Surface features which are common to the terrestrial planets include impact craters and volcanoes. Impact cratering occurs when a meteorite strikes the solid surface of a planet or satellite. The energy of the meteorite is transferred to the planet, causing a shock wave which fractures the rock below the impact. As the rocks return to their usual pressure, another shock wave travels upward toward the surface. It is this shock which causes the ejection of material from the crater basin creating an "ejecta blanket" as well as rays. The displacement of material not thrown completely out of the impact basin forms a rim around the basin. Volcanism is the process thorugh which molten rock and gases in a planet's interior are transferred to the surface. Many heat sources cause the melting of the rocks. Heating by the accretion of particles helped in the original melting of the planet; once this began, heat could be released during core formation. Residual heat from these processes kept some planetary material molten. Heat from radioactive decay in the interior of the bodies as well as tidal heating are sources which continue to affect some planets. The original atmospheres of all planets consisted of gases from the original solar nebula which were gravitationally trapped early in the planets' lifetimes. The terrestrial planets have lost this hydrogen and helium atmosphere due to a strong solar wind early in their histories and weak gravitational fields which could not hold the gases. Instead, these planets have secondary atmospheres formed when gases were released from the planets' interiors. As internal differentiation occured, elements such as carbon, nitrogen, and oxygen separated from the solids and formed gaseous envelopes of compounds such as CO2, N2, and H2O. All of the terrestrial planets are probably differentiated into cores, mantles, and crusts. The crust of each planet is the thin surface layer on which surface features are seen. The mantle is generally composed of relatively light compounds such as silicates. In some cases the mantle is thought to be at least partially molten. The core of a planet, if it exists, would be composed of heavy elements such as iron.
