The composition of a planet's atmosphere has roots deep beneath its surface. When mantle material melts to form magma, it traps subsurface carbon.
As magma moves upward toward the surface and pressure decreases, that carbon is released as a gas.
On Earth, carbon is trapped in magma as carbonate and degassed as carbon dioxide, a greenhouse gas that helps Earth's atmosphere trap heat from the sun.
But how carbon is transferred from underground to the atmosphere in other planets - and how that might influence greenhouse conditions - wasn't well understood.
"We know carbon goes from the solid mantle to the liquid magma, from liquid to gas and then out," Alberto Saal, professor of geological sciences at Brown and one of the study's authors, said.
"We want to understand how the different carbon species that are formed in the conditions that are relevant to the planet affect the transfer," he said.
This latest study, which also included researchers from Northwestern University and the Carnegie Institution of Washington, indicated that under conditions like those found in the mantles of Mars, the Moon and other bodies, carbon is trapped in the magmas mainly as a species called iron carbonyl and released as carbon monoxide and methane gas. Both gasses, methane especially, have high greenhouse potential.
The findings suggest that when volcanism was widespread early in Mars' history, it may have released enough methane to keep the planet significantly warmer than it is today.
The study is published in the Proceedings of the National Academy of Sciences.