock or Rocks may also refer to:
Places United Kingdom
* Rock, Caerphilly, a location in Wales ...
and
metal
A metal (from Greek μέταλλον ''métallon'', "mine, quarry, metal") is a material that, when freshly prepared, polished, or fractured, shows a lustrous appearance, and conducts electricity and heat relatively well. Metals are typicall ...
, the Kuiper belt is composed largely of
ice
Ice is water frozen into a solid state, typically forming at or below temperatures of 0 degrees Celsius or Depending on the presence of impurities such as particles of soil or bubbles of air, it can appear transparent or a more or less opaqu ...
s, such as
methane
Methane ( , ) is a chemical compound with the chemical formula (one carbon atom bonded to four hydrogen atoms). It is a group-14 hydride, the simplest alkane, and the main constituent of natural gas. The relative abundance of methane on Ea ...
,
ammonia
Ammonia is an inorganic compound of nitrogen and hydrogen with the formula . A stable binary hydride, and the simplest pnictogen hydride, ammonia is a colourless gas with a distinct pungent smell. Biologically, it is a common nitrogenous wa ...
, and
water
Water (chemical formula ) is an Inorganic compound, inorganic, transparent, tasteless, odorless, and Color of water, nearly colorless chemical substance, which is the main constituent of Earth's hydrosphere and the fluids of all known living ...
. The objects within the Kuiper belt, together with the members of the
scattered disc
The scattered disc (or scattered disk) is a distant circumstellar disc in the Solar System that is sparsely populated by icy small solar system bodies, which are a subset of the broader family of trans-Neptunian objects. The scattered-disc obje ...
and any potential
Hills cloud
In astronomy, the Hills cloud (also called the inner Oort cloud and inner cloud) is a vast theoretical circumstellar disc, interior to the Oort cloud, whose outer border would be located at around 20,000 to 30,000 astronomical units (AU) fr ...
or
Oort cloud
The Oort cloud (), sometimes called the Öpik–Oort cloud, first described in 1950 by the Dutch astronomer Jan Oort, is a theoretical concept of a cloud of predominantly icy planetesimals proposed to surround the Sun at distances ranging from ...
objects, are collectively referred to as
trans-Neptunian object
A trans-Neptunian object (TNO), also written transneptunian object, is any minor planet in the Solar System that orbits the Sun at a greater average distance than Neptune, which has a semi-major axis of 30.1 astronomical units (au).
Typically ...
s (TNOs).
Two TNOs have been visited and studied at close range,
Pluto
Pluto (minor-planet designation: 134340 Pluto) is a dwarf planet in the Kuiper belt, a ring of bodies beyond the orbit of Neptune. It is the ninth-largest and tenth-most-massive known object to directly orbit the Sun. It is the largest ...
and
486958 Arrokoth
Arrokoth (minor-planet designation 486958 Arrokoth; provisional designation ), formerly nicknamed Ultima Thule, is a trans-Neptunian object located in the Kuiper belt. Arrokoth became the farthest and most primitive object in the Solar System ...
.
See also
*
Lunar soil
Lunar soil is the fine fraction of the regolith found on the surface of the Moon. Its properties can differ significantly from those of terrestrial soil. The physical properties of lunar soil are primarily the result of mechanical disinteg ...
*
Martian soil
Martian soil is the fine regolith (a blanket of unconsolidated, loose, heterogeneous superficial deposits covering solid rock) found on the surface of Mars. Its properties can differ significantly from those of terrestrial soil, including its to ...
*
Water on terrestrial planets
References
External links
International Astronomical UnionSolar System Live(an interactive
orrery
An orrery is a mechanical model of the Solar System that illustrates or predicts the relative positions and motions of the planets and moons, usually according to the heliocentric model. It may also represent the relative sizes of these bodies ...
)
Solar System Viewer(animation)
Pictures of the Solar System
NASA Planet Quest*
ttp://www.space.com/scienceastronomy/060816_planet_qanda.html Q&A: The IAU's Proposed Planet DefinitionQ&A New planets proposalSolar system– About Space
Nine Planets InformationNASA's fact sheetPlanetary Science Research Discoveries
{{DEFAULTSORT:Geology Of Solar Terrestrial Planets
Planetary geology