Mars' Atmosphere: What Happened?
New research on the Red Planet has discovered that most of the carbon dioxide in Mars' original atmosphere may have been lost by the time the valley network began forming, disputing previous thoughts by scientists.
The valley network formations are branching networks of valleys on Mars that resemble terrestrial river drainage basins. They are found mainly cut into the land of the martian southern highlands, and are typically approximately four billion years old.
"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."
The Martian atmosphere is comprised mostly of carbon dioxide, and it can be pulled out of the air and sequestered, or pulled into the ground by chemical reactions with rocks to form carbonate minerals.
Many scientists originally expected to find large Martian deposits of carbonates containing much of the carbon from the original atmosphere. Instead, once missions to Mars began, scientists have found only low concentrations of carbonate widely dispersed, with few concentrated deposits available. A region called Nili Fossae, about the size of Delaware, contains the largest (by far) area of carbonate-rich deposit on the planet.
Christopher Edwards, a former Caltech researcher now with the U.S. Geological Survey in Flagstaff, Az., compared a tally with Ehlmann of sequestered carbon at Nili Fossae to the amount necessary for Mars' original atmosphere to be dense enough for surface waters during the time that rivers made the valley networks, according to the study.
By their figures, more than 35 carbonate deposits the size of the one examined at Nili Fossae would've been needed, inciting doubt that many large deposits could have been overlooked on all the detailed orbiter surveys of the planet. Granted, deposits from an even earlier time have potential to be still hidden, but they don't help with the thin-atmosphere mystery during the period the valleys formed.
"Maybe the atmosphere wasn't so thick by the time of valley network formation," Edwards said according to Science Daily. "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."
Mars' modern atmosphere isn't dense enough for liquid water to hold on the surface. A denser atmosphere on ancient Mars could have allowed for liquid water, and could have been warm enough to prevent freezing.
Scientist have wondered, if a thicker atmosphere existed, what happened to it? One possible explanation is that Mars did have a much denser atmosphere, but much of it was lost to outer space, drifting from the top of the atmosphere.
NASA's Curiosity rover found evidence of top-of-atmosphere loss, based on the modern atmosphere's ratio of heavier carbon to lighter carbon. However, there is much uncertainty about the timeline of the carbon loss. NASA's MAVEN orbiter, which has viewed Mars' outer atmosphere since 2014, is hoped to help solve that uncertainty.
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