Data from NASA's Curiosity rover on Mars has left scientists scratching their heads. On the one hand, the bot appears to have found evidence that water once flowed on Mars, but on the other hand, the readings suggest there couldn't have been.
The problem stems from carbon dioxide, or rather the lack of it. Curiosity has been trundling across the unforgiving dust world for nearly five years now, and so far it has found no carbonates. This potentially blows a hole in boffins' theories about the Martian atmosphere.
Current thinking is that about 3.8 billion years ago, Mars had running water on the surface. The Sun back then wasn't strong enough to warm the Martian surface to a temperature that would cause liquid water to form, so scientists postulated that there was enough carbon dioxide in the atmosphere to heat the surface using a greenhouse effect.
If that was the case, Martian rocks should be studded with carbonate material. Instead, Curiosity hasn't found any evidence of this after drilling into what is thought to be the bed of what was once a giant lake in the Gale Crater, according to new research to be published in the Proceedings of the National Academy of Science.
"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, in 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."
Based on the analysis of Curiosity's readings, and other Mars missions, the scientists estimate that when the lake existed there were no more than a few tens of millibars of carbon dioxide, based on the lack of carbonates. That wouldn't be enough carbon to produce a warming effect on the Red Planet.
"This analysis fits with many theoretical studies that the surface of Mars, even that long ago, was not warm enough for water to be liquid," said Robert Haberle, a Mars-climate scientist at NASA Ames and a co-author of the paper.
"It's really a puzzle to me. Some think perhaps the lake wasn't an open body of liquid water. Maybe it was liquid covered with ice. You could still get some sediment through to accumulate in the lakebed if the ice weren't too thick."
The problem with that ice-cover theory is that, on Earth, similar ice lakes produce clear indicators of ice wedges, or "dropstones," left in the geological record. These aren't showing up in Gale Crater.
There's plenty of evidence that Mars had running water, and there are some scientists who think it can even exist on the surface now under certain conditions. But the climate models must be missing something, the paper states. Perhaps there was another greenhouse gas besides CO2.
"It's been a mystery why there hasn't been much carbonate seen from orbit," Bristow said.
"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." ®