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W4206747154 abstract "Introduction: What key processes modify &#8220;out of equilibrium&#8221; 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 &#8220;climate optimum&#8221; resulting in fluvial erosion/VN [1-8]. Indeed, &#8220;Degraded craters are one of the main lines of evidence for a warmer climate on early Mars&#8221; [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].&#160; New missions, discoveries, models and data analysis make it opportune to revisit/xplore Mars crater degradation and landscape evolution.&#160; 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: &#160;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]. &#160;These major events impart a global legacy into the surface nature/morphology.&#160; 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 &#8220;cold and icy highlands&#8221; [16] with snow/ice accumulating above &#160;~+1km.&#160; VN, open/closed basin-lakes are attributed to transient heating/melting of snow and ice in the &#8220;icy highlands&#8221; [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 &#8220;warm/wet&#8221; 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.&#160; New Observational Data: &#160;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?&#160; 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?&#160; 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?&#160; 6) How widespread is the evidence for Noachian glaciation [23] and what are the implications for crater modification and degradation state?&#160; 7) How do eolian processes vary with atmospheric pressure and how does this influence crater degradation with time?&#160; 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.&#160; 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." @default.
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W4206747154 date "2021-07-21" @default.
W4206747154 modified "2023-10-18" @default.
W4206747154 title "Revisiting Noachian-Hesperian Crater Degradation Processes and Potential Climate Effects " @default.
W4206747154 doi "https://doi.org/10.5194/epsc2021-426" @default.
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