Offworld — Robin Parrish

The return of NASA's first manned mission to Mars was supposed to be a momentous day. But when the crew loses touch with ground control before entry, things look bleak. Safe after a treacherous landing, the crew emerges to discover the unthinkable--every man, woman, child, and animal has vanished without a trace. Alone now on their home planet, the crew sets out to discover where everyone has gone--and how to get them back--only to discover they may not be as alone as they thought.