A University of Colorado study, using data from the Mars Global Surveyor and Mars Odyssey orbiters, has concluded that the area surrounding the rover Opportunity’s landing site once contained a huge body of water — up to 330,000 square kilometers. That makes this Martian sea comparable to the Baltic or, as investigator Brian Hynek of the Laboratory for Atmospheric and Space Physics puts it, ‘all of the Great Lakes combined.’

Hynek’s paper on the Martian sea (“Implications for hydrologic processes on Mars from extensive bedrock outcrops throughout Terra Meridiani”) appears in the September 9 issue of Nature, and you can find CU’s press release on his work here.

The physicist used the thermal emission imaging system on-board Mars Odyssey to infer the size of rocks, studying how larger and smaller particles change temperature at different rates in daylight and at night. His thermal maps show wide areas of bedrock, and tell him that water once extended far beyond the landing area. To quote the press release:

Hynek speculated that if the outcrops at the landing site are the result of sea deposition, as believed, the body of water must have been deep enough and persisted long enough to build up sediments roughly one-third of a mile deep. “For this to occur, the ancient global climate of Mars must have been different from its present climate and have lasted for an extended period,” Hynek wrote in the Nature paper.

This is good news for the hunt for life, at least in microbial form, and implies that conditions were once far more favorable for its formation.