Geology of solar terrestrial planets
Encyclopedia
The geology of solar terrestrial planets 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. Only one terrestrial planet, Earth, is known to have an active hydrosphere
.
Terrestrial planets are substantially different from gas giant
s, which might not have solid surfaces and are composed mostly of some combination of hydrogen
, helium
, and water
existing in various physical state
s. 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 have numerous similarities to plutoids (objects like Pluto
), which also have a solid surface, but are composed of more icy materials. 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
.
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 atmosphere
s; all have impact crater
s and tectonic
surface features such as rift valley
s and volcano
es. 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).
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
. 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.
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 protoplanet
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
).
s and metal
s were cooked billions of years ago in cores of massive star
s.
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
and helium
, the Sun would have heated the gases and caused them to escape. Hence, solar terrestrial planets such as Mercury
, Venus
, Earth
, and Mars
are dense small worlds composed mostly from 2% of heavier elements contained in the solar nebula.
s 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 silicate
s which form their solid crusts
and semi-liquid mantles
, and metals such as iron
and nickel
, which form their cores
.
, smooth plain
s, craters
, and tectonic features.
Mercury's oldest surface is its intercrater plains, which are present (but much less
extensive) 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; 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.
At least 15 ancient basins have been identified on Mercury. Tolstoj is a true multi-ring basin, displaying
at least two, and possibly as many as four, concentric rings. It has a well-preserved ejecta blanket extending
outward as much as 500 kilometres (311 mi) from its rim. The basin interior is flooded with plains that clearly postdate the ejecta
deposits. Beethoven has only one, subdued massif-like rim 625 kilometres (388 mi) in diameter, but displays an impressive, well lineated
ejecta blanket that extends as far as 500 kilometres (311 mi). As at Tolstoj, Beethoven ejecta is asymmetric. The Caloris
basin is defined by a ring of mountains 1300 kilometres (808 mi) in diameter. Individual massifs are typically 30 kilometres (19 mi) to 50 kilometres (31 mi)
long; the inner edge of the unit is marked by basin-facing scarps. Lineated terrain extends for about 1000 kilometres (621 mi) 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
800 kilometres (497 mi) from the Caloris mountains. It consists of low, closely spaced to scattered hills about 0.3 to 1 kilometre (0.621372736649807 mi) 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.
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. 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. 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 and consist of sinuous to arcuate
scarps that transect preexisting plains and craters. They are most convincingly interpreted as thrust faults, 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.
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 altimeter
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 2 kilometres (1 mi) 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 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
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 meteor
ites before they hit the surface. The Venera and Magellan data agree: there are very few impact craters with a diameter less than 30 kilometres (19 mi), and data from Magellan show an absence of any craters less than 2 kilometres (1 mi) 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.
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 100 kilometres (62 mi) 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.
Venusian craters range from 3 kilometres (2 mi) to 280 kilometres (174 mi) 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.
The Earth's terrain
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
system, as well as undersea volcano
es, oceanic trench
es, submarine canyon
s, oceanic plateau
s, and abyssal plain
s. The remaining 29.2% not covered by water consists of mountains, deserts
, plain
s, plateau
s, 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 reef
s, and large meteorite impacts 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 ridge
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
s granite
and andesite
. Less common is basalt
, a denser volcanic rock that is the primary constituent of the ocean floors. Sedimentary rock
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
, 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 feldspar
s, amphibole
, mica
, pyroxene
, and olivine
. Common carbonate minerals include calcite
(found in limestone
), aragonite
, and dolomite
.
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. Close to 40% of the Earth's land surface is presently used for cropland and pasture, or an estimated 1.3 hectare
s (3.3 acre
s) of cropland and 3.4 hectares (8.4 acres) of pastureland.
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 (6695 kilometres (4,160 mi)) and the Amazon river in South America (6437 kilometres (4,000 mi)). 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).
The geological history of Earth
can be broadly classified into two periods namely:
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.
Analysis of the soil samples collected by the Viking landers in 1976 indicate iron-rich clay
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
, 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). 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 1800 kilometres (1,118 mi) deep, 3200 °C (5,792 °F), lava seas of the early planet, while Mars, with a lower lava temperature of 2200 °C (3,992 °F) 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 125 kilometres (78 mi), which is much thicker than Earth's crust which varies between 5 kilometres (3 mi) and 70 kilometres (43 mi). 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.
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
s 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 aeolian activity.
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.
The geological history of Mars can be broadly classified into many epochs, but the following are the three major ones:
An asteroid belt is located between Mars
and Jupiter
. It is made of thousands of rocky planetesimals from 1000 kilometres (621 mi) 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 meteorite
s 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.
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.
beyond the planets extending from the orbit
of Neptune
(at 30 AU
) to approximately 55 AU from the Sun
. It is similar to the asteroid belt
, 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
—Pluto
. But while the asteroid belt is composed primarily of rock
and metal
, the Kuiper belt is composed largely of ice
s, 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 object
s (TNOs).
Solar System
The Solar System consists of the Sun and the astronomical objects gravitationally bound in orbit around it, all of which formed from the collapse of a giant molecular cloud approximately 4.6 billion years ago. The vast majority of the system's mass is in the Sun...
namely, Mercury
Mercury (planet)
Mercury is the innermost and smallest planet in the Solar System, orbiting the Sun once every 87.969 Earth days. The orbit of Mercury has the highest eccentricity of all the Solar System planets, and it has the smallest axial tilt. It completes three rotations about its axis for every two orbits...
, Venus
Venus
Venus is the second planet from the Sun, orbiting it every 224.7 Earth days. The planet is named after Venus, the Roman goddess of love and beauty. After the Moon, it is the brightest natural object in the night sky, reaching an apparent magnitude of −4.6, bright enough to cast shadows...
, Earth
Earth
Earth is the third planet from the Sun, and the densest and fifth-largest of the eight planets in the Solar System. It is also the largest of the Solar System's four terrestrial planets...
