Noachian Crater Modification on Mars: Evidence for Cold-Based Crater Wall Glaciation and Endorheic Basin Formation
Authors: Boatwright Benjamin , Head James 
Published: October 2020

. Introduction. Mars is currently characterized by a hypothermal, hyperarid climate that is thought to have persisted throughout the Amazonian [1]. The distribution of water ice on Mars has changed over time due to periodic variations in spin axis obliquity [2]. During periods of relatively higher obliquity than present, water ice is mobilized from the poles and deposited as snow and ice in the mid-latitudes, producing cold-based glacial landforms (Fig. 1A-B) [3-6].Geologic evidence suggests that the ambient climate in the Noachian was significantly different. Fluvial and lacustrine features have been interpreted to indicate the presence of a warm and wet climate [7-8]. However, global climate models predict Noachian temperatures well below those required to sustain liquid water at the surface [9-10]. These models also predict that the distribution of water ice will mostly be controlled by altitude rather than latitude. Supporting geomorphic evidence for this hypothesis has been elusive because glaciation in such an environment is predicted to be cold-based and, as in the Amazonian, melting is limited to top-down (supraglacial) sources [11]. We can search for characteristic cold-based glacial morphologies preserved in high-altitude terrains in order to better assess the character of potential cold-based glacial processes in the Noachian [12].2. Geology of Crater B. We report on the geology of a 54-km Noachian-aged crater in the southern highlands that displays evidence for cold-based glaciation. The crater is located in ~800 km northwest of the Hellas basin rim in Terra Sabaea. In a study of eroded crater floors in the same region, Irwin et al. [13] noted that the interior of this crater, which they designated &#8220;B,&#8221; contains a characteristic dark-over-light stratigraphy that is repeated in many other nearby craters. Inverted ridges are preserved in the more erodible dark unit and are likely fluvial in origin.On the basis of this previous work, we mapped the interior of this crater in detail using CTX and HiRISE visible images. These data reveal an ensemble of additional features (Fig. 1C, 2) not previously described in Noachian-aged craters that indicate a history of cold-based glaciation and top-down melting.A series of arcuate, upslope-facing scarps occurs near the base of the crater wall; each segment typically corresponds to a single upslope alcove. These scarps are positive relief features consistent with the deposition of moraines originating from cold-based glaciers flowing downslope from the alcoves. The heads of the inverted fluvial ridges closely follow the orientation of the scarps as they bend around the alcoves. A thin debris apron consistent with proglacial sedimentation occurs along the base of the crater wall, superposing the scarps and inverted ridge heads. We derive a lower limit crater age date of ~3.5 Ga, or Late Noachian&#8211;Early Hesperian (LN&#8211;EH), for the ensemble of cold-based glacial features in the crater based upon the superposition of the debris apron over the other units.The largest system of inverted fluvial ridges extends 45 km circumferentially around the northeastern crater floor and terminates near the modern topographic low point of the crater. A second, smaller system of inverted ridges originates from the southern crater wall and terminates at the margin of a smaller deposit that sits ~30 m higher than the first basin.3. Discussion and Conclusions. We interpret the alcove-scarp-ridge system (Fig. 1C) as the result of top-down melting of a cold-based crater wall glacier, leading to proglacial outwash in the form of fluvial channels that transported glacially derived sediment into multiple low-lying basins in the crater floor. The fine-grained channel effluent was likely removed by eolian erosion, which could also have modified the paleotopographic surface such that a slight difference exists between the current elevation of the two depositional basins. Outcrops of a lighter-toned indurated substrate are coincident with the lowest and most extensively deflated areas of the crater floor.The endorheic crater B basin, with completely internal drainage, lies in contrast to the similarly-aged open- and closed-basin lakes on Mars [14-15]. The lack of significant exterior drainage into crater B argues against an origin for the inverted ridges as the depositional remnants of fluvial valley networks derived from areally distributed rainfall and runoff. Likewise, groundwater sapping is unlikely to produce the dense, high stream-order ridges we observe.We hypothesize that a brief period of atmospheric warming caused top-down melting of cold-based crater wall glaciers during the transition from Noachian climate conditions to conditions typical of the later Hesperian and Amazonian. Recognition and documentation of these features provides specific criteria to search for other examples of past glaciation in the southern highlands in order to further test hypotheses of Mars climate evolution.References. [1] Carr M.H., Head J.W. (2010) EPSL 294; [2] Laskar J. et al. (2004) Icarus 170; [3] Berman D.C. et al. (2005) Icarus 178; [4] Jawin E.R. et al. (2018) Icarus 309; [5] Berman D.C. et al. (2009) Icarus 200; [6] Fassett C.I. et al. (2010) Icarus 208; [7] Craddock R.A., Howard A.D. (2002) JGR 107; [8] Ramirez R.M., Craddock R.A. (2018) Nature Geo. 11; [9] Forget F. et al. (2013) Icarus 222; [10] Wordsworth R. et al. (2013) Icarus 222; [11] Fastook J.L., Head J.W. (2015) PSS 106; [12] Marchant D.R., Head J.W. (2007) Icarus 192; [13] Irwin R.P. et al. (2018) JGR 123; [14] Fassett C.I., Head J.W. (2008) Icarus 198; [15] Goudge T.A. et al. (2015) Icarus 260.Fig. 1. (A) Amazonian cold-based glacial morphologies in an unnamed crater in Terra Sirenum. Moraine scarps mark former locations of glacial lobes. (B) Amazonian glaciofluvial valleys in the crater Greg have been inverted into ridges on the crater floor. (C) Degraded remnants of lobes and scarps are still preserved in crater B. The scarps are closely associated with the position of the debris apron and inverted ridge heads.Fig. 2. CTX mosaic of crater B and sketch map overlay showing the major geologic features within crater B. Inverted tributary ridges begin near the crater wall base in association with upslope-facing scarps and debris apron and terminate in two depositional basins.