This group of images shows the evolution of the north polar cap on Mars between October 1996 and March 1997. The images are projected so that the north pole is at the center of each image, and 50°N latitude is at the edge of each image. The October 1996 image corresponds to the beginning of spring in the northern hemisphere. The polar cap is at its maximum extent, stretching to 60° latitude, a distance of about 1800 kilometers from the pole. The polar cap at this time is primarily carbon dioxide (CO2) frost, or “dry ice,” that has condensed out of the atmosphere and frozen to the ground due to the cold winter temperatures. The brown, comma-shaped feature near the 7 o'clock position is a dust cloud in the atmosphere.
The January 1997 image shows the situation in mid-spring. As the temperature rises, the CO2 frost sublimates back into the atmosphere, and the polar cap shrinks in size. The March 1997 image shows the view in early summer. By now, the CO2 frost has entirely returned to the atmosphere. The small remaining polar cap is water ice. The distribution of ice is very irregular at this season and is probably controlled by variations in the local topographic elevation and slope. The dark ring surrounding the summer polar cap is formed by a series of dune fields. These dunes are completely covered by the polar cap in the October 1996 image and are faintly visible in the January 1997 image.
Hubble Space Telescope image STScI-PR97-15b