, and Mars
Mars
Mars is the fourth planet from the Sun in the Solar System. The planet is named after the Roman god of war, Mars. It is often described as the "Red Planet", as the iron oxide prevalent on its surface gives it a reddish appearance...
and one terrestrial dwarf planet
Dwarf planet
A dwarf planet, as defined by the International Astronomical Union , is a celestial body orbiting the Sun that is massive enough to be spherical as a result of its own gravity but has not cleared its neighboring region of planetesimals and is not a satellite...
, Ceres. Only one terrestrial planet, Earth, is known to have an active hydrosphere
Hydrosphere
A hydrosphere in physical geography describes the combined mass of water found on, under, and over the surface of a planet....
.
Terrestrial planets are substantially different from gas giant
Gas giant
A gas giant is a large planet that is not primarily composed of rock or other solid matter. There are four gas giants in the Solar System: Jupiter, Saturn, Uranus, and Neptune...
s, which might not have solid surfaces and are composed mostly of some combination of hydrogen
Hydrogen
Hydrogen is the chemical element with atomic number 1. It is represented by the symbol H. With an average atomic weight of , hydrogen is the lightest and most abundant chemical element, constituting roughly 75% of the Universe's chemical elemental mass. Stars in the main sequence are mainly...
, helium
Helium
Helium is the chemical element with atomic number 2 and an atomic weight of 4.002602, which is represented by the symbol He. It is a colorless, odorless, tasteless, non-toxic, inert, monatomic gas that heads the noble gas group in the periodic table...
, and water
Water
Water is a chemical substance with the chemical formula H2O. A water molecule contains one oxygen and two hydrogen atoms connected by covalent bonds. Water is a liquid at ambient conditions, but it often co-exists on Earth with its solid state, ice, and gaseous state . Water also exists in a...
existing in various physical state
Phase (matter)
In the physical sciences, a phase is a region of space , throughout which all physical properties of a material are essentially uniform. Examples of physical properties include density, index of refraction, and chemical composition...
s. These planets have a compact, rocky surfaces, with the last three also having an atmosphere
Atmosphere
An atmosphere is a layer of gases that may surround a material body of sufficient mass, and that is held in place by the gravity of the body. An atmosphere may be retained for a longer duration, if the gravity is high and the atmosphere's temperature is low...
. Their size, radius, and density are all similar.
Terrestrial planets have numerous similarities to plutoids (objects like Pluto
Pluto
Pluto, formal designation 134340 Pluto, is the second-most-massive known dwarf planet in the Solar System and the tenth-most-massive body observed directly orbiting the Sun...
), which also have a solid surface, but are composed of more icy materials. 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
Solar nebula
In cosmogony, the nebular hypothesis is the most widely accepted model explaining the formation and evolution of the Solar System. There is evidence that it was first proposed in 1734 by Emanuel Swedenborg. Originally applied only to our own Solar System, this method of planetary system formation...
.
Terrestrial planets all have roughly the same structure—a central metallic core, mostly iron
Iron
Iron is a chemical element with the symbol Fe and atomic number 26. It is a metal in the first transition series. It is the most common element forming the planet Earth as a whole, forming much of Earth's outer and inner core. It is the fourth most common element in the Earth's crust...
, with a surrounding silicate mantle
Mantle (geology)
The mantle is a part of a terrestrial planet or other rocky body large enough to have differentiation by density. The interior of the Earth, similar to the other terrestrial planets, is chemically divided into layers. The mantle is a highly viscous layer between the crust and the outer core....
. The Moon
Moon
The Moon is Earth's only known natural satellite,There are a number of near-Earth asteroids including 3753 Cruithne that are co-orbital with Earth: their orbits bring them close to Earth for periods of time but then alter in the long term . These are quasi-satellites and not true moons. For more...
is similar, but lacks an iron core. Three of the four solar terrestrial planets (Venus, Earth and Mars) have substantial atmosphere
Atmosphere
An atmosphere is a layer of gases that may surround a material body of sufficient mass, and that is held in place by the gravity of the body. An atmosphere may be retained for a longer duration, if the gravity is high and the atmosphere's temperature is low...
s; all have impact crater
Impact crater
In the broadest sense, the term impact crater can be applied to any depression, natural or manmade, resulting from the high velocity impact of a projectile with a larger body...
s and tectonic
Tectonics
Tectonics is a field of study within geology concerned generally with the structures within the lithosphere of the Earth and particularly with the forces and movements that have operated in a region to create these structures.Tectonics is concerned with the orogenies and tectonic development of...
surface features such as rift valley
Rift valley
A rift valley is a linear-shaped lowland between highlands or mountain ranges created by the action of a geologic rift or fault. This action is manifest as crustal extension, a spreading apart of the surface which is subsequently further deepened by the forces of erosion...
s and volcano
Volcano
2. Bedrock3. Conduit 4. Base5. Sill6. Dike7. Layers of ash emitted by the volcano8. Flank| 9. Layers of lava emitted by the volcano10. Throat11. Parasitic cone12. Lava flow13. Vent14. Crater15...
es. 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
Immanuel Kant
Immanuel Kant was a German philosopher from Königsberg , researching, lecturing and writing on philosophy and anthropology at the end of the 18th Century Enlightenment....
and independently formulated by Pierre-Simon Laplace
Pierre-Simon Laplace
Pierre-Simon, marquis de Laplace was a French mathematician and astronomer whose work was pivotal to the development of mathematical astronomy and statistics. He summarized and extended the work of his predecessors in his five volume Mécanique Céleste...
. This theory holds that 4.6 billion years ago the Solar System formed from the gravitational collapse of a giant molecular cloud
Molecular cloud
A molecular cloud, sometimes called a stellar nursery if star formation is occurring within, is a type of interstellar cloud whose density and size permits the formation of molecules, most commonly molecular hydrogen ....
. 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
Sun
The Sun is the star at the center of the Solar System. It is almost perfectly spherical and consists of hot plasma interwoven with magnetic fields...
