"Mars is the most Earthlike of the other planets in its surface conditions, but Venus may be the most Earthlike in its activity. All three planets started out young and restless, with warm, churning interiors and water flowing on their surfaces. But they have gone their separate ways. Mars cooled off...Venus, too, lost its ocean, but it retains an active churning interior and a surface that has been reworked many times by processes that are apparently still ongoing."