They didn't find carbonates, which suggested that atmospheric levels of Carbon dioxide at the time were much lower than levels required to warm ancient Mars enough to maintain liquid water on its surface.
About the absence of carbonate in sedimentary rock at Gale Crater the rover has examined, "We've been particularly struck", said lead author Thomas Bristow of NASA's Ames Research Center in California and principal investigator for the chemistry and mineralogy (CheMin) instrument on Curiosity. 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.
Mars is now cold and dry, but scientists have plenty of evidence suggesting that its surface was once covered with oceans, rivers, streams, ponds and lakes.
The Curiosity Rover has previously found that the Martian environment was conducive to the presence of water, which hints that the planet may have possibly been able to support life.
The Curiosity rover has been cruising around on Mars since late 2012, so you'd think that scientists would have a pretty good handle on the kind of data it's been sending back by now. Using that evidence and the amount of other minerals present, the researchers calculated that the Mars atmosphere must have contained very little carbon dioxide. When carbon dioxide interacts with water on the surface of the Earth, it undergoes chemical reactions to produce carbonate, which can then interact with other elements and compounds to produce carbonate minerals at the bottom of bodies of water. Yet, the ancient sun was about one-third less warm and climate modelers struggle to produce scenarios that get the surface of Mars warm enough for keeping water unfrozen. "I think the key problem is how long these liquid water events lasted", Dr. Niles said, but determining the date of geologic features on Mars is hard.
The research, published in the Proceedings of the National Academy of Sciences, analyzes data from Curiosity's analysis of rocks from Gale Crater, which scientists believe was home to an ancient and long-lived lake.
For the past two decades, researchers have used spectrometers on Mars orbiters to search for carbonate that could have resulted from an early era of more abundant carbon dioxide. The current atmosphere of Mars is less than 10 millibars and about 95 percent carbon dioxide. One successful model proposes a thick carbon-dioxide atmosphere that also contains molecular hydrogen. "We still don't know", said Fairen.
"The downside of all these other greenhouse gases is that they tend to be quite reactive, so when you put them in the atmosphere, they don't hang out an especially long time", Bristow said.
The new study pins the puzzle to a particular place and time, with an on-the-ground check for carbonates in exactly the same sediments that hold the record of a lake about a billion years after the planet formed.
Additionally, the new NASA study found only tens of millibars (one one-thousandth of sea-level air pressure on Earth) of carbon dioxide present when the lake at Gale Crater existed. Researchers have certainly found the mineral on the Red Planet before. So it's looking more and more unlikely that Mars' ancient atmosphere was saturated with CO2.