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
Accretion (astrophysics)
In astrophysics, the term accretion is used for at least two distinct processes.The first and most common is the growth of a massive object by gravitationally attracting more matter, typically gaseous matter in an accretion disc. Accretion discs are common around smaller stars or stellar remnants...
.
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
Fragmentation
-In biology:* Fragmentation , a form of asexual reproduction* Fragmentation * Habitat fragmentation* Population fragmentation-Music:* Fragmented , the debut album from the Filipino independent band Up Dharma Down-Other:...
process and continued to slowly grow into protoplanet
Protoplanet
Protoplanets are large planetary embryos that originate within protoplanetary discs and have undergone internal melting to produce differentiated interiors. They are believed to form out of kilometer-sized planetesimals that attract each other gravitationally and collide...
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
The mass density or density of a material is defined as its mass per unit volume. The symbol most often used for density is ρ . In some cases , density is also defined as its weight per unit volume; although, this quantity is more properly called specific weight...
).
Terrestrial planets
In the warmer inner solar system, planetesimals formed from rockRock (geology)
In geology, rock or stone is a naturally occurring solid aggregate of minerals and/or mineraloids.The Earth's outer solid layer, the lithosphere, is made of rock. In general rocks are of three types, namely, igneous, sedimentary, and metamorphic...
s and metal
Metal
A metal , is an element, compound, or alloy that is a good conductor of both electricity and heat. Metals are usually malleable and shiny, that is they reflect most of incident light...
s were cooked billions of years ago in cores of massive star
Star
A star is a massive, luminous sphere of plasma held together by gravity. At the end of its lifetime, a star can also contain a proportion of degenerate matter. The nearest star to Earth is the Sun, which is the source of most of the energy on Earth...
s.
These elements constituted only 0.6% of the material in the solar nebula
Solar nebula
In cosmogony, the nebular hypothesis is the most widely accepted model explaining the formation and evolution of the Solar System. There is evidence that it was first proposed in 1734 by Emanuel Swedenborg. Originally applied only to our own Solar System, this method of planetary system formation...
. 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
Hydrogen is the chemical element with atomic number 1. It is represented by the symbol H. With an average atomic weight of , hydrogen is the lightest and most abundant chemical element, constituting roughly 75% of the Universe's chemical elemental mass. Stars in the main sequence are mainly...
and helium
Helium
Helium is the chemical element with atomic number 2 and an atomic weight of 4.002602, which is represented by the symbol He. It is a colorless, odorless, tasteless, non-toxic, inert, monatomic gas that heads 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
Mercury (planet)
Mercury is the innermost and smallest planet in the Solar System, orbiting the Sun once every 87.969 Earth days. The orbit of Mercury has the highest eccentricity of all the Solar System planets, and it has the smallest axial tilt. It completes three rotations about its axis for every two orbits...
, Venus
Venus
Venus is the second planet from the Sun, orbiting it every 224.7 Earth days. The planet is named after Venus, the Roman goddess of love and beauty. After the Moon, it is the brightest natural object in the night sky, reaching an apparent magnitude of −4.6, bright enough to cast shadows...
, Earth
Earth
Earth is the third planet from the Sun, and the densest and fifth-largest of the eight planets in the Solar System. It is also the largest of the Solar System's four terrestrial planets...
, and Mars
Mars
Mars is the fourth planet from the Sun in the Solar System. The planet is named after the Roman god of war, Mars. It is often described as the "Red Planet", as the iron oxide prevalent on its surface gives it a reddish appearance...
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 planetTerrestrial 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 are the inner planets closest to the Sun...
s have dense, rocky
Rock (geology)
In geology, rock or stone is a naturally occurring solid aggregate of minerals and/or mineraloids.The Earth's outer solid layer, the lithosphere, is made of rock. In general rocks are of three types, namely, igneous, sedimentary, and metamorphic...
compositions, few or no moons
Natural satellite
A natural satellite or moon is a celestial body that orbits a planet or smaller body, which is called its primary. The two terms are used synonymously for non-artificial satellites of planets, of dwarf planets, and of minor planets....
, and no ring systems
Planetary ring
A planetary ring is a ring of cosmic dust and other small particles orbiting around a planet in a flat disc-shaped region.The most notable planetary rings known in Earth's solar system are those around Saturn, but the other three gas giants of the solar system possess ring systems of their...
. They are composed largely of minerals with high melting points, such as the silicate
Silicate
A silicate is a compound containing a silicon bearing anion. The great majority of silicates are oxides, but hexafluorosilicate and other anions are also included. This article focuses mainly on the Si-O anions. Silicates comprise the majority of the earth's crust, as well as the other...
s which form their solid crusts
Crust (geology)
In geology, the crust is the outermost solid shell of a rocky planet or natural satellite, which is chemically distinct from the underlying mantle...
and semi-liquid mantles
Mantle (geology)
The mantle is a part of a terrestrial planet or other rocky body large enough to have differentiation by density. The interior of the Earth, similar to the other terrestrial planets, is chemically divided into layers. The mantle is a highly viscous layer between the crust and the outer core....
, and metals such as iron
Iron
Iron is a chemical element with the symbol Fe and atomic number 26. It is a metal in the first transition series. It is the most common element forming the planet Earth as a whole, forming much of Earth's outer and inner core. It is the fourth most common element in the Earth's crust...
and nickel
Nickel
Nickel is a chemical element with the chemical symbol Ni and atomic number 28. It is a silvery-white lustrous metal with a slight golden tinge. Nickel belongs to the transition metals and is hard and ductile...
, which form their cores
Planetary core
The planetary core consists of the innermost layer of a planet.The core may be composed of solid and liquid layers, while the cores of Mars and Venus are thought to be completely solid as they lack an internally generated magnetic field. In our solar system, core size can range from about 20% to...
.
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. Mercury's surface shows intercrater plains, basinsDepression (geology)
A depression in geology is a landform sunken or depressed below the surrounding area. Depressions may be formed by various mechanisms.Structural or tectonic related:...
