New Theory Explains A Warm, Wet Early Mars

A trio of scientists from MIT and Harvard have published a new theory that could help explain how Mars could have maintained an early warm and wet environment, while better tallying with the observations made by the Rover probes on the planet's surface.

Most recent observations have pointed to an early Martian history that featured considerable liquid water, even oceans, and a greenhouse effect similar to Earth's that helped trap heat, creating conditions needed for stable water to exist.

But the apparent absence of limestone on the surface has proved puzzling. Throughout the Earth's history, carbon dioxide has been a key element responsible for greenhouse warming, as it is released in volcanic eruptions, and then absorbed in seawater. Here it helps in the formation of calcium carbonate, or limestone, which shows up in ocean sediments.

Evidence of this hasn't shown up in the observations made by the Opportunity and Spirit rovers, however. Now researchers Maria Zuber and Itay Halevy of MIT, and Daniel Schrag of Harvard, say scientists may have been focusing on the wrong molecule.

"There is abundant evidence for a warmer climate, perhaps even a liquid water ocean, early in Martian history, between 3.5 and 4 billion years ago," says Schrag, the paper's senior author. "However, scientists have found it difficult to reconcile this evidence with our understanding of how the climate system is regulated on Earth."

Discovery of a mineral called jarosite, as well as other sulfur-rich materials, indicates that the early Martian greenhouse effect may have been driven by the emissions of sulfur dioxide, rather than primarily carbon dioxide, as on Earth, they say.

Like CO2, sulfur dioxide would have been absorbed easily by seawater. However, it would have prevented the creation of carbonate sediments, instead leading to the creation of silicate and sulfites, such as calcium sulfite. These minerals would have degraded fairly quickly, but would have led in turn to the formation of clays, which have indeed been found on the Martian surface today.

Because sulfur dioxide is a more powerful greenhouse gas than carbon dioxide, a relatively small amount in an atmosphere of mostly CO2 would go a long way towards explaining how the planet was able to maintain liquid water on its surface, the researchers say.

"The sulfur dioxide would essentially preempt the role of carbon dioxide in surface weathering reactions," says Halevy, the first author of the report. "The presence of even a small amount of sulfur dioxide in the atmosphere would contribute to the warmer climate, and also prevent limestone deposits from forming."

The researchers' analysis was published in the Dec. 21 issue of Science.

Sulfur Dioxide May Have Helped Maintain a Warm Early Mars[Harvard press release]

(Image: Gully channels, potentially the result of liquid water (but possibly indicating landslides of dry material), in the southern highlands of Mars. Credit: NASA/JPL/University of Arizona)