That's not cloud cover. It's polar ice on Mars, about 600 miles across and covered with deep etchings. The dark valley on the right, named Chasma Boreale, is about the size of the Grand Canyon. This riven Martian arctic was a mystery to scientists for over forty years. But data from NASA's Mars Reconnaissance Orbiter has given researchers some important clues to how the ice spirals formed. Their findings appear in twopapers published in the journal Nature.
Data from Mars now points to both the canyon and spiral troughs being created and shaped primarily by wind. Rather than being cut into existing ice very recently, the features formed over millions of years as the ice sheet grew. By influencing wind patterns, the shape of underlying, older ice controlled where and how the features grew. [NASA]
This image was made using Mars Orbiter Laser Altimeter data. A shaded-relief image, it shows clearly the pole's gorges. Chasma Boreale is a mile deep in some places. Related content: Bad Astronomy: A marvelous night for a Moon (and Mars) dance Bad Astonomy: Sand dunes march across Mars Bad Astronomy: Mars craters reveal ice 80beats: Mars Rover Sets Endurance Record: Photos From Opportunity’s 6 Years On-PlanetImage1: NASA/Caltech/JPL/E. DeJong/J. Craig/M. StetsonImage2: NASA/GSFC