It is striking that, at global scale, the topographies of both the Moon and Mars are dominated by clear dichotomies between old rough terrains (highlands on the Moon, concentrated on the far side, southern plateaus on Mars) and younger and smoother terrains (Mare on the Moon, northern plains on Mars). The first results of the orbital imaging of the northern hemisphere of Mercury by MESSENGER’s camera and laser altimeter also point to a similar dichotomy, between a large area of smoother volcanic plains at high Northern latitudes (Head et al., 2011; Zuber et al.,
2012) and much rougher terrains everywhere else. The aim of this study is to establish direct quantitative comparisons of the roughnesses of different terrains and different planets in the inner Solar System.