Mars: let it snow, let it snow....

Mars: Let it snow, let it snow...

The idea that snows might blanket the Martian poles first surfaced in the mid-1960s, when Earth-based studies showed Mars' seasonal polar caps consisted of frozen carbon dioxide (dry ice). Because those caps form in darkness, during the fall and winter, neither spacecraft nor Earth-based telescopes have observed whether that "ice" resulted from the condensation of frost or the falling of snow. However, new analyses of old spacecraft observations of Mars' poles now indicate snowstorms of "dry ice" are likely and probably very common, according to three California astronomers.

David A. Paige of the University of California, Los Angeles, and his colleagues reviewed infrared views of the poles recorded by the Mariner 9 Mars-orbited in 1971 and 1972 and by two Viking orbiters between 1976 and 1979. They show that clouds of carbon dioxide in the Martian sky "are very dense and extend from the surface to altitudes of over 30 kilometers," Paige reports. During the cold seasons, another 3 millimeters of frozen carbon dioxide accumulates on the polar surfaces daily, he says, building to a depth of more than 1 meter.

Paige's team interprets the Mariner 9 data as indicating that carbon dioxide particles condense at the cloud tops and rain out as snow. Relatively little surface frost even forms if the clouds are overhead, they say.

The seasonal variations in the thickness of these snowy polar deposits may result from changes in Mars' orbital motion around the sun, Paige's team says. The researchers suggest that core samples collected from the polar caps by future Marslanding craft may help further resolve the processes that affect climate change on Mars -- and on Earth.