Melosh explains what might have happened next : " Once life began on Mars , the Late Heavy Bombardment [ lots of big rocks crashed into Earth and Mars about 4 billion years ago ] would have provided abundant means of transport for the Mars-Earth diaspora . Given Mars ' current very hostile surface environment , I would not be surprised if it petered out later ( maybe some life still persists in the subsurface , living off the chemical and energetic gleanings from Mars ' still-active volcanism ) .