Mystery of 'missing' carbon on Mars decoded

Scientists have found that ultraviolet (UV) rays from Sun caused Mars to lose its atmospheric carbon dioxide, solving the long standing mystery of the 'missing' carbon on the red planet.
Mars is blanketed by a thin, mostly carbon dioxide atmosphere that is too thin to keep water from freezing or quickly evaporating. However, geological evidence has led scientists to conclude that ancient Mars was once a warmer, wetter place than it is today.
The solar wind stripped away much of Mars' ancient atmosphere and is still removing tonnes of it every day.
But scientists have been puzzled by why they have not found more carbon - in the form of carbonate - captured into Martian rocks.

Scientists from the California Institute of Technology (Caltech) and NASA's Jet Propulsion Laboratory (JPL) in US suggest that 3.8 billion years ago, Mars might have had a moderately dense atmosphere.
Such an atmosphere could have evolved into the current thin one, not only minus the "missing" carbon problem, but also consistent with the observed ratio of carbon-13 to carbon-12, which differ by number of neutrons in the nucleus.

The measurements of the ratio of the relative amounts of carbon-13 to carbon-12 isotopes in the atmosphere at different points in time can be used as a fingerprint to infer exactly what happened to the Martian atmosphere in the past, said lead author Renyu Hu, postdoctoral fellow at Caltech.

The first constraint is set by measurements of the ratio in meteorites that contain gases released volcanically from deep inside Mars, providing insight into the starting isotopic ratio of the original Martian atmosphere.
The modern ratio comes from measurements by the Sample Analysis at Mars (SAM) instrument on NASA's Curiosity rover.
NASA's MAVEN (Mars Atmosphere and Volatile Evolution) mission has yielded recent results indicating that about 100 grammes of particles every second are stripped from today's Martian atmosphere via sputtering, a precess that involves interactions between the solar wind and the upper atmosphere.
Measurements from NASA's Curiosity rover show that today's Martian atmosphere is far more enriched in carbon-13 - in proportion to carbon-12 - than it should be as a result of sputtering alone, so a different process must also be at work.

The researchers identified a mechanism that could have significantly contributed to the carbon-13 enrichment.
The process begins with UV light from the Sun striking a molecule of carbon dioxide in the upper atmosphere, splitting it into carbon monoxide and oxygen. Then, UV light hits the carbon monoxide and splits it into carbon and oxygen.
Some carbon atoms produced this way have enough energy to escape from the atmosphere, and the new study shows that carbon-12 is far more likely to escape than carbon-13.
The researchers found that a small amount of escape by this process leaves a large fingerprint in the carbon isotopic ratio. That allowed them to calculate that the atmosphere 3.8 billion years ago might have had a surface pressure a bit less thick than Earth's atmosphere today.

The study was published in the journal Nature Communications.