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 <273K for most of the year. Thus: 1) VN/lakes should not form through rainfall-related erosion, 2) rainsplash/runoff crater degradation is not predicted, and 3) a northern ocean is improbable. 

New Observational Data:  Global crater-wall steepest-slope distribution was used to assess magnitudes of degradational processes with latitude/altitude/time [22]: total LN crater-wall degradation is very small, interpreted to mean that LN climate was not characterized by persistent/continuous warm/wet conditions. MRO-CTX [23] reveals evidence for crater-wall cold-based glaciation, top-down glacial melting, fluvial crater floor meltwater drainage/endorheic crater-floor lake.

Outstanding Questions: A full understanding of Noachian crater degradation clearly requires addressing the following questions: 1) What is the magnitude of the role of the impact flux and its effect on crater degradation and diffusional processes, and how does this change with atmospheric pressure?  2) In a warm and wet/arid climate, what was the intensity of the rainfall required for infiltration and what is the rate transition to runoff? How does this vary with atmospheric pressure and substrate?  3) What causes the abrupt change from highly degraded craters to much less degraded craters at the end of the Noachian? 4) What role do EN basin-related torrential rainfall processes have [24] on setting the stage for LN crater formation and degradation? 5) What role do explosive [25] and effusive [26] volcanism play in the resurface of craters and filling of crater floors?  6) How widespread is the evidence for Noachian glaciation [23] and what are the implications for crater modification and degradation state?  7) How do eolian processes vary with atmospheric pressure and how does this influence crater degradation with time?  8) Can the observed fluvial activity and open and closed-basin lake degradation and filling be explained by transient heating phenomena in an otherwise cold and icy climate?

References: 1. Craddock&Maxwell, 1990, JGR95, 14625; 2. Ibid, 1993, JGR98, 3452; 3. Craddock et al., 1997, JGR102, 13321; 4. Craddock&Howard, 2002, JGR107, 5111; 5. Forsberg-Taylor et al., 2004, JGR109, E05002; 6. Howard et al., 2005, JGR 110, E12S14; 7. Irwin et al. 2005, JGR110, E12S15; 8.  Howard, 2007, Geomorphology91, 322; 9. Mangold et al., 2012. JGR117, E04003; 10. Fassett&Head, 2011, Icarus211, 1204; 11. Ibid., 2008, Icarus195, 61; 12. Toon et al., 2010, Ann Rev38, 303; 13. Turbet et al., 2019, Icarus335, 113419; 14. Palumbo&Head, 2018, MAPS53, 687; 15. Forget et al., 2013, Icarus222, 81; 16. Wordsworth et al., 2015, Icarus222, 1; 17. Head&Marchant, 2014, Antarctic Science26, 774; Fastook&Head, 2015, Icarus106, 82; 18. Palumbo et al., Icarus300, 261; 19. Weiss&Head, 2015, PSS117, 401; 20. Ibid., 2016, Icarus280, 205; 21. Palumbo&Head, 2018, GRL45, 10249; 22. Kreslavsky&Head, 2018, GRL45, 751; 23. Boatwright&Head, 2021, PSJ2, 1; 24. Palumbo&Head, this volume; 25. Kerber et al., 2013, Icarus 223, 149; 26. Whitten&Head, 2013, PSS 85, 24.