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Thick Clouds Cast Shadows Over Mars

Irene Klotz, Discovery News

Photos

Cloudy Skies

Jan. 24, 2008 -- The last thing researchers expected to find in the red Martian skies are big, thick clouds with enough substance to cast shadows on the ground below.

Yet data collected by an instrument aboard Europe's orbiting Mars Express satellite paints just such a scene.

With an atmosphere dominated by carbon dioxide, the clouds are most likely dry ice -- frozen carbon dioxide -- that form primarily around the planet's equatorial belt due to extreme shifts in temperatures over relatively short periods of time.

For example, temperatures during warm summertime afternoons on the Martian equator can reach 65 degrees Fahrenheit, but they plunge to freezing at sunset. During the night, the temperature at the same spot could reach 100 degrees below zero.

As temperatures climb again in the morning, bubbles of warming gas rise above the surface, said French researcher Franck Montmessin with Service d'Aeronomie at the University of Versailles and the lead author of a paper about the Martian clouds in last week's Journal of Geophysical Research.

"There is potential for large-scale convection, particularly as the morning sun warms the ground," Montmessin said.

As the bubbles rise, they condense and freeze, releasing heat that boosts the gas and ice particles even higher. Eventually the clouds grow to sufficient mass to block up to 40 percent of the sun's brightness, the researchers said.

"This means that they cast quite a dense shadow and this has a noticeable effect on the local ground temperature," Montmessin said.