Revisiting Noachian-Hesperian Crater Degradation Processes and Potential Climate Effects

Introduction: What key processes modify “out of equilibrium” landforms (impact craters) on Mars and how do we model them quantitatively [1-8]? Amazonian-Late Hesperian craters display generally fresh and pristine morphologies. Noachian-Early Hesperian craters show fundamental morphological differences (e.g., general absence/subdued nature ejecta, elevated crater rim-crests being low or missing, shallower flat floors, missing central peaks, and often textured/grooved walls). These differences were interpreted to be due to relatively higher Noachian erosion rates attributed to landform degradation by rainfall (pluvial activity), in a warmer/wetter climate with a LN “climate optimum” resulting in fluvial erosion/VN [1-8]. Indeed, “Degraded craters are one of the main lines of evidence for a warmer climate on early Mars” [9]. Further analysis of 281 >20 km craters in two highland regions [9] confirmed earlier findings, revealing three classes: Type III: Fresh craters with ejecta/central peaks; Type II: Gently degraded with fluvial landforms/alluvial fans; Type I: Strongly degraded, without ejecta/central peak, with fluvial erosion. Type I were formed/degraded during the Noachian, Type II between EH-EA, and Type III formed subsequently. A sharp transition is seen between Types I/II, interpreted to indicate a rapid change in climate conditions [9].  New missions, discoveries, models and data analysis make it opportune to revisit/xplore Mars crater degradation and landscape evolution. Perspectives on Noachian Geologic Sequence and History: A synthesis of sequence/timing of conditions on early Mars [10] showed 1) distinctive separation of EN basin-forming period from MN-LN during which no basins formed, 2) LN-EH when valley networks (VN) formed [11], unrelated to basins [12], 3) lack of correlation between phyllosilicates/VN formation.Role and Legacy of Impact Basin Formation:  Recent studies of impact basin effects on climate and EN surface modification show that the threshold diameter for radical atmosphere effects is in the basin-size range [13-14]; collective effects of basin-scale atmospheric/surface effects (ICASE) are: 1) globally distributed very high temperature rainfall; 2) extremely high (~2m/yr) rainfall/runoff rates; 3) significant degradation of crater rims, filling of interiors, regional smoothing; 4) significant influence on mineralogical alteration of the crust [14].  These major events impart a global legacy into the surface nature/morphology. Models of Noachian Climate: Atmospheric general circulation models [15-16] suggest ~225K mean annual temperature (MAT), a distinctive alternative to the generally warm/wet/arid pluvial climate [1-8] implied by earlier models [1-8]; an adiabatic cooling effect predicts a “cold and icy highlands” [16] with snow/ice accumulating above  ~+1km.  VN, open/closed basin-lakes are attributed to transient heating/melting of snow and ice in the “icy highlands” [17-18]. The influence of substrate snow/ice on cratering and degradation [19-20] includes: 1) Amazonian-like double-layered ejecta/pedestal craters; 2) shallower underlying target-rock cavities in the, lower post-ice rims; 3) modification by rim-crest backwasting, ice melting and fluvial erosion. Removal of surface snow/ice could eliminate smaller craters, drastically modifying size-frequency distributions.GCMs of a “warm/wet” climate (MAT ~275K)[21]: rainfall is limited in abundance/areal distribution, precipitation is snowfall-dominated, and highlands are