How and when Mars lost its atmosphere is a question that continues to remain unanswered. But new research provides an alternative to existing theories. 

The Red Planet is believed to have gone through one or several wet periods, with lakes, seas and rivers present on the surface. These are believed to have sculpted some of the features on Mars, such as its valleys, while the atmosphere was still present, keeping the temperature warm enough for water to flow. Eventually, the atmosphere disappeared, and the water soon froze or evaporated. Only ice remains on Mars, mostly at its poles.

A new study in the journal Geology, though, suggests Mars lost its atmosphere when water was still present, before the period of valley formation. This would mean that the valleys were formed not by water, but by ice and snow, with the water freezing much earlier than expected. Brief periods of the temperature rising above the freezing point of water would have allowed this ice to thaw slightly and flow on the surface, forming the valleys.

"Maybe the atmosphere wasn't so thick by the time of valley network formation," said Christopher Edwards of the U.S. Geological Survey in Arizona, lead author on the study, in a statement. "Instead of a Mars that was wet and warm, maybe it was cold and wet with an atmosphere that had already thinned. How warm would it need to have been for the valleys to form? Not very. In most locations, you could have had snow and ice instead of rain. You just have to nudge above the freezing point to get [ice] to thaw and flow occasionally, and that doesn't require very much atmosphere."

Evidence for this theory comes from the amount of carbon that is detectable on the surface. Most of the Martian atmosphere was, and still is, made of carbon dioxide. The gas can be pulled out of the air and left to dissipate, or pulled into the ground by rocks, forming carbonate minerals. So scientists had expected to find large deposits of such minerals on the surface, left over from the time when the atmosphere was much thicker, but this is not the case.

The biggest carbonate deposit on Mars, Nili Fossae, has just twice the current levels of carbon in it as the Martian atmosphere. Combining it with other known carbonate deposits on Mars is simply not enough to explain how the atmosphere was once so thick.

“Even if you combined all known carbon reservoirs together, it is still nowhere near enough to sequester the thick atmosphere that has been proposed for the time when there were rivers flowing on the Martian surface,” Bethany Ehlmann, of the California Institute of Technology (Caltech) and NASA’s Jet Propulsion Laboratory (JPL), said in the statement.

It would take 35 deposits the size of Nili Fossae to account for the large atmosphere needed for water to form the valleys, and the scientists say it is unlikely such a large number of unknown deposits exist on the surface. Known deposits are found using instruments on spacecraft orbiting Mars. Deeper carbon deposits from when the atmosphere was thicker and water flowed may be hiding on Mars, but this would have been a time before valley formation.

The study therefore suggests Mars lost its atmosphere much longer ago than thought. While not giving a figure in their study, this would likely be in the billions of years; the most recent flow of water on Mars is thought to be 600 million years ago.

This isn’t the only evidence for glaciers forming Martian valleys, rather than water. Last year, the discovery of a mineral called jarosite high on the walls of Valles Marineris suggested that it had been transported there by ice, as it was too high for a river.

The mystery deepens, but evidence is mounting that Mars was less wet than thought in its relatively recent history.

^{Image in text: Carbonate-rich deposits (green) at Nili Fossae. NASA/JPL-Caltech/JHUAPL/University of Arizona.}