Carbon dioxide rivers? Ancient Mars' liquid may not all have been water

Scientists are rethinking a long-held belief that the liquid responsible for shaping Mars' surface must have been water.

For decades, evidence like massive outflow channels, ancient river valleys, deltas and lakebeds have suggested Mars had a watery past, as these formations resemble those shaped by water on Earth. These widespread features would seem to narrow the possibilities to liquid water — but there are cracks in this theory.

Another possibility is liquid carbon dioxide. Under the dense atmosphere of early Mars, carbon dioxide could have liquefied and plausibly flowed across the Red Planet, carving its surface in ways similar to water. In a new study, a team of researchers argue that our extensive understanding of water-based systems on Earth, combined with limited knowledge of liquid carbon dioxide systems, may have led us to prematurely dismiss a scenario that could have fundamentally shaped Mars as we know it today.

"It's difficult to say how likely it is that this speculation about early Mars is actually true," said Michael Hecht, principal investigator of the MOXIE instrument aboard the NASA Mars Rover Perseverance, in an interview with MIT News. "What we can say, and we are saying, is that the likelihood is high enough that the possibility should not be ignored."

They reference earlier experiments from carbon sequestration research that investigated how carbon dioxide interacts with minerals in the presence of brine and supercritical or liquid carbon dioxide —a phase of carbon dioxide that occurs at specific temperatures and pressures in which it exhibits the properties of both a gas and a liquid.

These studies demonstrated widespread carbonation processes, where carbon dioxide is incorporated into minerals as carbonates, under conditions relevant to early Mars. "Geologic sequestration on Earth has revealed a surprising degree of chemical reactivity between [carbon dioxide] fluid and minerals if the fluid is water-saturated, as it would probably have been on Mars," the researchers write in a new study. "The resulting alteration products — carbonates, phyllosilicates and possibly sulfates — are consistent with minerals found on Mars today."

Current mineralogy and surface features could have formed from stable liquid carbon dioxide melting beneath carbon dioxide glaciers, or even subsurface reservoirs.

However, the researchers emphasize moving away from the idea of a single warm, wet environment, instead highlighting a range of brief, unstable, and subsurface processes.

This could also mean that a combination of both liquid water and liquid CO₂ may have worked together to shape Mars' landscape. It's not necessarily an either/or scenario—and this is the core message the scientists aim to convey. Understanding what might have happened on Mars requires thinking beyond the confines of Earth and exploring possibilities outside traditional assumptions.

"Understanding how sufficient liquid water was able to flow on early Mars to explain the morphology and mineralogy we see today is probably the greatest unsettled question of Mars science," Hecht said. "There is likely no one right answer, and we are merely suggesting another possible piece of the puzzle."

The team's research was published in the journal Nature Geoscience.