Observations of the north polar region of Mars from the Mars Orbiter LaserAltimeter

Elevations from the Mars Orbiter Laser Altimeter (MOLA) have been used to c onstruct a precise topographic map of the martian north polar region. The n orthern ice cap has a maximum elevation of 3 kilometers above its surroundi ngs but Lies within a 5-kilometer-deep hemispheric depression that is conti guous with the area into which most outflow channels emptied, Polar cap top ography displays evidence of modification by ablation, flow, and wind and i s consistent with a primarily H2O composition. Correlation of topography wi th images suggests that the cap was more spatially extensive in the past. T he cap volume of 1.2 x 10(6) to 1.7 x 10(6) cubic kilometers is about half that of the Greenland ice cap. Clouds observed over the polar cap are Likel y composed of CO2 that condensed out of the atmosphere during northern hemi sphere winter. Many clouds exhibit dynamical structure Likely caused by the interaction of propagating wave fronts with surface topography.