After four and a half years of exploring Mars, NASA’s Curiosity rover has made a new discovery that only deepens a long-standing mystery about the Red Planet — namely, how the world used to be so wet. Pretty much all Mars scientists agree that billions of years ago the planet had flowing rivers and lakes on its surface. But there’s a problem: no one can quite explain how ancient Mars was warm enough back then to support liquid water. And Curiosity is unearthing clues that only make things more confusing.

Since its landing in a region called Gale Crater, the rover has found critical signs that liquid water once pooled on the Martian surface. Curiosity has been scouring over hundreds of meters of sedimentary rocks that are thought to have been deposited by a lake that existed in the crater 3.5 billion years ago. But there’s an issue with timing: back when Mars supposedly had water on its surface, the Sun wasn’t cranking out that much heat. It’s a conundrum known as the “faint young Sun paradox,” and it’s the idea that our Solar System’s star was only producing about 70 percent of the energy as it does today. That means there must have been some other factor that warmed Mars up enough so that surface water existed as a liquid.

One leading idea is that the Martian atmosphere was thick with carbon dioxide, a potent greenhouse gas that traps heat. If the amount of CO2 was high enough, it could have theoretically warmed Mars up to the necessary temperature for lakes and rivers to flow on the surface. But Curiosity has all but torpedoed that theory. A new study, published this week in the Proceedings of the National Academy of Science, reveals that the rover hasn’t found a crucial byproduct of an ancient carbon dioxide atmosphere.

If Mars did have a thick carbon dioxide atmosphere, the gas would have dissolved into the water on Mars and formed something called carbonic acid. This acid tends to weather rocks underneath the water, eventually producing a substance known as carbonate minerals. However, these minerals haven’t been found by Curiosity. “Under a very thick CO2-rich atmosphere, the sediment that forms in lakes and rivers are expected to have carbonate minerals,” Thomas Bristow, a scientist at NASA’s Ames Research Center and lead author on the study, tells The Verge. “But we haven’t seen them in any of the rocks at Gale Crater.”

It doesn’t mean that carbonates are absent from Mars. Researchers have certainly found the mineral on the Red Planet before. The new findings just means that Curiosity’s CheMin instrument — a tool that beams X-rays onto samples of rocks to look for minerals — hasn’t been able to detect any of this substance in Gale Crater. The CheMin instrument can pick up carbonates if they make up just a few percent of a rock that’s being analyzed. But since Curiosity hasn’t picked up any signs of carbonates, the mineral is under the detection limits of the instrument. “If there are carbonates there, they’re in teeny tiny amounts,” says Bristow.

That doesn’t bode well for the thick CO2 atmosphere theory. In order for ancient Mars to be warm enough to support liquid water, the planet would have needed carbon dioxide at a pressure of one bar. That’s about one Earth atmosphere’s worth of CO2. However, given the detection limit on Curiosity’s CheMin instrument and the lack of carbonates found, NASA estimates that the atmospheric carbon was tens to hundreds of times less than one bar of pressure.

It’s a finding that backs up what spacecraft have found in orbit around Mars. Numerous vehicles circulating the planet haven’t found as many carbonates on Mars as researchers had expected either, and Curiosity’s findings seem to confirm that the compound isn’t as abundant as previously thought on the Red Plant. “When you make your observations from space, there’s always a way to kind of explain the lack of carbonate that’s been found. Perhaps the satellites are looking in the wrong place,” says Bristow. “But now we’re able to ground the truth in what’s been seen from space.” So it’s looking more and more unlikely that Mars’ ancient atmosphere was saturated with CO2.

Of course, there are numerous other greenhouse gases that could have heated up ancient Mars, such as sulfur dioxide, methane, nitrous oxide, and even hydrogen. There’s one theory that early Mars was plagued by many volcanic eruptions, which put sulfur dioxide in the atmosphere and kept the planet warm. But that model doesn’t seem to totally fit, since such a warming from eruptions would have only lasted for hundreds to thousands of years, says Bristow. “That’s not what we see in the rock records,” he says. “It implies the surface was warm for hundreds of thousands or millions of years.”

There’s also the idea that perhaps Mars wasn’t that hot, and that the lake in Gale Crater was capped off with a layer of ice. That ice may have acted like a blanket and kept the water warm enough underneath to remain as a liquid. But this theory also isn’t satisfying, says Bristow, because Curiosity hasn’t seen any signs of ancient glacial processes in the sedimentary rocks at Gale Crater.

And then there’s the idea that perhaps Mars’ orbit changed and that the planet somehow got closer to the Sun billions of years ago. That concept is a bit hard to test, though, which means this mystery is far from being solved. “It really is a puzzle,” says Bristow. “People have been thinking about carbon dioxide-rich atmospheres on Mars for a really long time... Perhaps there is some kind of localized mechanism for keeping lakes warm that hasn’t been considered before.”