SIMULATIONS OF CRATERING AND EROSION ON THE MARTIAN HIGHLANDS
The simulation domain is 256x256 scaled to .4 km/cell, so that the simulated area is about 102x102 km. Both the vertical and lateral boundaries are periodic, so that the left side is (computationally) contiguous with the right side, and the top with the bottom. This is done to avoid needing to specify external boundary conditions. Simulations start from a simulated saturated cratered surface with craters from 1 to 50 km in size. Simulated processes include weathering, creep, threshold mass wasting, fluvial detachment, sediment transport and fluvial deposition. Some simulations include groundwater sapping.
In this simulation the initial weathered regolith is initially 1 m thick, and when mass wasting or fluvial erosion exposes bedrock, the erodibility drops by a factor of 10. There is no critical shear stress for fluvial erosion, and creep rates are low enough to be ineffectual. However, mass wasting increases without limit as regolith slopes approach a gradient of 0.8 and when bedrock slopes approach a gradient of 2.7. These simulations employ a "desert" hydrology such that no ponded water is allowed (water "evaporates" when it reaches enclosed depressions). In the first few simulations fluvial runoff is "Hortonian" overland flow due to excess of ranfall rate over infiltration capacity and runoff yield is assumed to be spatially uniform.
In this simulation the central area is higher than the top and bottom boundaries. The north and south borders become alluviated by fans. Fans also established in the interiors of larger craters. Notice that deep, transient valleys develop when an upland basin becomes breached and dissected. Erosion and valley development erode the exterior rim of large craters until they often become nearly as steep as the interior crater wall. Note that the alluviated floors of individual craters or groups of craters on upland areas can become high plateaus that are primarily eroded around their edges -- often the original crater rims have been removed, so that the upland flats seem "inexplicable".