Scientists studying Mars have stumbled upon a paradox that says the Red Planet was once wet with flowing water, but that it may not have actually been warm enough to keep water unfrozen.
A new analysis of data collected by the Curiosity rover shows that Mars did not have enough carbon dioxide in its atmosphere about 3.5 billion years ago to cause a greenhouse effect that would thaw frozen water ice.
Curiosity found sediments from the bed of what was once a lake on Mars, where microbes could have existed.
But at that same site, the rover discovered no carbonate minerals in samples of the bedrock.
Analysis of these samples has concluded that there could not have been much carbon dioxide on Mars when the lake existed.
In fact, Curiosity has made no conclusive detection of carbonates in any of the lakebed rocks it has sampled since landing on the Red Planet in 2011.
"We've been particularly struck with the absence of carbonate minerals in sedimentary rock the rover has examined," says Thomas Bristow of NASA's Ames Research Center in 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."
Adding to the paradox is the fact that the ancient Sun was about a third less warm than it is today, meaning it would have struggled to warm the planet enough to keep water unfrozen.
One theory says that perhaps the former lake studied by Curiosity was liquid water covered with ice, but this would have created deep cracks called ‘dropstones’ , which so far have not been found.
"It's been a mystery why there hasn't been much carbonate seen from orbit," says Bristow.
"You could get out of the quandary by saying the carbonates may still be there, but we just can't see them from orbit because they're covered by dust, or buried, or we're not looking in the right place.
The Curiosity results bring the paradox to a focus.
This is the first time we've checked for carbonates on the ground in a rock we know formed from sediments deposited under water."