Dry ice, wetter Mars

Newfound cache of frozen carbon dioxide could periodically thicken Red Planet’s atmosphere

A newfound reservoir of dry ice on Mars suggests that the planet’s surface has been wetter in the relatively recent past, though not necessarily warmer than it is today.

CAPPED The thickness of a newfound reservoir of frozen carbon dioxide at Mars’ south polar cap varies from a few meters (blue) to more than 500 meters (red). JPL-Caltech, U. of Rome, Southwest Research Institute

The new study adds to evidence that Mars once had a carbon dioxide atmosphere thick enough to keep liquid water on the surface from evaporating. It’s unclear whether the planet would have been hospitable for life, however, because temperatures on Mars may actually have been slightly colder during times when the atmosphere had a greater amount of carbon dioxide.

Roger Phillips of the Southwest Research Institute in Boulder, Colo., and his colleagues base their findings on radar studies by the Mars Reconnaissance Orbiter of the layered deposits at Mars’ south polar cap. Earlier studies had indicated that a veneer of frozen carbon dioxide sits atop part of the cap with a thin layer of water ice beneath it. But a detailed analysis of radar reflected from different layers of the cap reveals that beneath the frozen water lies a volume of carbon dioxide ice 30 times greater than previously estimated, the team reports online April 21 in Science.

This unexpected reservoir of dry ice is intriguing, Phillips says, because about every 100,000 years Mars is known to dramatically tilt its spin axis. During these periods of high polar tilt, enough sunlight falls on the poles to vaporize the frozen carbon dioxide and release it into the atmosphere, roughly doubling the atmospheric pressure on the Red Planet. With a denser atmosphere, liquid water could persist on the surface rather than evaporating, and might account for some of the features on Mars that appear to have carved by water, such as channels and gullies, Phillips notes.

Although the newly found reservoir could nearly double the mass of carbon dioxide in Mars’s atmosphere, the resulting climate alterations would be “modest” and would not generate a warmer, wetter Mars, notes Peter Thomas of Cornell University in a commentary also posted online April 21 in Science.

Phillips concurs and notes that during times of higher tilt, more carbon dioxide frost would settle on the planet’s surface. The reflectivity of the surface frost, along with other effects, would offset any greenhouse warming from the extra gas in the atmosphere, and would tend to maintain the chilly temperatures now typical on the Red Planet.

Warmer conditions would require a much thicker carbon dioxide atmosphere supplied by an additional source of the compound, such as carbonates in Martian rocks, says Thomas. The abundance of carbonates in the rocks is still under exploration.

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