A Piece of the Rock

It’s been a busy time for NASA: Just before the first rover landed on Mars (page 10), the Stardust spacecraft achieved a dazzling double score, collecting pieces of comet Wild 2 (the first sample of a body beyond the moon) and sending back pictures of the comet’s 3.1-mile-wide icy core. The surface is pockmarked with lumps and strange depressions. Daniel Britt, a planetary scientist at the University of Central Florida, interprets the pits as impact craters that have been widened by eruptions of evaporating gas and dust. Compared with Borrelly, an older comet visited by the Deep Space 1 probe in 2001, Wild 2 appears to be a relative newcomer to the inner solar system, its surface far less chewed up by the sun’s heat. “It seems less evolved than Borrelly,” Britt says. If so, it may contain material that has changed little since the planets formed 4.6 billion years ...