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Growth and Form of the Mound in Gale Crater, Mars: Slope

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Growth and Form of the Mound in Gale Crater, Mars: Slope 1 Growth and form of the mound in Gale Crater, Mars: Slope- 2 wind enhanced erosion and transport 3 Edwin S. Kite1, Kevin W. Lewis,2 Michael P. Lamb1 4 1Geological & Planetary Science, California Institute of Technology, MC 150-21, Pasadena CA 5 91125, USA. 2Department of Geosciences, Princeton University, Guyot Hall, Princeton NJ 6 08544, USA. 7 8 Abstract: Ancient sediments provide archives of climate and habitability on Mars (1,2). Gale 9 Crater, the landing site for the Mars Science Laboratory (MSL), hosts a 5 km high sedimentary 10 mound (3-5). Hypotheses for mound formation include evaporitic, lacustrine, fluviodeltaic, and 11 aeolian processes (1-8), but the origin and original extent of Gale’s mound is unknown. Here we 12 show new measurements of sedimentary strata within the mound that indicate ~3° outward dips 13 oriented radially away from the mound center, inconsistent with the first three hypotheses. 14 Moreover, although mounds are widely considered to be erosional remnants of a once crater- 15 filling unit (2,8-9), we find that the Gale mound’s current form is close to its maximal extent. 16 Instead we propose that the mound’s structure, stratigraphy, and current shape can be explained 17 by growth in place near the center of the crater mediated by wind-topography feedbacks. Our 18 model shows how sediment can initially accrete near the crater center far from crater-wall 19 katabatic winds, until the increasing relief of the resulting mound generates mound-flank slope- 20 winds strong enough to erode the mound. Our results indicate mound formation by airfall- 21 dominated deposition with a limited role for lacustrine and fluvial activity, and potentially 22 limited organic carbon preservation. Morphodynamic feedbacks between wind and topography 1 23 are widely applicable to a range of sedimentary mounds and ice mounds across the Martian 24 surface (9-15), and possibly other planets. 25 26 Main text: Most of Mars’ known sedimentary rocks are in the form of intra-crater or canyon 27 mounded deposits like the Gale mound (2,9), but identifying the physical mechanism(s) that 28 explain mound growth and form has proved challenging, in part because these deposits have no 29 clear analog on Earth. The current prevailing view on the formation of intra-crater mounds is that 30 sedimentary layers (i.e., beds) completely filled each crater at least to the summit of the present- 31 day mound (2). Subsequent aeolian erosion, decoupled from the deposition of the layers, is 32 invoked to explain the present-day topography (8,9). If the sedimentary rocks formed as 33 subhorizontal layers in an evaporitic playa-like setting, then >>106 km3 must have been removed 34 to produce the modern moats and mounds (8,9). These scenarios predict near-horizontal or 35 slightly radially-inward dipping layers controlled by surface or ground water levels. To test this, 36 we made 81 bed-orientation measurements from six one-meter-scale stereo elevation models, 37 finding that layers have shallow but significant dips away from the mound center (Figures 1,S1), 38 implying 3-4 km of pre-erosional stratigraphic relief. In addition, measurements of a dark-toned, 39 erosionally resistant marker bed that varies in elevation by >1 km indicates beds are not planar. 40 Postdepositional tilting with this pattern is unlikely (Supplementary Text), and these 41 measurements permit only a minor role (6) for deposition mechanisms that preferentially fill 42 topographic lows (e.g., playa, fluviodeltaic or lacustrine sedimentation), but are entirely 43 consistent with aeolian processes (Figure S2). This suggests the mound grew with its modern 44 elliptical shape, and that the processes sculpting the modern mound also molded the growing 45 mound. 2 46 Mars is a windy place: saltating sand-sized particles are in active motion on Mars, at rates 47 that predict aeolian erosion of bedrock at 10-50 µm/yr (16). Aeolian erosion of rock has 48 occurred within the last ~1-10 Ka (17) and is probably ongoing. The inability of General 49 Circulation Models (GCMs) to reproduce these observations shows that small-scale winds, not 50 the regional-to-global winds resolved by GCMs, are responsible for saltation. Because of Mars’ 51 thin atmosphere, slope winds dominate the circulation in craters and canyons (18). Downslope- 52 oriented yardangs, crater statistics, exposed layers, and lag deposits show that sedimentary 53 mounds in Gale and Valles Marineris are being actively eroded by slope winds (9). Slope- 54 enhanced winds also define both the large-scale and small-scale topography and stratigraphy of 55 the polar layered deposits (13,14), and radar sounding of intracrater ice mounds near the north 56 polar ice sheet proves that these grew from a central core, suggesting a role for slope winds (15). 