The Mars Orbiter Laser Altimiter (MOLA) instrument flies aboard the Mars Global Surveyor (MGS) spacecraft, which just finished aerobraking and is currently in a circular orbit around Mars. Surface elevations obtained from MOLA have been used to construct both a precise topographic map of Mars' north polar region (above) and the first 3-D image of the north polar cap. Measuring the topography provides information about the cap's composition, the abundances of water and carbon dioxide, and the interactions between the surface and atmosphere that are key to understanding past and present hydrologic cycles. The northern ice cap has a maximum elevation of 3 km (1.9 mi) but lies within a depression 5 km (3 mi) deep; many outflow channels appear to have emptied into this large depression. Comparing the topography with images suggests that the cap covered a much larger area in the past. Presently, the volume is about half of Greenland's ice cap. The topographic features show the polar cap has been modified by erosional processes (ablation, flow, and wind) and imply that the cap is composed primarily of water ice. Clouds have been observed over the polar cap, and appear to be composed of carbon dioxide that condensed out of the atmosphere during the northern winter.

LINKS:
Full text of the Science publication
NASA press release
The Mars Orbiter Laser Altimiter (MOLA) homepage