A large reservoir of carbon dioxide trapped in the soil of Mars may account for apparent drastic changes in the red planet's climate over billions of years, two scientists at Caltech's Jet Propulsion Laboratory told their colleagues today.
Dr. Fraser P. Fanale and W. A. Cannon told meeting of the American Geophysical Union in Washington, D.C., that exchange of carbon dioxide between the Martian regolith -- layer of soil up to kilometer (0.62 miles) thick -- and the Martian atmosphere may be the key to climate changes on Mars -- from warm and wet in the past to the planet's present ice age.
"Our results suggest," Fanale said, "that the regolith acts as sort of 'sponge' for large amounts of carbon dioxide that are expelled into the atmosphere during warm epochs and return to the regolith during cold epochs."
A variety of evidence points to denser atmosphere in Mars' past -- primarily deeply carved water channels on the surface. But water cannot flow in large amounts for any length of time on Mars under current conditions; the atmosphere is too thin and temperatures are too cold to allow water to remain on the surface as liquid.
In the past, scientists answered the problem by proposing that the residual polar caps on Mars -- those parts that remain frozen through the summer months -- were huge reservoirs of frozen carbon dioxide. When small changes in the Martian orbit, or changes in the sun's output, heated the planet, so the theory went, these polar caps thawed and the carbon dioxide went into the atmosphere. Most investigators agree that, if enough CO2 were supplied to the atmosphere, transition to "warm-wet" climate would be likely.
However, results from Mariner 9 and from Viking showed that the residual (or permanent) polar caps are not frozen carbon dioxide, but are frozen water. That would not permit the atmosphere to build to the density required to allow large quantities of liquid water on the surface, Fanale and Cannon agree.
They suggest in their AGU paper that the thick soil layer or regolith could provide enough presently hidden but exchangeable CO2 to "pump up" the atmosphere in response to changes in the amount of solar energy reaching the surface.
Their studies of C02 adsorption by pulverized rocks suggest that the soil on Mars can absorb very large quantities of carhon dioxide in much the same way as charcoal adsorbs odorous gases in refrigerator.
Further, they say, the carbon dioxide thus held in the Martian soil is exchangable with the atmosphere: once the regolith warms up, the carbon dioxide can be released into the atmosphere. When the regolith cools again the carbon dioxide is adsorbed again.
Although carbon dioxide may exist in the form of carbonate rocks in the regolith, Fanale and Cannon say, it is not usable in the exchange process, since it is chemically bound in the rocks; simple warming of the regolith will not free it.
The Fanale-Cannon exchange process needs between 100,000 and 1 million years to release all the CO2, since it probably takes that long to warm the regolith to the suggested depth of 1 kilometer.
In this model, they say, it appears that all essential ingredients for development of definitive Mars climate-change model are available. The validity of climate changes on Mars need not be doubted, Fanale and Cannon say, just because no apparent large C02 reservoir is identifiable at the polar caps.
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