But how did such a small, cold world ever have lakes and rivers of liquid water? The reigning theory was that the thin atmosphere once contained so much carbon dioxide that the greenhouse effect made the planet warm enough. To test this theory, NASA sent the Curiosity rover to the bed of an ancient lake (above) to sample the sedimentary rocks within it. Rocks that formed in an atmosphere dense with CO2 ought to show evidence of that, in the form of carbonate minerals.
The same Martian bedrock in which Curiosity found sediments from an ancient lake where microbes could have thrived is the source of the evidence adding to the quandary about how such a lake could have existed. Curiosity detected no carbonate minerals in the samples of the bedrock it analyzed. The new analysis concludes that the dearth of carbonates in that bedrock means Mars' atmosphere when the lake existed -- about 3.5 billion years ago -- could not have held much carbon dioxide.So it's back to the drawing board in trying to model the ancient atmosphere of Mars.
"We've been particularly struck with the absence of carbonate minerals in sedimentary rock the rover has examined," said Thomas Bristow of NASA's Ames Research Center, Moffett Field, California. "It would be really hard to get liquid water even if there were a hundred times more carbon dioxide in the atmosphere than what the mineral evidence in the rock tells us."