Recent planetary missions by NASA, the European Space Agency, and other national agencies have reaffirmed that the geological processes which are familiar from our studies of the Earth operate on many solid planets and satellites. Common threads link the internal structure, thermal evolution, and surface character of both rocky and icy worlds. Volcanoes, impact craters, ice caps, dunes, rift valleys, rivers, and oceans are features of extra-terrestrial worlds as diverse as Mercury and Titan. New data reveals that many of the supposedly inert planetary bodies were recently subject to...