a b s t r a c tThe deflated surfaces of the alluvial fans in Saheki crater reveal the most detailed record of fan stratigraphy and evolution found, to date, on Mars. During deposition of at least the uppermost 100 m of fan deposits, discharges from the source basin consisted of channelized flows transporting sediment (which we infer to be primarily sand-and gravel-sized) as bedload coupled with extensive overbank mud-rich flows depositing planar beds of sand-sized or finer sediment. Flow events are inferred to have been of modest magnitude (probably less than $60 m 3 /s), of short duration, and probably occupied only a few distributaries during any individual flow event. Occasional channel avulsions resulted in the distribution of sediment across the entire fan. A comparison with fine-grained alluvial fans in Chile's Atacama Desert provides insights into the processes responsible for constructing the Saheki crater fans: sediment is deposited by channelized flows (transporting sand through boulder-sized material) and overbank mudflows (sand size and finer) and wind erosion leaves channels expressed in inverted topographic relief. The most likely source of water was snowmelt released after annual or epochal accumulation of snow in the headwater source basin on the interior crater rim during the Hesperian to Amazonian periods. We infer the Saheki fans to have been constructed by many hundreds of separate flow events, and accumulation of the necessary snow and release of meltwater may have required favorable orbital configurations or transient global warming.

[1] A hydrologic routing model has been applied to the Noachian cratered highlands of Mars to establish the climatic conditions required to maintain exit breached lakes on early Mars and the likely fraction of the upland surface that would have hosted lakes whether they overflowed or not. The climatic conditions were expressed as a ratio of net evaporative loss from lakes to the surface runoff from uplands (the "X ratio"). Simulations were conducted using 16 different X ratios. The lake area, volume, and number of overflowing lakes decrease as climate becomes drier (larger X ratio). The modal frequency of the X ratio for the overflow of highland basins with eroded exit breaches was 5.0, which is comparable to that of the Great Basin region in the western United States during the Last Glacial Maximum (LGM). This indicates that lakes on early Mars were likely to have been at least as extensive as those in the Great Basin region during the LGM.

[1] Widespread occurrences of valley networks on Mars provide geomorphic evidence for an active hydrologic cycle. To constrain the climatic conditions capable of forming the valley networks, a hydrological model was used to analyze the valley incision depth and volume of eroded valleys. Because the absolute magnitudes of precipitation, runoff, and evaporation are uncertain, we have used the ratio of these quantities (the X-ratio) to express climatic conditions. The spatial distribution and strength of the correlations between (1) the estimated depth and volume of eroded material and (2) estimated flood magnitude and valley gradients were investigated as a function of the assumed X-ratio. We also conducted an analysis of conditions required to have appreciable discharge in selected valley networks, which provided the most definitive constraint on Martian paleoclimate. The other methods show a relatively weak dependency of incision depths and volumes upon the assumed X-ratio. The multiple regression analyses indicate that incision depth is strongly influenced by gradient and weakly related to modeled flood discharge. The factors determining relative depth of incision depend partly on the type of channel bed. However, postflow modification of the valley networks by mass wasting, cratering, aeolian infilling, and ice-related processes precludes direct determination of bed morphology. Our hydrological analyses suggest that climatic conditions on early Mars were at least as moist as those that occurred in the Great Basin region of the U.S. when large lakes were present during the Pleistocene in terms of the balance of runoff and lake evaporation.

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