[1] Valley networks provide compelling evidence that past geologic processes on Mars were different than those seen today. The generally accepted paradigm is that these features formed from groundwater circulation, which may have been driven by differential heating induced by magmatic intrusions, impact melt, or a higher primordial heat flux. Although such mechanisms may not require climatic conditions any different than today's, they fail to explain the large amount of recharge necessary for maintaining valley network systems, the spatial patterns of erosion, or how water became initially situated in the Martian regolith. In addition, there are no clear surface manifestations of any geothermal systems (e.g., mineral deposits or phreatic explosion craters). Finally, these models do not explain the style and amount of crater degradation. To the contrary, analyses of degraded crater morphometry indicate modification occurred from creep induced by rain splash combined with surface runoff and erosion; the former process appears to have continued late into Martian history. A critical analysis of the morphology and drainage density of valley networks based on Mars Global Surveyor data shows that these features are, in fact, entirely consistent with rainfall and surface runoff. The necessity for a cold, dry early Mars has been predicated on debatable astronomical and climatic arguments. A warm, wet early climate capable of supporting rainfall and surface runoff is the most plausible scenario for explaining the entire suite of geologic features in the Martian cratered highlands.
To explain the much higher denudation rates and valley network development on early Mars (>∼3.6 Gyr ago), most investigators have invoked either steady state warm/wet (Earthlike) or cold/dry (modern Mars) end‐member paleoclimates. Here we discuss evidence that highland gradation was prolonged, but generally slow and possibly ephemeral during the Noachian Period, and that the immature valley networks entrenched during a brief terminal epoch of more erosive fluvial activity in the late Noachian to early Hesperian. Observational support for this interpretation includes (1) late‐stage breaching of some enclosed basins that had previously been extensively modified, but only by internal erosion and deposition; (2) deposition of pristine deltas and fans during a late stage of contributing valley entrenchment; (3) a brief, erosive response to base level decline (which was imparted as fretted terrain developed by a suite of processes unrelated to surface runoff) in fluvial valleys that crosscut the highland‐lowland boundary scarp; and (4) width/contributing area relationships of interior channels within valley networks, which record significant late‐stage runoff production with no evidence of recovery to lower‐flow conditions. This erosion appears to have ended abruptly, as depositional landforms generally were not entrenched with declining base level in crater lakes. A possible planetwide synchronicity and common cause to the late‐stage fluvial activity are possible but remain uncertain. This increased activity of valley networks is offered as a possible explanation for diverse features of highland drainage basins, which were previously cited to support competing warm, wet and cold, dry paleoclimate scenarios.
Abstract. Degraded craters in the southern highlands are indicative of an early martian climate much different than the present. Using a photoclinometric model, analyses of degraded crater morphometry have revealed the stages of crater modification and, for the first time, allow a quantitative assessment of the amount of material eroded in the highlands. Central peaks of fresh craters are removed early by degradational processes. The sharp rims of fresh craters also become rounded while the interior slopes become shallower. Continued degradation causes the crater rim to lower, and infilling produces a broad, flat crater floor. Contrary to earlier observations, the degree of rim modification does not appear to be dependent on the presence of ancient valley networks. During 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 with prior observations. These models suggest that larger diameter (i.e., 50 km) craters were enlarged a greater amount than smaller diameter craters, which is opposite to what should be observed. To explain this discrepancy, a ~ 10 rn thick deposit, presumably aeolian in origin, must have been emplaced within the crater interiors following cessation of the degradational process. By the terminal stage of degradation, crater diameters appear to have been enlarged by 30%. In addition, a deposit ~60 rn average thickness must have been emplaced within these rimless craters to explain the discrepancy in crater enlargement. Because this deposit is contained only within the highly eroded, rimless craters, this material most likely originated from erosion of the surrounding terrain. The measured crater morphometry has allowed us to develop equations describing the amount of material eroded at any given stage of degradation. Applying these equations to craters within the Margaritifer Sinus and Sinus Sabaeus region indicates that an equivalent of ~200 rn 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 that erosion rates were on the order of-0.0003 to 0.0005 mm/yr. These erosion rates are equivalent to those determined for terrestrial periglacial environments. Two-dimensional simulations of some possible degradational processes suggest that fluvial erosion and deposition combined with diffusional creep come closest to producing equivalent degrees of modification through the range of crater diameters investigated in this study (20 to 50 km). However, these processes are inefficient at producing the amount of crater enlargement observed, suggesting that crater interior slopes may have also been undermined by sapping. These results imply that geologic processes related to precipitation dominated the early martian environment. Our working hypothesis is that this pre...
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