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. It has a well-preserved ejecta blanket extending outward as much as from its rim. The basin interior is flooded with plains that clearly postdate the ejecta deposits.
Beethoven has only one, subdued massif-like rim in diameter, but displays an impressive, well lineated ejecta blanket that extends as far as . As at Tolstoj, Beethoven ejecta is asymmetric. The Caloris basin is defined by a ring of mountains in diameter. Individual massifs are typically to long; the inner edge of the unit is marked by basin-facing scarps. impact at its antipodal point. 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. Such relations strongly support a volcanic origin for the mercurian smooth plains, even in the absence of diagnostic landforms. 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. 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 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 have 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 primarily 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 , and data from
Magellan show an absence of any craters less than in diameter. However, there are also fewer of the large craters, and those appear relatively young; they are rarely filled with lava, showing that they happened after volcanic activity in the area, and radar shows that they are rough and have not had time to be eroded down. s in Venus's
Alpha Regio Much of Venus' surface appears to have been shaped by volcanic activity. Overall, Venus has several times as many volcanoes as Earth, and it possesses some 167 giant volcanoes that are over 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 to in diameter. There are no craters smaller than 3 km, because of the effects of the dense atmosphere on incoming objects. Objects with less than a certain
kinetic energy are slowed down so much by the atmosphere that they do not create an impact crater.
Earth and
bathymetry. Data from the
National Geophysical Data Center's TerrainBase Digital Terrain Model. The Earth's
terrain varies greatly from place to place. About 70.8% of the surface is covered by water. The
sea floor 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. 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. of the surface of the Earth—approximately 71% of the Earth's surface is covered with water. 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 of cropland and 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 () and the Amazon river in South America (). Deserts cover about 20% of the total land area. The largest is the
Sahara, which covers nearly one-third of Africa. The elevation of the land surface of the Earth varies from the low point of −418 m (−1,371 ft) at the
Dead Sea, to a 2005-estimated maximum altitude of 8,848 m (29,028 ft) at the top of
Mount Everest. 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: extends for approximately 90% of geologic time, from 4.6 billion years ago to the beginning of the Cambrian Period (539
Ma). It is generally believed that small proto-continents existed prior to 3000 Ma, and that most of the Earth's landmasses collected into a single
supercontinent around 1000 Ma. •
Phanerozoic: the current eon in the geologic timescale. It covers 539 million years. During this time, continents drifted about, eventually collected into a single landmass known as
Pangea 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 lava has a very low viscosity, typical of basalt. which is much thicker than Earth's crust which varies between and . As a result, Mars' crust does not easily deform, as was shown by the recent radar map of the south polar ice cap which does not deform the crust despite being about 3 km thick. 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 vaporize 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==