Three and a half billion years ago Mars used to have rivers and lakes, but the Sun was a lot dimmer that it is today, so it shouldn't have been warm enough for them. Scientists thought the Red Planet must have been in full greenhouse-mode, with plenty of carbon dioxide warming up its otherwise frigid atmosphere.
But NASA’s Curiosity has delivered a carbon conundrum for astronomers. The rover is studying the bed of an ancient lake and it wasn’t able to find any trace of carbonate minerals, suggesting that the atmosphere was quite devoid of carbon dioxide. These results are published in the Proceedings of the National Academy of Sciences.
"We've been particularly struck by the absence of carbonate minerals in sedimentary rock the rover has examined," said Thomas Bristow from NASA's Ames Research Center in a statement. "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."
Curiosity can identify carbonates even if they make up just a few percent of the rocks, but has failed to make any definitive detection – quite the opposite of what the researchers were expecting. When carbonates were failed to be seen from orbit, planetary scientists suggested that they might have been buried, but the new data shows that Mars couldn’t have been kept warm by carbon dioxide.
“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,” added Bristow.
The team proposed both different global and local scenarios. For example, the ancient lake that Curiosity is exploring (now known as Gale Crater) might have been covered in ice, although they have not found any evidence supporting this explanation.
Global explanations require a radical rethink of the young Mars atmospheric model. Carbon dioxide and hydrogen were considered the major players in the primordial atmosphere of the Red Planet, but the data doesn’t support this.
Four billion years ago, the Sun was emitting only 70 percent of the light it emits today. The conditions of the terrestrial planets of the Solar System are closely linked to the evolution of our Sun, so it’s complicated enough to explain how liquid water was possible on Earth, let alone Mars.
