1 Growth and form of the mound in Gale Crater, Mars: Slope-wind
2 enhanced erosion and transport
3 Edwin S. Kite1, Kevin W. Lewis2, Michael P. Lamb1, Claire E. Newman,3 and Mark I.
4 Richardson3
5
6 1Geological and Planetary Sciences, California Institute of Technology, MC 150-21, Pasadena CA
7 91125, USA.
8 2Department of Geosciences, Princeton University, Guyot Hall, Princeton NJ 08544, USA.
9 3Ashima Research, Pasadena CA 91125, USA.
10
11 ABSTRACT
12 Ancient sediments provide archives of climate and habitability on Mars. Gale Crater, the landing
13 site for the Mars Science Laboratory (MSL), hosts a 5 km high sedimentary mound (Mt. Sharp /
14 Aeolis Mons). Hypotheses for mound formation include evaporitic, lacustrine, fluviodeltaic, and
15 aeolian processes, but the origin and original extent of Gale’s mound is unknown. Here we show
16 new measurements of sedimentary strata within the mound that indicate ~3° outward dips oriented
17 radially away from the mound center, inconsistent with the first three hypotheses. Moreover,
18 although mounds are widely considered to be erosional remnants of a once crater-filling unit, we
19 find that the Gale mound’s current form is close to its maximal extent. Instead we propose that the
20 mound’s structure, stratigraphy, and current shape can be explained by growth in place near the
21 center of the crater mediated by wind-topography feedbacks. Our model shows how sediment can
22 initially accrete near the crater center far from crater-wall katabatic winds, until the increasing
23 relief of the resulting mound generates mound-flank slope-winds strong enough to erode the
1
24 mound. The slope-wind enhanced erosion and transport (SWEET) hypothesis indicates mound
25 formation dominantly by aeolian deposition with limited organic carbon preservation potential, and
26 a relatively limited role for lacustrine and fluvial activity. Morphodynamic feedbacks between
27 wind and topography are widely applicable to a range of sedimentary and ice mounds across the
28 Martian surface, and possibly other planets.
29
30 INTRODUCTION
31 Most of Mars’ known sedimentary rocks are in the form of intra-crater or canyon mounded
32 deposits like the mound in Gale crater (Hynek et al., 2003), but identifying the physical
33 mechanism(s) that explain mound growth and form has proved challenging, in part because these
34 deposits have no clear analog on Earth. The current prevailing view on the formation of intra-crater
35 mounds is that sedimentary layers (i.e., beds) completely filled each crater at least to the summit of
36 the present-day mound (Malin and Edgett, 2000). Subsequent aeolian erosion, decoupled from the
37 deposition of the layers, is invoked to explain the present-day topography (Andrews-Hanna et al.,
38 2010; Murchie et al., 2009). Evaporitic, lacustrine, fluviodeltaic, and aeolian processes have each
39 been invoked to form the layers (e.g., Anderson and Bell, 2010; Andrews-Hanna et al., 2010; ;
40 Irwin et al., 2005; Niles and Michalski, 2009; Pelkey et al., 2004; Thomson et al, 2011). If the
41 sedimentary rocks formed as subhorizontal layers in an evaporitic playa-like setting, then >>106
42 km3 must have been removed to produce the modern moats and mounds (Zabrusky et al., 2012).
43 These scenarios predict near-horizontal or slightly radially-inward dipping layers controlled by
44 surface or ground water levels.
