The Martian atmosphere was once a lot thicker than it is today, according to observations from NASA’s Mars Atmosphere and Volatile Evolution Mission — MAVEN for short. But charged particles from the sun knocked away some of the lighter isotopes of argon and other molecules. These molecules are like a lid on the upper atmosphere. When present, they keep other gases circulating at lower altitudes. But in their absence, it's easier for the atmosphere to escape into space. What's left is a very thin atmosphere.
Recent observations from the European Space Agency’s Mars Express spacecraft showed in the most detail yet how in warm weather, water circulates from the Martian polar caps into the lower atmosphere, which is in turned coupled to the more tenuous outer atmosphere that slowly leaks into space. In cold weather, water in the atmosphere freezes out and lands again on the caps. The cycle is complex, researchers said, but their work showed that the atmosphere of Mars "behaves as a single system."
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Scientists searched for decades for the presence of carbonates on the surface of Mars. Finally, in 2008, NASA’s Mars Reconnaissance Orbiter (MRO) found deposits of magnesium carbonate in Nili Fossae, a zone that is near one of the planet's largest impact basins. Both MRO and the space agency's Mars Odyssey have observed carbon dioxide in the planet’s ice caps, which could be another possible source for warming up the atmosphere.
The authors tallied the inventory of CO2 stores on Mars, which for the most part include the caps and Nili Fossae. They found that CO2 in the soil would require either strip mining or a large amount of heating to release into the atmosphere. The massive ice caps would basically have to be evaporated completely to remove their carbon stores.
Their conclusion is surely a letdown for advocates of terraforming Mars: For all the effort, there’s just not enough carbon available to trigger the necessary atmospheric changes to support life.
"People at first thought there must be a lot of carbon dioxide [on the surface], because of Mars's thicker atmosphere in the past," Jakosky told Seeker. "But we haven't been able to identify very much."
He said even the Nili Fossae carbonate plains — the single largest source of carbon on Mars — have tough rocks that would make it difficult to harvest the CO2 on a large scale.