, smooth plain
Plain
In geography, a plain is land with relatively low relief, that is flat or gently rolling. Prairies and steppes are types of plains, and the archetype for a plain is often thought of as a grassland, but plains in their natural state may also be covered in shrublands, woodland and forest, or...
s, craters
Impact crater
In the broadest sense, the term impact crater can be applied to any depression, natural or manmade, resulting from the high velocity impact of a projectile with a larger body...
, and tectonic features.
Mercury's oldest surface is its intercrater plains, which are present (but much less
extensive) on the Moon
Moon
The Moon is Earth's only known natural satellite,There are a number of near-Earth asteroids including 3753 Cruithne that are co-orbital with Earth: their orbits bring them close to Earth for periods of time but then alter in the long term . These are quasi-satellites and not true moons. For more...
. The intercrater plains are level to gently rolling terrain
Terrain
Terrain, or land relief, is the vertical and horizontal dimension of land surface. When relief is described underwater, the term bathymetry is used...
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; 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.
at least two, and possibly as many as four, concentric rings. It has a well-preserved ejecta blanket extending
outward as much as 500 kilometres (311 mi) from its rim. The basin interior is flooded with plains that clearly postdate the ejecta
deposits. Beethoven has only one, subdued massif-like rim 625 kilometres (388 mi) in diameter, but displays an impressive, well lineated
ejecta blanket that extends as far as 500 kilometres (311 mi). As at Tolstoj, Beethoven ejecta is asymmetric. The Caloris
basin is defined by a ring of mountains 1300 kilometres (808 mi) in diameter. Individual massifs are typically 30 kilometres (19 mi) to 50 kilometres (31 mi)
long; the inner edge of the unit is marked by basin-facing scarps. Lineated terrain extends for about 1000 kilometres (621 mi) 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
800 kilometres (497 mi) from the Caloris mountains. It consists of low, closely spaced to scattered hills about 0.3 to 1 kilometre (0.621372736649807 mi) 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.
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. 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. 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 and consist of sinuous to arcuate
scarps that transect preexisting plains and craters. They are most convincingly interpreted as thrust faults, 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 topographyTopography
Topography is the study of Earth's surface shape and features or those ofplanets, moons, and asteroids...
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
Oceanic basin
Hydrologically, an oceanic basin may be anywhere on Earth that is covered by seawater, but geologically ocean basins are large geologic basins that are below sea level...
to the Himalayas is about 20 kilometres (12.4 mi).
According to the data of the altimeter
Altimeter
An altimeter 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 underwater.-Pressure altimeter:...
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 2 kilometres (1 mi) from the median radius.
Impact crater
In the broadest sense, the term impact crater can be applied to any depression, natural or manmade, resulting from the high velocity impact of a projectile with a larger body...
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
Impact crater
In the broadest sense, the term impact crater can be applied to any depression, natural or manmade, resulting from the high velocity impact of a projectile with a larger body...
, and the Venera 15
Venera 15
Venera 15 was a spacecraft sent to Venus by the Soviet Union. This unmanned orbiter was to map the surface of Venus using high resolution imaging systems...
and Venera 16
Venera 16
Venera 16 was a spacecraft sent to Venus by the Soviet Union. This unmanned orbiter was to map the surface of Venus using high resolution imaging systems...
probes identified almost 150 such features of probable impact origin. Global coverage from Magellan subsequently made it possible to identify nearly 900 impact craters.
Mercury (planet)
Mercury is the innermost and smallest planet in the Solar System, orbiting the Sun once every 87.969 Earth days. The orbit of Mercury has the highest eccentricity of all the Solar System planets, and it has the smallest axial tilt. It completes three rotations about its axis for every two orbits...
, the Moon
Moon
The Moon is Earth's only known natural satellite,There are a number of near-Earth asteroids including 3753 Cruithne that are co-orbital with Earth: their orbits bring them close to Earth for periods of time but then alter in the long term . These are quasi-satellites and not true moons. For more...
and other such bodies, Venus has very few craters. In part, this is because Venus's dense atmosphere burns up smaller meteor
METEOR
METEOR is a metric for the evaluation of machine translation output. The metric is based on the harmonic mean of unigram precision and recall, with recall weighted higher than precision...
ites before they hit the surface. The Venera and Magellan data agree: there are very few impact craters with a diameter less than 30 kilometres (19 mi), and data from Magellan show an absence of any craters less than 2 kilometres (1 mi) 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.
Hawaii (island)
The Island of Hawaii, also called the Big Island or Hawaii Island , is a volcanic island in the North Pacific Ocean...
of Hawaii
Hawaii
Hawaii is the newest of the 50 U.S. states , and is the only U.S. state made up entirely of islands. It is the northernmost island group in Polynesia, occupying most of an archipelago in the central Pacific Ocean, southwest of the continental United States, southeast of Japan, and northeast of...
. 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
Subduction
In geology, subduction is the process that takes place at convergent boundaries by which one tectonic plate moves under another tectonic plate, sinking into the Earth's mantle, as the plates converge. These 3D regions of mantle downwellings are known as "Subduction Zones"...
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.
Venusian craters range from 3 kilometres (2 mi) to 280 kilometres (174 mi) 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
Kinetic energy
The kinetic energy of an object is the energy which 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 acceleration, the body maintains this kinetic energy unless its speed changes...
are slowed down so much by the atmosphere that they do not create an impact crater.
