The red planet Mars may have had a warmer and wetter state long ago compared to its current colder and drier conditions, scientists said. They suggested that Mars may have been once shrouded by an atmosphere that is thicker than that of the Earth. If it were true, scientists wonder: where did all the carbon go?

A team of experts from California Institute of Technology (Caltech) and NASA's Jet Propulsion Laboratory (JPL) believe they have a plausible explanation for the phenomenon.

About 3.8 billion years ago, Mars might have had only a moderately dense atmosphere. The researchers found a photochemical process that could have helped such a moderately dense atmosphere to evolve into its current state without causing the carbon to go missing completely. The process is also incredibly consistent with carbon isotopic measurements that exist today.

"With this new mechanism, everything that we know about the Martian atmosphere can now be pieced together into a consistent picture of its evolution," explained Renyu Hu, the study's lead author.

Researchers said there were two plausible mechanisms that could explain the removal of excess carbon dioxide in the planet. The first one was that the CO2 incorporated into minerals in rocks known as carbonates, while the second one was that the CO2 had become lost in space.

A previous study on carbon sequestration assessed data from satellites that orbit Mars in order to create an inventory of carbonates in the planet. The data revealed that there were not enough carbonates in the upper kilometer of crust in the planet that would equate to the missing carbon from the thick atmosphere which might have existed before.

The current team of scientists examined the ratio of carbon-12 and carbon-13. These are stable isotopes that contain the same number of protons but have different numbers of neutrons, as well as different masses.

Hu explained that since processes can alter the amounts of these two isotopes in the atmosphere, they used the measurements of the ratios at several points in time to infer what occurred to the Martian atmosphere.

Scientists used measurements of the ratio of the carbon isotope found in Martian meteorites which contain gases from deep within Mars' mantle. These measurements provide insight into the ratio of isotopes of the earlier atmosphere in the red planet.

From that, the team compared measurement of isotopes in the current Martian atmosphere collected by the Curiosity rover and the measurement of isotopes in that of the original. They found that the atmosphere is unusually abundant in carbon-13.

Previously, scientists thought carbon in Mars would be ejected through sputtering, which involves interactions between solar wind and the upper atmosphere. But the new photochemical mechanism describes a process in which the sun's ultraviolet light particle strikes a CO2 molecule in the upper atmosphere. This molecule absorbs energy from the photon. The molecule then splits into carbon monoxide and oxygen. The ultraviolet particle hits the carbon monoxide once more, and it dissociates into an atomic carbon and oxygen. Some carbon particles had enough energy to escape, and that carbon-12 particles were far more likely to go into the atmosphere than carbon-13, the team said.

The findings are published in the journal Nature Communications.