The four largest planets in our Solar System are gas giants, with very deep atmospheres and no detectable solid ‘surface’. All of the smaller bodies, the terrestrial planets, asteroids, moons and comets, have solid surfaces. These bodies display geological features that yield clues about their formation, as well as past and current geological activity. The surface reflectivity varies dramatically from one body to another; some surfaces have very low albedos (such as the maria on the Moon, carbonaceous asteroids, comet nuclei), while others are highly reflective (Europa, Enceladus). Large albedo variations may even be seen on a single object (Iapetus). Some bodies are almost completely covered by impact craters (Moon, Mercury, Mimas), while others showlittle or no sign of impacts (Io, Europa, Earth). The terrestrial planets and many of the larger moons show clear evidence of past volcanic activity, and some (Earth, Io, Enceladus, Triton) are active even today. Past volcanic activity may be seen in the form of volcanoes of different shapes and size (Earth, Mars, Venus) or large solidified lava lakes (Moon).