These "planetary protection" requirements impose heavy financial burdens on Mars missions, partially explaining why no robots have searched for life on the Red Planet's surface since NASA's twin Viking landers ceased operations three decades ago, researchers Alberto Fairen of Cornell University and Dirk Schulze-Makuch of Washington State University write in a commentary published online today (June 27) in the journal Nature Geoscience.

Further, such restrictions are unnecessary, because Earth life has doubtless made it to the neighboring Red Planet already inside chunks of rock blasted off our planet by asteroid strikes, the scientists say.