Everything we’ve observed so far about the surface of Mars points to an ancient past that was warmer, wetter, and very possibly habitable for life as we know it. From the scars of enormous floods and vast branching river deltas that are etched into the Martian surface to the rounded pebbles of ancient stream beds to the chemical signatures of materials formed only in the presence of water, the evidence for Mars’ wet history seems overwhelming. But there’s one big question that still stymies scientists: what happened to all of Mars’ carbon dioxide?

Even though Mars’ atmosphere is 96% CO2 today, it is incredibly thin—only 1% as dense as Earth’s. It’s thought that Mars once had a much thicker atmosphere in its early history, but was there enough of the greenhouse gas even then to keep it warm enough (with a cooler young Sun) to maintain liquid water on its surface? According to a new analysis of data from NASA’s Curiosity rover, Mars just didn’t have enough carbon dioxide 3.5 billion years ago to provide enough warming to prevent water from freezing solid.

“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. “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.”

But with all the physical evidence pointing at liquid water—even without the CO2—could something else have been keeping Mars warm?

Read the full story from NASA here: NASA’s Curiosity Rover Sharpens Paradox of Ancient Mars