57 Most of the ancient stratigraphy explored by the Opportunity rover is aeolian (19), and aeolian 58 deposits likely represent a volumetrically significant component of the sedimentary rock record, 59 including within the strata of the Gale mound (4). Evidence for fluvial reworking within 60 sedimentary mounds is comparatively limited and/or localized (4-6). Quasi-periodic bedding at 61 many locations including the upper portion of Gale’s mound implies slow (~30 µm/yr) orbitally- 62 paced accumulation (10). These rates are comparable to the modern gross atmospherically- 63 transported sediment deposition rate (101-2 µm/yr ; 20), suggesting that aeolian processes may be 64 responsible for the layers. Sedimentary strata within Valles Marineris are meters-to-decameters 65 thick, laterally continuous, have horizontal-to-draping layer orientations, and display very few 66 angular unconformities (11). These data suggest that sedimentary deposits formed by the 67 accretion of atmospherically-transported sediment (ash, dust, impact ejecta, ice nuclei, or 68 rapidly-saltating sand) were common on both modern and early Mars (1,9,21). 3 69 Slope-wind erosion of indurated or lithified aeolian deposits cannot explain layer 70 orientations at Gale unless the topographic depression surrounding the mound (i.e., the moat) 71 seen in Figure 1 was present during mound growth. This implies a coupling between mound 72 primary layer orientations, slope winds, and mound relief. 73 To explore this feedback, we aimed to develop the simplest possible model that can 74 account for the structure and stratigraphy of Mars' equatorial sedimentary rock mounds (see 75 Supplementary Information for details). In one horizontal dimension (x), topographic change 76 dz/dt is given by 77 78 dz/dt = D – E (1) 79 80 where D is an atmospheric source term and E(x,t) is erosion rate. Initial model topography 81 (Figure 2) is a basalt (nonerodible) crater/canyon with a flat floor of half-width R and long, 20° 82 slopes. To highlight the role of slope winds, we initially assume D is constant and uniform (for 83 example, uniform airfall sediment concentration multiplied by a uniform settling velocity, with 84 little remobilization; 7,12-13). E typically has a power-law dependence on maximum shear 85 velocity magnitude at the air-sediment interface, U: 86 E = k U α (2) 87 where k is an erodibility factor that depends on substrate grainsize and 88 induration/cementation (22,23), and α ~ 3-4 for sand transport, soil erosion, and rock abrasion 89 (24). We assume eroded material does not pile up in the moat but is instead removed from the 90 crater, for example through breakdown to easily-mobilized dust-sized particles (25). Shear 91 velocity magnitude is given by 4 92 (3) 93 which is the sum of a background bed shear velocity Uo and the component of shear velocity due 94 to slope winds. The max|±()| operator returns the maximum of downslope (nighttime) or upslope 95 (daytime) winds, z' is local topography, x and x' are distances from the crater center, and L is a 96 slope-wind correlation length scale that represents the effects of inertia. The slope winds are 97 affected by topography throughout the model domain, but are most sensitive to slopes within L 98 of x. 99 Model output characteristically produces Gale-like mound structure and stratigraphy 100 (Figures 2,S3). Katabatic winds flowing down the crater walls inhibit sediment layer 101 accumulation both on the crater walls and for an inertial run-out length on the floor that scales 102 with L. Layer accumulation in the quiet crater interior is not inhibited, so layers can be deposited 103 there. Greater wind speeds close to the walls increase sediment erosion and entrainment. The 104 gradient in slope-wind shear velocity causes a corresponding gradient in sediment accumulation, 105 which over time defines a moat and a growing mound. Mound aggradation rate does not change 106 significantly upsection, consistent with observations that show no systematic decrease in layer 107 thickness with height (21,10). Growth does not continue indefinitely: when the relief of the 108 mound becomes comparable to that of the crater walls, slope winds induced by the mound itself 109 become strong enough to erode earlier deposits at the toe of the mound. This erosional front 110 steepens the topography and further strengthens winds, so erosion propagates inward from the 111 edge of the mound, leading to a late-stage net erosional state (Supplementary Information). 112 This evolution does not require any change in external forcing with time; however, 113 simulating discrete, alternating erosional and depositional events with a constant, short 5 114 characteristic timescale produces the same model output. For the continuous case, the modeled 115 bedding contacts should be interpreted as timelines, whereas for discrete events the bedding 116 contacts represent small-scale unconformities.
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