Google has put up Mars maps (to go along with their Earth and Moon maps). Pretty neat; they have an elevation view which goes from -9 to +21 km. This is 2.4 times the maximum elevation on Earth (Everest, at 8844 m). At some point when I acquired an interest in landscape simulations I learned that this is because of a phenomenon called "mass wasting", which basically refers to the shear stress on a surface as a result of the force of gravity acting on it. Since the gravity is comparatively greater on Earth than on Mars, this means that the same surface will experience greater shear stress on Earth; or, conversely, we would expect a comparable shear force for steeper surfaces on Mars (assuming the material is roughly the same). This means weaker erosion on Mars, and thus higher peaks. The equatorial surface gravity is about 2.65 times stronger on Earth than on Mars, so there's some rough agreement there. Although, as is obvious if you tool around on the map, there's clearly much less volcanic activity on Mars than there is on Earth.