CRATER MORPHOMETRY AND MODIFICATION IN THE SINUS SABAEUS AND MARGARITIFER SINUS REGIONS OF MARS

Degraded craters in the southern highlands are indicative of an early martian climate much different than the present. Using a photoclinomet ric model, analyses of degraded crater morphometry have revealed the s tages of crater modification and, for the first time, allow a quantita tive assessment of the amount of material eroded in the highlands. Cen tral peaks of fresh craters are removed early by degradational process es. The sharp rims of fresh craters also become rounded while the inte rior slopes become shallower. Continued degradation causes the crater rim to lower, and infilling produces a broad; flat crater floor. Contr ary to earlier observations, the degree of rim modification does not a ppear to be dependent on the presence of ancient valley networks. Duri ng degradation, the diameter of the impact craters also increases due to backwasting. A simple algebraic model balancing the measured amount of infilling with that eroded from the interior slopes suggests that the crater diameters were enlarged by 7 to 10% initially, agreeing wit h prior observations. These models suggest that larger diameter (i.e., 50 km) craters were enlarged a greater amount than smaller diameter c raters, which is opposite to what should be observed. To explain this discrepancy, a similar to 10 m thick deposit, presumably aeolian in or igin, must have been emplaced within the crater interiors following ce ssation of the degradational process. By the terminal stage of degrada tion, crater diameters appear to have been enlarged by 30%. In additio n, a deposit similar to 60 m average thickness must have been emplaced within these rimless craters to explain the discrepancy in crater enl argement. Because this deposit is contained only within the highly ero ded, rimless craters, this material most likely originated from erosio n of the surrounding terrain. The measured crater morphometry has allo wed us to develop equations describing the amount of material eroded a t any given stage of degradation. Applying these equations to craters within the Margaritifer Sinus and Sinus Sabaeus region indicates that an equivalent of similar to 200 m of highland material was eroded and redistributed within the study area. Depending upon model chronology, degradation operated for either 400 or 600 million years, suggesting t hat erosion rates were on the order of similar to 0.0003 to 0.0005 mm/ yr. These erosion rates are equivalent to those determined for terrest rial periglacial environments. Two-dimensional simulations of some pos sible degradational processes suggest that fluvial erosion and deposit ion combined with diffusional creep come closest to producing equivale nt degrees of modification through the range of crater diameters inves tigated in this study (20 to 50 km). However, these processes are inef ficient at producing the amount of crater enlargement observed, sugges ting that crater interior slopes may have also been undermined by sapp ing. These results imply that geologic processes related to precipitat ion dominated the early martian environment. Our working hypothesis is that this precipitation was due to the presence of a primordial atmos phere which condensed and collapsed (i.e., precipitated) into the mart ian regolith; a process which ceased during the late Hesperian/early A mazonian (3.5 to 1.8 Ga).