The geology of solar terrestrial planets mainly
deals with the geological aspects of the four
terrestrial planets of the Solar System – Mercury,
Venus, Earth, and Mars – and one terrestrial
dwarf planet: Ceres. Earth is the only terrestrial
planet known to have an active hydrosphere.
Terrestrial planets are substantially different
from the giant planets, which might not have
solid surfaces and are composed mostly of
some combination of hydrogen, helium, and
water existing in various physical states.
Terrestrial planets have a compact, rocky
surfaces, and Venus, Earth, and Mars each
also have 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 primarily composed
of 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.
The 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 a substantial
iron core. Three of the four solar terrestrial
planets (Venus, Earth, and Mars) have substantial
atmospheres; all have impact craters and tectonic
surface features such as rift valleys and
volcanoes.
The term inner planet should not be confused
with inferior planet, which 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 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 protoplanets
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).
=== Terrestrial planets ===
In the warmer inner Solar System, planetesimals
formed from rocks and metals 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 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.
== Surface geology of inner solar planets
==
The four inner or terrestrial planets have
dense, rocky compositions, few or no moons,
and no ring systems. They are composed largely
of minerals with high melting points, such
as the silicates which form their solid crusts
and semi-liquid mantles, and metals such as
iron and nickel, which form their cores.
=== Mercury ===
The Mariner 10 mission (1974) mapped about
half the surface of Mercury. On the basis
of that data, scientists have a first-order
understanding of the geology and history of
the planet. Mercury's surface shows intercrater
plains, basins, smooth plains, 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 1,300 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
1,000 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 (1 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.
=== Venus ===
The surface of Venus is comparatively very
flat. When 93% of the topography was mapped
by Pioneer Venus, scientists found that the
total distance from the lowest point to the
highest point on the entire surface was about
13 kilometres (8 mi), while on the Earth the
distance from the basins to the Himalayas
is about 20 kilometres (12.4 mi).
According to the data of the altimeters of
the Pioneer, nearly 51% of the surface is
found located within 500 metres (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 craters has
been utilized as a dating method to approximately
date the Venusian surface (since there are
thus far no known samples of Venusian rock
to be dated by more reliable methods). Dates
derived are the dominantly in the range ~500
Mya–750Mya, although ages of up to ~1.2
Gya have been calculated. This research has
led to the fairly well accepted hypothesis
that Venus has undergone an essentially complete
volcanic resurfacing at least once in its
distant past, with the last event taking place
approximately within the range of estimated
surface ages. While the mechanism of such
an impressionable thermal event remains a
debated issue in Venusian geosciences, some
scientists are advocates of processes involving
plate motion to some extent. There are almost
1,000 impact craters on Venus, more or less
evenly distributed across its surface.
Earth-based radar surveys made it possible
to identify some topographic patterns related
to craters, and the Venera 15 and Venera 16
probes identified almost 150 such features
of probable impact origin. Global coverage
from Magellan subsequently made it possible
to identify nearly 900 impact craters.
Crater counts give an important estimate for
the age of the surface of a planet. Over time,
bodies in the Solar System are randomly impacted,
so the more craters a surface has, the older
it is. Compared to Mercury, the Moon and other
such bodies, Venus has very few craters. In
part, this is because Venus's dense atmosphere
burns up smaller meteorites before they hit
the surface. The Venera and Magellan data
agree: there are very few impact craters with
a diameter less than 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.
=== Earth ===
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
volcanoes, oceanic trenches, submarine canyons,
oceanic plateaus, and abyssal plains. The
remaining 29.2% not covered by water consists
of mountains, deserts, plains, plateaus, and
other geomorphologies.
The planetary surface undergoes reshaping
over geological time periods due to the effects
of tectonics and erosion. The surface features
built up or deformed through plate tectonics
are subject to steady weathering from precipitation,
thermal cycles, and chemical effects. Glaciation,
coastal erosion, the build-up of coral reefs,
and large meteorite impacts also act to reshape
the landscape.
As the continental plates migrate across the
planet, the ocean floor is subducted under
the leading edges. At the same time, upwellings
of mantle material create a divergent boundary
along mid-ocean ridges. The combination of
these processes continually recycles the ocean
plate material. Most of the ocean floor is
less than 100 million years in age. The oldest
ocean plate is located in the Western Pacific,
and has an estimated age of about 200 million
years. By comparison, the oldest fossils found
on land have an age of about 3 billion years.The
continental plates consist of lower density
material such as the igneous rocks granite
and andesite. Less common is basalt, a denser
volcanic rock that is the primary constituent
of the ocean floors. 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 feldspars, 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 13 million square kilometres
(5.0 million square miles) of cropland and
34 million square kilometres (13 million square
miles) 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 (6,695 kilometres or
4,160 miles) and the Amazon river in South
America (6,437 kilometres or 4,000 miles).
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:
Precambrian: includes approximately 90% of
geologic time. It extends from 4.6 billion
years ago to the beginning of the Cambrian
Period (about 570 Ma). It is generally believed
that small proto-continents existed prior
to 3000 Ma, and that most of the Earth's landmasses
collected into a single supercontinent around
1000 Ma.
Phanerozoic: is the current eon in the geologic
timescale. It covers roughly 545 million years.
During the period covered, continents drifted
about, eventually collected into a single
landmass known as Pangea and then split up
into the current continental landmasses.
=== Mars ===
The surface of Mars is thought to be primarily
composed of basalt, based upon the observed
lava flows from volcanos, the Martian meteorite
collection, and data from landers and orbital
observations. The lava flows from Martian
volcanos show that that lava has a very low
viscosity, typical of basalt.
Analysis of the soil samples collected by
the Viking landers in 1976 indicate iron-rich
clays 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 1,800 kilometres
(1,118 mi) deep, 3,200 °C (5,792 °F), lava
seas of the early planet, while Mars, with
a lower lava temperature of 2,200 °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 craters are common on Mars
but uncommon on the Moon, which may indicate
the presence of near-surface volatiles (ice
and water) on Mars. Degraded impact structures
record variations in volcanic, fluvial, and
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:
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.[a]
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 1026 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 ===
The asteroid belt is located between Mars
and Jupiter. It is made of thousands of rocky
planetesimals from 1,000 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 meteorites
and provide information about the primordial
solar nebula. Most of these fragments have
the size of sand grains. They burn up in the
Earth's atmosphere, causing them to glow like
meteors.
=== Comets ===
A comet is a small Solar System body that
orbits the Sun and (at least occasionally)
exhibits a coma (or atmosphere) and/or a tail—both
primarily from the effects of solar radiation
upon the comet's nucleus, which itself is
a minor body composed of rock, dust, and ice.
=== Kuiper belt ===
The Kuiper belt, sometimes called the Edgeworth–Kuiper
belt, is a region of the Solar System beyond
the planets extending from the orbit of Neptune
(at 30 AU) 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, which may
be geologically active. But while the asteroid
belt is composed primarily of rock and metal,
the Kuiper belt is composed largely of ices,
such as methane, ammonia, and water. The objects
within the Kuiper belt, together with the
members of the scattered disc and any potential
Hills cloud or Oort cloud objects, are collectively
referred to as trans-Neptunian objects (TNOs).
Two TNOs have been visited and studied at
close range, Pluto and Ultima Thule.
== See also ==
Lunar soil
Martian soil
Water on terrestrial planets
