Areal density of D > 20 km craters for Mercury, the Moon and Mars. For each target the three physical evolution models are plotted using the same colours as in Fig. 4. The upper cross is for the steeper size distribution of the small disk objects, and the lower cross is for the shallower one. The low mass disk is used in both cases. The lines at the common level of 100 craters per million km² mark the observational reference data.