Today’s Martian atmosphere is tenuous. What little carbon dioxide atmosphere Mars has isn’t enough to keep water on its surface. But geological evidence points to a time when Mars was warmer and wetter. For a warmer climate to have existed, many researchers suggest the planet was once covered in a thicker carbon dioxide atmosphere.

We know solar wind stripped away most of Mars atmosphere. And it continues to this day. But scientists are having a tough time explaining why they haven’t found more carbon, or carbonate, in Martian rocks.

Scientists from Caltech and NASA’s JPL tackled the problem in a paper published this week. Their solution? Mars’ atmosphere was thicker than it was today. But they describe it as a “moderately dense atmosphere.” Surface pressure would have been about the same or less than on Earth. Their research shows Mars’ ancient atmosphere could have evolved into the one we see today and solves the missing carbon problem. It also explains the ratios between carbon-13 to carbon-12 seen.

From lead author Renyu Hu of Caltech:

“Our paper shows that transitioning from a moderately dense atmosphere to the current thin one is entirely possible. It is exciting that what we know about the Martian atmosphere can now be pieced together into a consistent picture of its evolution — and this does not require a massive undetected carbon reservoir.”

There are two possible processes that could explain the transition from a thicker Mars atmosphere to the very thin one we see today. One is the carbon dioxide was captured by rocks in the form of carbonate. Or, it was lost to space.

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A recent study rules out the carbonate scenario. Bethany Ehlmann from Caltech and NASA’s JPL explained:

“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.”

The loss to space scenario has its issues. Sputtering is one way carbon dioxide escapes to space. This happens as the solar wind interacts with the upper atmosphere. And NASA’s MAVEN orbiter recently showed about 100 grams of particles are stripped from Mars’ atmosphere every second.

The issue? Sputtering causes more loss of carbon-12 compared to carbon-13. But not in substantial amounts. The Curiosity rover shows today’s Martian atmosphere contains much more carbon-13 in proportion to carbon-12.

So what’s going on? Hu and his colleagues found a mechanism that could explain the higher amounts of carbon-13. It’s called ultraviolet photodissociation. As ultraviolet radiation (hv) interacts with a CO2 molecule, it breaks the bonds to form carbon monoxide (CO) and carbon (C). The ratio of carbon isotopes is then affected as carbon atoms are lost the space. The lighter carbon-12 isotope is more likely to be lost to space than the carbon-13 isotope. The infographic below visualizes this process.

mars carbon loss

The researchers then modeled the long-term effects of ultraviolet photodissociation. They found a small amount of escape led to a large fingerprint in the carbon isotopic ratio. Using the models, the researchers were able to figure out Mars’ atmosphere 3.9 billion years ago might have a had a surface pressure slightly thinner than Earth’s atmosphere today.

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Ehlmann, who co-authored this week’s study, explains the results.

“This solves a long-standing paradox. The supposed very thick atmosphere seemed to imply that you needed this big surface carbon reservoir, but the efficiency of the UV photodissociation process means that there actually is no paradox. You can use normal loss processes as we understand them, with detected amounts of carbonate, and find an evolutionary scenario for Mars that makes sense.”

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