... Credit: Mars Exploration Rover Mission , JPL , NASA Explanation: How prevalent was water on Mars? Results from the Spirit rover now indicate that Gusev crater likely had a wet past, a result that comes shortly after Spirit's twin rover Opportunity uncovered clear evidence of past water at Meridiani Planum on the other side of Mars . ... The amount of Mars once covered by ancient water remains unknown, as both rovers landed in regions thought likely to once be underwater. ... Mars in View . ...