Earth
Terrain
Terrain, or land relief, is the vertical and horizontal dimension of land surface. When relief is described underwater, the term bathymetry is used...
varies greatly from place to place. About 70.8% of the surface is covered by water, with much of the continental shelf
Continental shelf
The continental shelf is the extended perimeter of each continent and associated coastal plain. Much of the shelf was exposed during glacial periods, but is now submerged under relatively shallow seas and gulfs, and was similarly submerged during other interglacial periods. The continental margin,...
below sea level. The submerged surface has mountainous features, including a globe-spanning mid-ocean ridge
Mid-ocean ridge
A mid-ocean ridge is a general term for an underwater mountain system that consists of various mountain ranges , typically having a valley known as a rift running along its spine, formed by plate tectonics. This type of oceanic ridge is characteristic of what is known as an oceanic spreading...
system, as well as undersea volcano
Volcano
2. Bedrock3. Conduit 4. Base5. Sill6. Dike7. Layers of ash emitted by the volcano8. Flank| 9. Layers of lava emitted by the volcano10. Throat11. Parasitic cone12. Lava flow13. Vent14. Crater15...
es, oceanic trench
Oceanic trench
The oceanic trenches are hemispheric-scale long but narrow topographic depressions of the sea floor. They are also the deepest parts of the ocean floor....
es, submarine canyon
Submarine canyon
A submarine canyon is a steep-sided valley on the sea floor of the continental slope. Many submarine canyons are found as extensions to large rivers; however there are some that have no such association. Canyons cutting the continental slopes have been found at depths greater than 2 km below sea...
s, oceanic plateau
Oceanic plateau
An oceanic plateau is a large, relatively flat submarine region that rises well above the level of the ambient seabed. While many oceanic plateaus are composed of continental crust, and often form a step interrupting the continental slope, some plateaus are undersea remnants of large igneous...
s, and abyssal plain
Abyssal plain
An abyssal plain is an underwater plain on the deep ocean floor, usually found at depths between 3000 and 6000 metres. 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. They are among the flattest, smoothest...
s. The remaining 29.2% not covered by water consists of mountains, deserts
Déserts
Déserts is a piece by Edgard Varèse for brass , percussion , piano, and tape. Percussion instruments are exploited for their resonant potential, rather than used solely as accompaniment...
, plain
Plain
In geography, a plain is land with relatively low relief, that is flat or gently rolling. Prairies and steppes are types of plains, and the archetype for a plain is often thought of as a grassland, but plains in their natural state may also be covered in shrublands, woodland and forest, or...
s, plateau
Plateau
In geology and earth science, a plateau , also called a high plain or tableland, is an area of highland, usually consisting of relatively flat terrain. A highly eroded plateau is called a dissected plateau...
s, and other geomorphologies
Geomorphology
Geomorphology is the scientific study of landforms and the processes that shape them...
.
The planetary surface undergoes reshaping over geological time periods due to the effects of tectonics and erosion
Erosion
Erosion is when materials are removed from the surface and changed into something else. It only works by hydraulic actions and transport of solids in the natural environment, and leads to the deposition of these materials elsewhere...
. The surface features built up or deformed through plate tectonics are subject to steady weathering
Weathering
Weathering is the breaking down of rocks, soils and minerals as well as artificial materials through contact with the Earth's atmosphere, biota and waters...
from precipitation
Precipitation (meteorology)
In meteorology, precipitation In meteorology, precipitation In meteorology, precipitation (also known as one of the classes of hydrometeors, which are atmospheric water phenomena is any product of the condensation of atmospheric water vapor that falls under gravity. The main forms of precipitation...
, thermal cycles, and chemical effects. Glaciation, coastal erosion
Coastal erosion
Coastal erosion is the wearing away of land and the removal of beach or dune sediments by wave action, tidal currents, wave currents, or drainage...
, the build-up of coral reef
Coral reef
Coral reefs are underwater structures made from calcium carbonate secreted by corals. Coral reefs are colonies of tiny living animals found in marine waters that contain few nutrients. Most coral reefs are built from stony corals, which in turn consist of polyps that cluster in groups. The polyps...
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
In geology, subduction is the process that takes place at convergent boundaries by which one tectonic plate moves under another tectonic plate, sinking into the Earth's mantle, as the plates converge. These 3D regions of mantle downwellings are known as "Subduction Zones"...
under the leading edges. At the same time, upwellings of mantle material create a divergent boundary
Divergent boundary
In plate tectonics, a divergent boundary or divergent plate boundary is a linear feature that exists between two tectonic plates that are moving away from each other. Divergent boundaries within continents initially produce rifts which produce rift valleys...
along mid-ocean ridge
Mid-ocean ridge
A mid-ocean ridge is a general term for an underwater mountain system that consists of various mountain ranges , typically having a valley known as a rift running along its spine, formed by plate tectonics. This type of oceanic ridge is characteristic of what is known as an oceanic spreading...
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
Igneous rock is one of the three main rock types, the others being sedimentary and metamorphic rock. Igneous rock is formed through the cooling and solidification of magma or lava...
s granite
Granite
Granite is a common and widely occurring type of intrusive, felsic, igneous rock. Granite usually has a medium- to coarse-grained texture. Occasionally some individual crystals are larger than the groundmass, in which case the texture is known as porphyritic. A granitic rock with a porphyritic...
and andesite
Andesite
Andesite is an extrusive igneous, volcanic rock, of intermediate composition, with aphanitic to porphyritic texture. In a general sense, it is the intermediate type between basalt and dacite. The mineral assemblage is typically dominated by plagioclase plus pyroxene and/or hornblende. Magnetite,...
. Less common is basalt
Basalt
Basalt is a common extrusive volcanic rock. It is usually grey to black and fine-grained due to rapid cooling of lava at the surface of a planet. It may be porphyritic containing larger crystals in a fine matrix, or vesicular, or frothy scoria. Unweathered basalt is black or grey...
, a denser volcanic rock that is the primary constituent of the ocean floors. Sedimentary rock
Sedimentary rock
Sedimentary rock are types of rock that are formed by the deposition of material at the Earth's surface and within bodies of water. Sedimentation is the collective name for processes that cause mineral and/or organic particles to settle and accumulate or minerals to precipitate from a solution....
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 rock
Metamorphic rock is the transformation of an existing rock type, the protolith, in a process called metamorphism, which means "change in form". The protolith is subjected to heat and pressure causing profound physical and/or chemical change...
