Geology of solar terrestrial planets
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 giants, 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 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 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 designates those planets which are closer to the Sun than Earth is (i.e. Mercury and Venus).