Mars' Missing Carbon: Where Its Atmosphere Disappeared To Over Billions of Years

Mars is blanketed by a thin, carbon dioxide atmosphere. In fact, this atmosphere is so thin that it can't prevent large amounts of water from subliming or evaporating. Now, researchers are looking at how this atmosphere became so thin and what happened to the planet's missing carbon.

About 3.8 billion years ago, Mars might have had a moderately dense atmosphere. This atmosphere would explain how the planet had lakes and rivers on it billions of years ago.

But what happened to this atmosphere and how did the moderate early atmosphere evolve into the current thin one without creating the problem of "missing" carbon?

There are two possible mechanisms that may explain how Mars transitioned to its current state. Either the CO2 was incorporated into minerals in rocks called carbonates, or it was lost to space. In order to find out which mechanism it was, the scientists used data from several Mars-orbiting satellites.

The new findings show that there are not enough carbonates in the upper kilometer of crust to contain the missing carbon from a very thick early atmosphere that may have existed about 3.8 billion years ago.

In order to study the escape-to-space scenario, the scientists examined the ratio of carbon-12 and carbon-13, two stable isotopes of the element carbon that have the same number of protons in their nuclei, but different numbers of neutrons and thus different masses. Because various processes can change the relative amounts of those two isotopes in the atmosphere, researchers can use measurements of the ratio to fingerprint exactly what happened to the Martian atmosphere in the past.

In order to establish a starting point, the researchers used measurements of the carbon isotope ratio in Martian meteorites that contain gases that originated deep in the planet's mantle. The scientists then compared those values to isotopic measurements of the current Martian atmosphere.

In this case, the researchers believe that a new mechanism may be responsible. A particle of ultraviolet light from the sun strikes a molecule of CO2 in the upper atmosphere. That molecule absorbs the photon's energy and divides into carbon monoxide (CO) and oxygen. Then another ultraviolet particle hits the CO, causing it to dissociate into atomic carbon (C) and oxygen. Some carbon atoms produced in this way have enough energy to escape the atmosphere.

"The efficiency of this new mechanism shows that there is in fact no discrepancy between Curiosity's measurements of the modern enriched value for carbon in the atmosphere and the amount of carbonate rock found on the surface of Mars," said Bethany Ehlmann, one of the researchers, in a news release. "With this mechanism, we can describe an evolutionary scenario for Mars that makes sense of the apparent carbon budget, with no missing processes or reservoirs."

The findings are published in the journal Nature Communications.

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