The solid surfaces of planets, moons, asteroids and comets defines a boundary layer that separates the interior of a planetary body from the surrounding atmosphere and space. Planetary surfaces interact with the solid matter of the substratum as well as with the overlaying volatile matter and the influence of the interplanetary space and thus are affected by endogenic and exogenic processes. The results of surface processes are recorded in the geology, morphology, structure and composition and stratigraphy and that provide insight into the origin and evolution of planetary bodies. The two main types of solid planetary surfaces are either rocky as the siliceous crusts of the inner solar system or icy as the crusts of the moons of the outer solar system.
Planetary Geology investigates the origin and evolution of planetary surfaces and the alteration of planetary crusts by geological processes such as volcanism, tectonics, weathering, erosion and impact.