, 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
Quartz is the second-most-abundant mineral in the Earth's continental crust, after feldspar. It is made up of a continuous framework of SiO4 silicon–oxygen tetrahedra, with each oxygen being shared between two tetrahedra, giving an overall formula SiO2. There are many different varieties of quartz,...
, the feldspar
Feldspar
Feldspars are a group of rock-forming tectosilicate minerals which make up as much as 60% of the Earth's crust....
s, amphibole
Amphibole
Amphibole is the name of an important group of generally dark-colored rock-forming inosilicate minerals, composed of double chain tetrahedra, linked at the vertices and generally containing ions of iron and/or magnesium in their structures.-Mineralogy:...
, mica
Mica
The mica group of sheet silicate minerals includes several closely related materials having highly perfect basal cleavage. All are monoclinic, with a tendency towards pseudohexagonal crystals, and are similar in chemical composition...
, pyroxene
Pyroxene
The pyroxenes are a group of important rock-forming inosilicate minerals found in many igneous and metamorphic rocks. They share a common structure consisting of single chains of silica tetrahedra and they crystallize in the monoclinic and orthorhombic systems...
, and olivine
Olivine
The mineral olivine is a magnesium iron silicate with the formula 2SiO4. It is a common mineral in the Earth's subsurface but weathers quickly on the surface....
. Common carbonate minerals include calcite
Calcite
Calcite is a carbonate mineral and the most stable polymorph of calcium carbonate . The other polymorphs are the minerals aragonite and vaterite. Aragonite will change to calcite at 380-470°C, and vaterite is even less stable.-Properties:...
(found in limestone
Limestone
Limestone is a sedimentary rock composed largely of the minerals calcite and aragonite, which are different crystal forms of calcium carbonate . Many limestones are composed from skeletal fragments of marine organisms such as coral or foraminifera....
), aragonite
Aragonite
Aragonite is a carbonate mineral, one of the two common, naturally occurring, crystal forms of calcium carbonate, CaCO3...
, and dolomite
Dolomite
Dolomite is a carbonate mineral composed of calcium magnesium carbonate CaMg2. The term is also used to describe the sedimentary carbonate rock dolostone....
.
Pedosphere
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. The sum total of all the organisms, soils, water and air is termed as the "pedosphere"...
is the outermost layer of the Earth that is composed of soil
Soil
Soil is a natural body consisting of layers of mineral constituents of variable thicknesses, which differ from the parent materials in their morphological, physical, chemical, and mineralogical characteristics...
and subject to soil formation processes
Pedogenesis
Pedogenesis is the science and study of the processes that lead to the formation of soil ' and first explored by the Russian geologist Vasily Dokuchaev , the so called grandfather of soil science, who determined that soil formed over time as a consequence of...
. It exists at the interface of the lithosphere
Lithosphere
The lithosphere is the rigid outermost shell of a rocky planet. On Earth, it comprises the crust and the portion of the upper mantle that behaves elastically on time scales of thousands of years or greater.- Earth's lithosphere :...
, atmosphere
Earth's atmosphere
The atmosphere of Earth is a layer of gases surrounding the planet Earth that is retained by Earth's gravity. The atmosphere protects life on Earth by absorbing ultraviolet solar radiation, warming the surface through heat retention , and reducing temperature extremes between day and night...
, hydrosphere
Hydrosphere
A hydrosphere in physical geography describes the combined mass of water found on, under, and over the surface of a planet....
, and biosphere
Biosphere
The biosphere is the global sum of all ecosystems. It can also be called the zone of life on Earth, a closed and self-regulating system...
. 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 1.3 hectare
Hectare
The hectare is a metric unit of area defined as 10,000 square metres , and primarily used in the measurement of land. In 1795, when the metric system was introduced, the are was defined as being 100 square metres and the hectare was thus 100 ares or 1/100 km2...
s (3.3 acre
Acre
The acre is a unit of area in a number of different systems, including the imperial and U.S. customary systems. The most commonly used acres today are the international acre and, in the United States, the survey acre. The most common use of the acre is to measure tracts of land.The acre is related...
s) of cropland and 3.4 hectares (8.4 acres) of pastureland.
The physical features of land are remarkably varied. The largest mountain ranges—the Himalayas
Himalayas
The Himalaya Range or Himalaya Mountains Sanskrit: Devanagari: हिमालय, literally "abode of snow"), usually called the Himalayas or Himalaya for short, is a mountain range in Asia, separating the Indian subcontinent from the Tibetan Plateau...
in Asia and the Andes
Andes
The Andes is the world's longest continental mountain range. It is a continual range of highlands along the western coast of South America. This range is about long, about to wide , and of an average height of about .Along its length, the Andes is split into several ranges, which are separated...
in South America—extend for thousands of kilometres. The longest rivers are the river Nile in Africa (6695 kilometres (4,160 mi)) and the Amazon river in South America (6437 kilometres (4,000 mi)). Deserts cover about 20% of the total land area. The largest is the Sahara
Sahara
The Sahara is the world's second largest desert, after Antarctica. At over , it covers most of Northern Africa, making it almost as large as Europe or the United States. The Sahara stretches from the Red Sea, including parts of the Mediterranean coasts, to the outskirts of the Atlantic Ocean...
, 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
Dead Sea
The Dead Sea , also called the Salt Sea, is a salt lake bordering Jordan to the east and Israel and the West Bank to the west. Its surface and shores are below sea level, the lowest elevation on the Earth's surface. The Dead Sea is deep, the deepest hypersaline lake in the world...
, to a 2005-estimated maximum altitude of 8,848 m (29,028 ft) at the top of Mount Everest
Mount Everest
Mount Everest is the world's highest mountain, with a peak at above sea level. It is located in the Mahalangur section of the Himalayas. The international boundary runs across the precise summit point...
. The mean height of land above sea level is 686 m (2,250 ft).
