Astronomy Picture of the Day

Explanation: 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 kilometres 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.