Does Mars need protection from our microbes? Conventional wisdom says yes, as does space law—the United Nations Outer Space Treaty prohibits the contamination of potentially fertile worlds with earthly bacteria. Yet some researchers disagree: Mars will be just fine on its own, they say, and the stringent safeguards now in place discourage scientists from exploring the Red Planet. On missions dedicated to searching for life, costs “could easily double because of planetary protection procedures,” says Cornell University astrobiologist Alberto G. Fairén.

Protecting Mars is not worth the effort and expense, Fairén and Dirk Schulze-Makuch of Washington State University argue in a recent issue of Nature Geoscience. After all, some Earth bacteria are probably already there, having hitched a ride on debris from ancient meteor impacts or more recently on NASA's Viking landers. Besides, any life-form already on Mars would easily fight off the poorly adapted invasive microbes.

The odds of NASA changing course are low. “If you want to study life elsewhere, you have to make sure not to bring Earth materials along” or else risk mistaking stowaways for alien life, says Catharine Conley, NASA's planetary protection officer.

John Rummel, Conley's predecessor at NASA, says simulations and experiments suggest Earth bacteria actually could survive on Mars. Adds Rummel: “We don't know everything that Earth organisms can do.”