The Mystery of What Happened to the Early Mars Atmosphere

Scientists may be Closer to Solving the Mystery of Mars Atmosphere

Scientists believe Mars changed from a world with plenty of surface water billions of years ago into the dry desert of a planet it is today.

And now, they may be closer to understanding why.

A new analysis of the largest known deposit of carbonate minerals on Mars suggests that the original Mars atmosphere may have already lost most of its carbon dioxide by the era of valley network formation.

Bet​hany Ehlmann, of the California Institute of Technology and NASA Jet Propulsion Laboratory, said:

The biggest carbonate deposit on Mars has, at most, twice as much carbon in it as the current Mars atmosphere.  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 Mars atmosphere. That gas can be pulled out of the air and sequestered or pulled into the ground by chemical reactions with rocks to form carbonate minerals. Years before the series of successful Mars missions, many scientists expected to find large Martian deposits of carbonates holding much of the carbon from the planet's original atmosphere.

Mars Spirit Rover

B5GEE5 Spirit Mars Exploration Rover on the flank of Husband Hill

Instead, these missions 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 at least the size of Delaware, and maybe as large as Arizona, in a region called Nili Fossae.

Christopher Edwards, a former Caltech researcher now with the U.S. Geological Survey in Flagstaff, Arizona, and Ehlmann reported the findings and analysis in a paper posted online by the journal Geology.

Their estimate of how much carbon is locked into the Nili Fossae carbonate deposit uses observations from numerous Mars missions, including the Thermal Emission Spectrometer (TES) on NASA's Mars Global Surveyor orbiter, the mineral-mapping Compact Reconnaissance Imaging Spectrometer for Mars (CRISM) and two telescopic cameras on NASA's Mars Reconnaissance Orbiter, and the Thermal Emission Imaging System (THEMIS) on NASA's Mars Odyssey orbiter.

Edwards and Ehlmann compare their tally of sequestered carbon at Nili Fossae to what would be needed to account for an early Mars atmosphere dense enough to sustain surface waters during the period when flowing rivers left their mark by cutting extensive river-valley networks. By their estimate, it would require more than 35 carbonate deposits the size of the one examined at Nili Fossae, which they deem unlikely, given the numerous and detailed surveys of the planet from orbit.

And while deposits from earlier in the history of Mars atmosphere could be deeper and better hidden, they won't help solve the thin-atmosphere conundrum at the time the river-cut valleys formed. 

Put it this way. The modern Martian atmosphere is too tenuous for liquid water to persist on the surface. And a denser atmosphere would have kept water from immediately evaporating. But if the atmosphere was once thicker and denser, what happened to it?

One possible explanation is that the early Mars atmosphere was much more dense during its flowing-rivers period, and then was lost into rather than by sequestration into ground minerals.

As Edwards said:​

Maybe the atmosphere wasn't so thick by the time of valley network formation. 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 water to thaw and flow occasionally, and that doesn't require very much atmosphere.

NASA's Curiosity Mars rover mission has found evidence of ancient top-of-atmosphere loss, based on the ratio of heavier carbon to lighter carbon in the modern Mars atmosphere. Uncertainty remains about how much of that loss occurred before the period of valley formation; much may have happened earlier. NASA's MAVEN orbiter, which has been studying the outer atmosphere of Mars since late 2014, may help reduce that uncertainty.

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