Seasonal and inter-annual changes of volume density of martian CO2 snow from time-variable elevation and gravity

The martian atmosphere seasonally exchanges CO2 with the surface by repeating condensation and sublimation, causing seasonal growth and decay of the polar CO2 snowcaps. These processes leave two kinds of geodetic signatures, i.e. seasonal changes of the martian gravity field and of surface elevation of the snow-covered regions. Here we study gradual increase of the volume density of the martian snow due to compaction, by combining these two data sets during 1999-2001 covering three martian winters. We found that light fresh snow of ∼0.1×103kgm−3 slowly becomes denser reaching ∼1.0×103kgm−3 or more immediately before it thaws. The maximum snow density varies slightly from year to year, and between hemispheres. In the second southern winter, the density became as high as ∼1.6×103kgm−3. This might have been caused by a dust storm activity, e.g. increased mixing of silicate particles and/or enhancement of sintering.