The geological history of Earth
Geological history of Earth
The geological history of Earth follows the major events in Earth's past based on the geologic time scale, a system of chronological measurement based on the study of the planet's rock layers...
can be broadly classified into two periods namely:
- PrecambrianPrecambrianThe Precambrian is the name which describes the large span of time in Earth's history before the current Phanerozoic Eon, and is a Supereon divided into several eons of the geologic time scale...
: 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 supercontinentSupercontinentIn geology, a supercontinent is a landmass comprising more than one continental core, or craton. The assembly of cratons and accreted terranes that form Eurasia qualifies as a supercontinent today.-History:...
around 1000 Ma. - PhanerozoicPhanerozoicThe Phanerozoic Eon is the current eon in the geologic timescale, and the one during which abundant animal life has existed. It covers roughly 542 million years and goes back to the time when diverse hard-shelled animals first appeared...
: 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
Mars
Mars is the fourth planet from the Sun in the Solar System. The planet is named after the Roman god of war, Mars. It is often described as the "Red Planet", as the iron oxide prevalent on its surface gives it a reddish appearance...
is thought to be primarily composed of basalt
Basalt
Basalt is a common extrusive volcanic rock. It is usually grey to black and fine-grained due to rapid cooling of lava at the surface of a planet. It may be porphyritic containing larger crystals in a fine matrix, or vesicular, or frothy scoria. Unweathered basalt is black or grey...
, 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.
Analysis of the soil samples collected by the Viking landers in 1976 indicate iron-rich clay
Clay
Clay is a general term including many combinations of one or more clay minerals with traces of metal oxides and organic matter. Geologic clay deposits are mostly composed of phyllosilicate minerals containing variable amounts of water trapped in the mineral structure.- Formation :Clay minerals...
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
Basalt is a common extrusive volcanic rock. It is usually grey to black and fine-grained due to rapid cooling of lava at the surface of a planet. It may be porphyritic containing larger crystals in a fine matrix, or vesicular, or frothy scoria. Unweathered basalt is black or grey...
, 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
Iron(III) oxide
Iron oxide or ferric oxide is the inorganic compound with the formula Fe2O3. It is one of the three main oxides of iron, the other two being iron oxide , which is rare, and iron oxide , which also occurs naturally as the mineral magnetite. As the mineral known as hematite, Fe2O3 is the main...
(rust). 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 1800 kilometres (1,118 mi) deep, 3200 °C (5,792 °F), lava seas of the early planet, while Mars, with a lower lava temperature of 2200 °C (3,992 °F) was too cool for this to happen.
The core is surrounded by a silicate mantle
Mantle (geology)
The mantle is a part of a terrestrial planet or other rocky body large enough to have differentiation by density. The interior of the Earth, similar to the other terrestrial planets, is chemically divided into layers. The mantle is a highly viscous layer between the crust and the outer core....
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 125 kilometres (78 mi), which is much thicker than Earth's crust which varies between 5 kilometres (3 mi) and 70 kilometres (43 mi). 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.
Pit crater
A pit crater is a depression formed by a sinking of the ground surface lying above a void or empty chamber, rather than by the eruption of a volcano or lava vent. It is often found in chains or troughs. Several craters may merge into a linear alignment...
s are common on Mars but uncommon on the Moon
Moon
The Moon is Earth's only known natural satellite,There are a number of near-Earth asteroids including 3753 Cruithne that are co-orbital with Earth: their orbits bring them close to Earth for periods of time but then alter in the long term . These are quasi-satellites and not true moons. For more...
, which may indicate the presence of near-surface volatiles
Volatiles
In planetary science, volatiles are that group of chemical elements and chemical compounds with low boiling points that are associated with a planet's or moon's crust and/or atmosphere. Examples include nitrogen, water, carbon dioxide, ammonia, hydrogen, and methane, all compounds of C, H, O...
(ice and water) on Mars. Degraded impact structures record variations in volcanic, fluvial
Fluvial
Fluvial is used in geography and Earth science to refer to the processes associated with rivers and streams and the deposits and landforms created by them...
, and aeolian activity.
The Yuty crater is an example of a Rampart crater
Rampart crater
Rampart craters are a specific type of Martian impact crater which are accompanied by distinctive fluidized ejecta features. A Martian Rampart crater displays an ejecta with a low ridge along its edge. Usually, rampart craters show a lobate outer margin, as if material moved along the surface,...
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 TerraNoachis TerraNoachis Terra is an extensive southern landmass of the planet Mars. It lies west of the giant Hellas impact basin, roughly between the latitudes −20° and −80° and longitudes 30° west and 30° east, centered on ....
): 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 PlanitiaAmazonis PlanitiaAmazonis Planitia is one of the smoothest plains on Mars. It is located between the Tharsis and Elysium volcanic provinces to the west of Olympus Mons in the Valles Marineris region of the Memnonia quadrangle, centered at...
): 1.8 billion years ago to present. Amazonian regions have few meteorite impact craters but are otherwise quite varied. Olympus MonsOlympus MonsOlympus Mons is a large volcanic mountain on the planet Mars. At a height of almost , it is one of the tallest mountains in the Solar System, three times as tall as Mount Everest and more than twice the height of Mauna Kea the tallest mountain on Earth. Olympus Mons is the youngest of the large...
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
Mars
Mars is the fourth planet from the Sun in the Solar System. The planet is named after the Roman god of war, Mars. It is often described as the "Red Planet", as the iron oxide prevalent on its surface gives it a reddish appearance...
and Jupiter
Jupiter
Jupiter is the fifth planet from the Sun and the largest planet within the Solar System. It is a gas giant with mass one-thousandth that of the Sun but is two and a half times the mass of all the other planets in our Solar System combined. Jupiter is classified as a gas giant along with Saturn,...
