Researchers : `` This may have happened just beneath the surface relatively recently , '' Arvidson said , `` or it may have happened deeper below ground longer ago and then , by serendipity , erosion stripped away material above it and made it accessible to our wheels . '' Mars Opportunity rover 's wheel broke off a piece of rock . That piece is the 'mystery rock ' that looks like a doughnut . Comment:
Researchers : Mars Opportunity rover 's wheel broke off a piece of rock . That piece is the 'mystery rock ' that looks like a doughnut . Comment: