A Polar Martian Dust Storm.

On August 29, a large dust storm was photographed erupting out from the north polar cap of Mars . Such dust storms are not uncommon as summer advances in the north. In the above picture taken by the Mars Global Surveyor spacecraft currently orbiting Mars , the white material is frozen carbon dioxide that covers much of the extreme north. As the north polar cap region begins to thaw, a temperature difference occurs between the cold frost region and recently thawed surface, resulting in swirling winds between the adjacent regions . Visible in the storm is a strong central jet about 900 kilometers long that is creating symmetric swirling vortices. Although winds can reach 100 km/hour, the thin atmosphere of Mars usually makes such storms less destructive than similar storms on Earth .