This classification is based on their structure, surfaces and atmospheres. The terrestrials are generally small, high density and have rocky surfaces. Mars Three fundamental processes determine the surface of terrestrial planets - impact cratering, volcanism and tectonism. All terrestrial planets are cratered, for the first 600 million years or so of their history this was an incessant process. On the Earth the effects have been wiped out to a large extent by the influence of the atmosphere, oceans and geological activity.
Volcanism appears to be a consequence of radioactive decay within the interiors.
Tectonic activity is the movement of areas of crust seen on the terrestrial planets. The figure below shows the internal structure of the Earth compared with that of Mercury. These processes vary in importance from planet to planet so atmospheres can be markedly different although the physical processes are the same. The two larger planets, Jupiter and Saturn, have small, solid cores surrounded by a layer of liquid metallic hydrogen. Uranus and Neptune do not have liquid metallic hydrogen zones because the pressure is not sufficient, but as on Jupiter and Saturn, differentiation has caused virtually all their heavier elements to sink into the core.