Picture of the Week—The Swirls of Mars

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The atmosphere on Mars is very different from Earth's. It is composed primarily of carbon dioxide, which condenses into dry ice at the poles during winter. And it's thin, with only one percent of the pressure of the Earth's atmosphere. But it sure is pretty at times, especially as seen through the lens of the HiRISE camera on the Mars Reconnaissance Orbiter.

Bad Astronomy's Phil Plait chose this image of the Mars atmosphere as his second best for the year (enjoying only a Lunar Reconnaissance Orbiter image of the Apollo 11 landing site more) and described it as follows:

The image above shows a region of Mars near its mid-lower northern latitudes. It’s a close-up of the bed of a crater, and you can see the ripples of sand dunes, endemic on the Martian surface. The sand is similar to beach sand here on Earth, but is dark in color because it’s made of basalt, a greyish rock. Then why is Mars so red? It’s because of much finer-grain dust, which is reddish in hue. The dust lies on top of the sand, making everything look red.
But then there’s that thin Martian air. Rising heat from the plains can blow through cooler air above, forming vortices like mini-tornadoes called dust devils. These then roll across the surface, picking up the lighter red dust but leaving behind the heavier, darker sand grains. What remains, as seen from above, are these gorgeous swirls, the fingerprints of the geology and weather of Mars.

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(Image: NASA/JPL/University of Arizona)

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