"Finding the signatures of an ancient Martian biosphere means exploring old rocks that might preserve traces of life for millions or billions of years," Farmer notes. Among the best places to look on Mars, he says, are deposits left by springs and former lakes in the heavily cratered highlands. "The rocks there date from a period in Martian history when liquid water was common at the surface." In fact, says Farmer, conditions on Mars then were likely similar to those on the early Earth at the time when life began.

"Besides water, life also requires energy sources and organic chemical building blocks," Farmer explains. "The Mars Exploration Rover Opportunity found ample evidence for water in ancient rocks at Meridiani Planum, but the rovers' instruments can't detect organic materials." However, NASA's next rover, the Mars Science Laboratory, will carry instruments to analyze traces of organic substances. It is due for launch in 2009.

Recognizing a Martian fossil may be difficult. "We're not talking about stumbling over dinosaur bones," Farmer says.

Instead, the discovery may involve finding biologically formed structures in old sedimentary deposits, perhaps like stromatolites found here on Earth. Stromatolites are distinctive structures that form in shallow oceans, lakes, or streams where microbial colonies trap sediments to form thin repeating layers.