. It is made of thousands of rocky planetesimals from 1000 kilometres (621 mi) 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 meteorite
Meteorite
A meteorite is a natural object originating in outer space that survives impact with the Earth's surface. Meteorites can be big or small. Most meteorites derive from small astronomical objects called meteoroids, but they are also sometimes produced by impacts of asteroids...
s 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 SunSun
The Sun is the star at the center of the Solar System. It is almost perfectly spherical and consists of hot plasma interwoven with magnetic fields...
and (at least occasionally) exhibits a coma
Coma (cometary)
frame|right|The [[153P/Ikeya-Zhang|comet Ikeya-Zhang]] exhibiting a bright, condensed coma In astronomy, a coma is the nebulous envelope around the nucleus of a comet. It is formed when the comet passes close to the Sun on its highly elliptical orbit; as the comet warms, parts of it sublimate...
(or atmosphere) and/or a tail—both primarily from the effects of solar radiation upon the comet's nucleus
Comet nucleus
The nucleus is the solid, central part of a comet, popularly termed a dirty snowball. A cometary nucleus is composed of rock, dust, and frozen gases. When heated by the Sun, the gases sublimate and produce an atmosphere surrounding the nucleus known as the coma...
, 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 SystemSolar System
The Solar System consists of the Sun and the astronomical objects gravitationally bound in orbit around it, all of which formed from the collapse of a giant molecular cloud approximately 4.6 billion years ago. The vast majority of the system's mass is in the Sun...
beyond the planets extending from the orbit
Orbit
In physics, an orbit is the gravitationally curved path of an object around a point in space, for example the orbit of a planet around the center of a star system, such as the Solar System...
of Neptune
Neptune
Neptune is the eighth and farthest planet from the Sun in the Solar System. Named for the Roman god of the sea, it is the fourth-largest planet by diameter and the third largest by mass. Neptune is 17 times the mass of Earth and is slightly more massive than its near-twin Uranus, which is 15 times...
(at 30 AU
Astronomical unit
An astronomical unit is a unit of length equal to about or approximately the mean Earth–Sun distance....
) to approximately 55 AU from the Sun
Sun
The Sun is the star at the center of the Solar System. It is almost perfectly spherical and consists of hot plasma interwoven with magnetic fields...
. It is similar to the asteroid belt
Asteroid belt
The asteroid belt is the region of the Solar System located roughly between the orbits of the planets Mars and Jupiter. It is occupied by numerous irregularly shaped bodies called asteroids or minor planets...
, 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
Small solar system body
A small Solar System body is an object in the Solar System that is neither a planet nor a dwarf planet, nor a satellite of a planet or dwarf planet:...
(remnants from the Solar System's formation) and at least one dwarf planet
Dwarf planet
A dwarf planet, as defined by the International Astronomical Union , is a celestial body orbiting the Sun that is massive enough to be spherical as a result of its own gravity but has not cleared its neighboring region of planetesimals and is not a satellite...
—Pluto
Pluto
Pluto, formal designation 134340 Pluto, is the second-most-massive known dwarf planet in the Solar System and the tenth-most-massive body observed directly orbiting the Sun...
. But while the asteroid belt is composed primarily of rock
Rock (geology)
In geology, rock or stone is a naturally occurring solid aggregate of minerals and/or mineraloids.The Earth's outer solid layer, the lithosphere, is made of rock. In general rocks are of three types, namely, igneous, sedimentary, and metamorphic...
and metal
Metal
A metal , is an element, compound, or alloy that is a good conductor of both electricity and heat. Metals are usually malleable and shiny, that is they reflect most of incident light...
, the Kuiper belt is composed largely of ice
Ice
Ice is water frozen into the solid state. Usually ice is the phase known as ice Ih, which is the most abundant of the varying solid phases on the Earth's surface. It can appear transparent or opaque bluish-white color, depending on the presence of impurities or air inclusions...
s, such as methane
Methane
Methane is a chemical compound with the chemical formula . It is the simplest alkane, the principal component of natural gas, and probably the most abundant organic compound on earth. The relative abundance of methane makes it an attractive fuel...
, ammonia
Ammonia
Ammonia is a compound of nitrogen and hydrogen with the formula . It is a colourless gas with a characteristic pungent odour. Ammonia contributes significantly to the nutritional needs of terrestrial organisms by serving as a precursor to food and fertilizers. Ammonia, either directly or...
, and water
Water
Water is a chemical substance with the chemical formula H2O. A water molecule contains one oxygen and two hydrogen atoms connected by covalent bonds. Water is a liquid at ambient conditions, but it often co-exists on Earth with its solid state, ice, and gaseous state . Water also exists in a...
.
The objects within the Kuiper belt, together with the members of the scattered disc
Scattered disc
The scattered disc is a distant region of the Solar System that is sparsely populated by icy minor planets, a subset of the broader family of trans-Neptunian objects. The scattered-disc objects have orbital eccentricities ranging as high as 0.8, inclinations as high as 40°, and perihelia greater...
and any potential Hills cloud or Oort cloud
Oort cloud
The Oort cloud , or the Öpik–Oort cloud , is a hypothesized spherical cloud of comets which may lie roughly 50,000 AU, or nearly a light-year, from the Sun. This places the cloud at nearly a quarter of the distance to Proxima Centauri, the nearest star to the Sun...
objects, are collectively referred to as trans-Neptunian object
Trans-Neptunian object
A trans-Neptunian object is any minor planet in the Solar System that orbits the Sun at a greater distance on average than Neptune.The first trans-Neptunian object to be discovered was Pluto in 1930...
s (TNOs).
External links
- International Astronomical Union
- Solar System Live (an interactive orreryOrreryAn orrery is a mechanical device that illustrates the relative positions and motions of the planets and moons in the Solar System in a heliocentric model. Though the Greeks had working planetaria, the first orrery that was a planetarium of the modern era was produced in 1704, and one was presented...
) - Solar System Viewer (animation)
- Pictures of the Solar System
- Renderings of the planets
- NASA Planet Quest
- Illustration comparing the sizes of the planets with each other, the sun, and other stars
- Q&A: The IAU's Proposed Planet Definition
- Q&A New planets proposal
- Solar system – About Space
- Atlas of Mercury – NASA
- Nine Planets Information
- NASA’s fact sheet
- Planetary Science Research Discoveries