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 has an atmosphere. Their size, radius, and density are all similar.

Terrestrial planets have numerous similarities to dwarf planets (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, 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 up to a few kilometers across in a few million years, a small time in 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 a strong 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 planets 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.Mariner 10 Special Issue (1975) JGR 80.Vilas F. et al., eds. (1988) Mercury. Univ. Arizona Press, 794 pp. Mercury's surface shows intercrater plains, basins, smooth plains, craters, and tectonic features.

Mercury's oldest surface is its intercrater plains,Gault D. E. et al. (1975) JGR 80, 2444. which are present (but much 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;Spudis P.D. and Guest J.E. (1988) in Mercury, 118-164. they probably formed by widespread volcanism early in Mercurian history.

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

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.Schaber G.G. et al. (1977) PEPI 15, 189. It has a well-preserved ejecta blanket extending outward as much as from its rim. The basin interior is flooded with plains that clearly postdate the ejecta deposits. Beethoven has only one, subdued massif-like rim in diameter, but displays an impressive, well lineated ejecta blanket that extends as far as . As at Tolstoj, Beethoven ejecta is asymmetric. The Caloris basin is defined by a ring of mountains in diameter.McCauley J.F. (1977) PEPI 15, 220.McCauley J.F. et al. (1981) Icarus 47, 184 Individual massifs are typically to long; the inner edge of the unit is marked by basin-facing scarps. Lineated terrain extends for about out from the foot of a weak discontinuous scarp on the outer edge of the Caloris mountains; this terrain is similar to the sculpture surrounding the Imbrium basin on the Moon. Hummocky material forms a broad annulus about from the Caloris mountains. It consists of low, closely spaced to scattered hills about 0.3 to across and from tens of meters to a few hundred meters high. The outer boundary of this unit is gradational with the (younger) smooth plains that occur in the same region. A hilly and furrowed terrain is found antipodal to the Caloris basin, probably created by antipodal convergence of intense seismic waves generated by the Caloris impact.Schultz, P.H. and Gault, D.E. (1975) The Moon 12, 159-177.

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

Lobate scarps are widely distributed over MercuryMelosh H.J. and McKinnon W.B. (1988) In Mercury, 374-400. and consist of sinuous to arcuate scarps that transect preexisting plains and craters. They are most convincingly interpreted as thrust faults, 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 from the median radius.