Gravitational equipotential surfaces (the geoid) on the Earth are independent of continent positions. However, highs are correlated with the Tharsis region and lows with Chryse and Amazonis basins on Mars. Therefore, the topography must be compensated by a mass deficit in the crust or mantle. The observed gravity for Tharsis is much smaller than that predicted from topography alone, however, so the crust must be strong or the compensation deep.
Take a crustal block of density extending a height d below the surface and a height h above it. Archimedes' principle then states that the buoyancy force is given by
where A is the cross sectional area and g is the gravitational acceleration. This must balance the weight, so
