Seasonal variations of snow depth on Mars

Using topography collected over one martian year from the Mars Orbiter Lase r Altimeter on the Mars Global Surveyor (MGS) spacecraft, we have measured temporal changes in the elevation of the martian surface that correlate wit h the seasonal cycle of carbon dioxide exchange between the surface and atm osphere. The greatest elevation change (1.5 to 2 meters) occurs at high lat itudes ( above 80 degrees), whereas the bulk of the mass exchange occurs at tower latitudes (below 75 degrees N and below 73 degrees S). An unexpected period of sublimation was observed during northern hemisphere autumn, coin cident with dust storms in the southern hemisphere. Analysis of MGS Doppler tracking residuals revealed temporal variations in the flattening of Mars that correlate with elevation changes. The combined changes in gravity and elevation constrain the average density of seasonally deposited carbon diox ide to be 910 +/- 230 kilograms per cubic meter, which is considerably dens er than terrestrial snow.