Where did Mars’ atmosphere go? NASA rules out one theory

Carbon dioxide from the atmosphere on early Mars reacted with surface rocks to form carbonate, thinning the atmosphere by sequestering the carbon in the rocks, but nowhere near enough to account for total atmosphere loss, a new analysis suggests.

“The biggest carbonate deposit on Mars has, at most, twice as much carbon in it as the current Mars atmosphere,” said Bethany Ehlmann of the California Institute of Technology and NASA Jet Propulsion Laboratory, both in Pasadena.

“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.”

Carbon dioxide makes up most of the Martian atmosphere and until recently, scientists expected to find large deposits of carbonates holding much of the carbon from the planet’s original atmosphere.

Instead, recent missions such as NASA’s Mars Reconnaissance Orbiter have found low concentrations of carbonate distributed widely, and only a few concentrated deposits.

By far the largest known carbonate-rich deposit on Mars covers an area about the size of Arizona, in a region called Nili Fossae. Scientists estimate there would need to be 35 deposits this size to account for total sequestration of the atmosphere.

The latest discoveries have also caused some rethinking on the formation of the modern Martian landscape.

The current atmosphere is too tenuous for liquid water to persist on the surface, but a denser atmosphere on ancient Mars could have kept water from immediately evaporating and could have held in enough warmth to keep liquid water from freezing.

One possible explanation is that Mars did have a much denser atmosphere during its flowing-rivers period, and then lost most of it to outer space from the top of the atmosphere, rather than by sequestration in minerals.

“Maybe the atmosphere wasn’t so thick by the time of valley network formation,” says Christopher Edwards, a former Caltech researcher now with the U.S. Geological Survey.

“Instead of 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 water to thaw and flow occasionally, and that doesn’t require very much atmosphere.”

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