Animated gif image from the Surface Stereo Imager onboard NASA's Phoenix Mars Lander showing clouds drifting across the horizon in the early morning on the 119th sol, or Martian day, since landing (September 25, 2008). Clouds were observed each night after Sol 80 (August 15, 2008) as the atmospheric temperature decreased. Image Credit: NASA/JPL-Caltech/University Arizona/Texas A&M University.

Source: http://www.nasa.gov/mission_pages/phoenix/images/press/16405.html