Mars is blanketed by a thin, mostly carbon dioxide atmosphere--one that is far too thin to prevent large amounts of water on the surface of the planet from subliming or evaporating. But many researchers have suggested that the planet was once shrouded in an atmosphere many times thicker than Earth's. For decades that left the question, "Where did all the carbon go?" Now a team of scientists from Caltech and JPL thinks they have a possible answer. The researchers suggest that 3.8 billion years ago, Mars might have had only a moderately dense atmosphere. They have identified a photochemical process that could have helped such an early atmosphere evolve into the current thin one without creating the problem of "missing" carbon and in a way that is consistent with existing carbon isotopic measurements.

The scientists describe their findings in a paper that appears in the November 24 issue of the journal Nature Communications.

"With this new mechanism, everything that we know about the martian atmosphere can now be pieced together into a consistent picture of its evolution," says Renyu Hu, a postdoctoral scholar at JPL, a visitor in planetary science at Caltech, and lead author on the paper.

When considering how the early martian atmosphere might have transitioned to its current state, there are two possible mechanisms for the removal of excess carbon dioxide (CO2). Either the CO2 was incorporated into minerals in rocks called carbonates or it was lost to space.

A separate recent study coauthored by Bethany Ehlmann, assistant professor of planetary science and a research scientist at JPL, used data from several Mars-orbiting satellites to inventory carbonate rocks, showing that there are not enough carbonates in the upper kilometer of crust to contain the missing carbon from a very thick early atmosphere that might have existed about 3.8 billion years ago.