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 ∼0.1×103 kgm−3 slowly becomes denser reaching ∼1.0×103 kgm−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 ∼1.6×103 kgm−3. This might have been caused by a dust storm activity, e.g. increased mixing of silicate particles and/or enhancement of sintering.