THE GEOMORPHIC RECORD OF CLIMATE CHANGE ON EARLY MARS

One of the major contributions to planetary geology by Alan Howard is a clearer understanding of climate change on early Mars. Analyses of degraded impact craters, fluvial valley networks, overflowed basins, and sedimentary deposits suggest a complex erosional history. During the heavy bombardment of the Noachian Period, large impacts and slow crater degradation occurred simultaneously, such that stratigraphically older craters became more degraded than younger ones of the same size. Changes in the cross-sectional morphology of impact craters with time are more consistent with fluvial erosion than other processes. However, Noachian geomorphic processes collectively tended to smooth high-frequency features in the landscape, removing small craters and valleys rather than incising dense fluvial networks. Around the Noachian/Hesperian transition, conditions were more favorable for incision and preservation of fluvial valleys. This epoch of valley development likely reflects either an increase in stream power (more precipitation or less infiltration and evaporation of water) or a reduction in sediment supply (less weathering or more induration of surface materials), resulting in stream downcutting. Basin overflows around this time require a paleoclimate roughly comparable to that of the Pleistocene Great Basin in the western United States in terms of water balance. As landscape denudation rates declined during the Hesperian Period, these valley networks appear to have become inactive, and they experienced some wall retreat and infilling. During the later part of the Hesperian Period, one or more intervals of fluvial erosion formed smaller entrenched channels or valleys inside the larger, older valleys. This activity was lower in magnitude or duration than the earlier valley development. Calculations based on paleochannel dimensions suggest dominant discharges similar to the two-year flood in terrestrial humid to semiarid regions. A number of mostly Late Noachian to Hesperian impact craters developed large alluvial fans or deltas at this time. This late fluvial erosion appears to have declined abruptly relative to the Pleistocene to Holocene climate change in the Great Basin, as late dissection of deltas and fans was minor.