Planetary surface

A planetary surface is where the solid (or liquid) material of the outer crust on certain types of astronomical objects contacts the atmosphere or outer space; planetary surfaces are found on solid objects of planetary mass (PMO)s, including terrestrial planets, dwarf planets, natural satellites, planetesimals and many other Small Solar System bodies (SSSB)s. The study of planetary surfaces is a field of planetary geology known as surface geology but also a focus of a number of fields including planetary cartography, topography, geomorphology, atmospheric sciences and astronomy. Land (or ground) is the term given to non-liquid planetary surfaces. The term "landing" is used to describe the collision of an object with a planetary surface and is usually at a velocity in which the object can remain intact and remain attached.

In differentiated bodies, the surface is where the crust meets the planetary boundary layer. Anything below this is regarded as being sub-surface or sub-marine. Most bodies more massive than Super-Earths, including stars and gas giants, as well as smaller gas dwarfs, transition contiguously between phases including gas, liquid and solids. As such they are generally regarded as lacking surfaces.

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