Greenhouse Effect on Early Mars Was Probably Much Stronger Than Known

Giant active volcanoes in the early history of Mars blew huge amounts of gas in the atmosphere, with a significant climate warming effect. This is old news, but recent experiments provide evidence that the greenhouse effect could have been much larger, because instead of carbon dioxide the much more potent greenhouse gas methane would form in the specific geological environment of Mars (and other planets). The findings suggest that even a thin atmosphere early in Mars' history might have created conditions warm enough for liquid water on the surface. 

The findings are from a study that investigated how carbon is trapped and released by iron-rich volcanic magma, offering clues about the early atmospheric evolution on Mars and other terrestrial bodies.

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 this carbon-transfer from the underground to the atmosphere works on other planets—and how that might influence greenhouse conditions—hasn’t been well understood.

“We know carbon goes from the solid mantle to the liquid magma, from liquid to gas and then out,” says Alberto Saal, professor of geological sciences at Brown University. “We want to understand how the different carbon species that are formed in the conditions that are relevant to the planet affect the transfer.”

The latest results, published in the Proceedings of the National Academy of Sciences, suggest 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.

A key difference between conditions in Earth’s mantle and the mantles of other terrestrial bodies is what scientists refer to as oxygen fugacity, the amount of free oxygen available to react with other elements. Earth’s mantle today has a relatively high oxygen fugacity, but in bodies like the Moon and early Mars, it is very low.

To find out how that lower oxygen fugacity affects carbon transfer, the researchers set up a series of experiments using volcanic basalt similar to those found on the Moon and Mars.

They melted the volcanic rock at varying pressures, temperature, and oxygen fugacities, using a powerful spectrometer to measure how much carbon was absorbed by the melt and in what form. They found that at low oxygen fugacities, carbon was trapped as iron carbonyl, something previous research hadn’t detected. At lower pressures, iron carbonyl degassed as carbon monoxide and methane.

“We found that you can dissolve in the magma more carbon at low oxygen fugacity than what was previously thought,” says graduate student and lead author Diane Wetzel. “That plays a big role in the degassing of planetary interiors and in how that will then affect the evolution of atmospheres in different planetary bodies.”

Researchers from Northwestern University and the Carnegie Institution of Washington contributed to the research, which was supported by NASA, the National Science Foundation, the David and Lucile Packard Foundation, and the Deep Carbon Observatory.