45
46 GALE MOUND LAYER ORIENTATIONS
2
47 To test this, we obtained bed-orientation measurements from six one-meter-scale stereo elevation
48 models using planar fits to extracted bedding profiles via the technique of Lewis and Aharonson
49 (2006). Each elevation model is constructed from a High Resolution Imaging Science Experiment
50 (HiRISE) stereopair using the method of Kirk et al. (2008). Individual measurements were rejected
51 where the angular regression error was greater than 2°, and the 81 remaining measurements were
52 averaged for each site to reduce uncertainty further, with the results shown in Figure 1. We find
53 that layers have shallow but significant dips away from the mound center, implying 3-4 km of pre-
54 erosional stratigraphic relief if these dips are extrapolated to the rim. Measurements of the marker
55 bed of Milliken et al. (2010) show that its elevation varies by >1km, confirming that beds are not
56 planar. Postdepositional radially-outward tilting is unlikely. Differential compaction of porous
57 sediments, flexural response to the mound load, or flexural response to excavation of material from
58 the moat would tilt layers inward, not outward. Layers targeted by MSL near the base of Gale’s
59 mound show no evidence for halotectonics or karstic depressions at kilometer scale, and
60 deformation by mantle rebound would require the Gale mound to accumulate extremely quickly
61 (Figure 2).Therefore, these measurements permit only a minor role for deposition mechanisms that
62 preferentially fill topographic lows (e.g., playa, fluviodeltaic or lacustrine sedimentation), but are
63 consistent with aeolian processes (Figure 2). This suggests the mound grew with its modern shape,
64 and that the processes sculpting the modern mound may have molded the growing mound.
65
66 SLOPE WIND EROSION ON MARS
67 Mars is a windy place; saltating sand-sized particles are in active motion on Mars, at rates that
68 predict aeolian erosion of bedrock at 10-50 µm/yr (Bridges et al., 2012). Aeolian erosion of rock
69 has occurred within the last ~1-10 Ka (Golombek et al., 2010) and is probably ongoing. Because of
70 Mars’ thin atmosphere, slope winds are expected to dominate the circulation in craters and canyons
3
71 (Spiga and Forget, 2009). We have performed mesoscale (~4km horizontal resolution) simulations
72 of Gale Crater using the MarsWRF general circulation model (Richardson et al., 2007; Toigo et al.,
73 2012) with embedded high-resolution nests, and these provide further evidence that winds in Gale
74 are expected to peak on the steep crater wall and mound slopes. Downslope-oriented yardangs,
75 crater statistics, exposed layers, and lag deposits suggest that sedimentary mounds in Valles
76 Marineris (e.g. Murchie et al., 2009) and Gale are being actively eroded by slope winds. Slope-
77 enhanced winds appear to define both the large-scale and small-scale topography and stratigraphy
78 of the polar layered deposits (e.g. Holt et al., 2010; Smith and Holt, 2010), and radar sounding of
79 intracrater ice mounds near the north polar ice sheet proves that these grew from a central core,
80 suggesting a role for slope winds (Conway et al., 2012). Most of the ancient stratigraphy explored
81 by the Opportunity rover is aeolian (Metz et al., 2009), and aeolian deposits likely represent a
82 volumetrically significant component of the sedimentary rock record, including within the strata of
83 the Gale mound (Anderson and Bell, 2010). Evidence for fluvial reworking within sedimentary
84 mounds is comparatively limited and/or localized (e.g., Thomson et al., 2011; Irwin et al., 2005).
85 Quasi-periodic bedding at many locations including the upper portion of Gale’s mound implies
86 slow (~30 µm/yr) orbitally-paced accumulation (Lewis et al., 2008). These rates are comparable to
87 the modern gross atmospherically-transported sediment deposition rate (101-2 µm/yr ; Drube et al.,
88 2010), suggesting that aeolian processes may be responsible for the layers. Sedimentary strata
89 within Valles Marineris are meters-to-decameters thick, laterally continuous, have horizontal-to-
90 draping layer orientations, and display very few angular unconformities (Fueten et al., 2010).
91 These data suggest that sedimentary deposits formed by the accretion of atmospherically-
92 transported sediment (ash, dust, impact ejecta, ice nuclei, or rapidly-saltating sand) formed readily
93 on early Mars as well as in the more recent past (Grotzinger et al., 2010; Cadieux et al., 2011).
4
94 Slope-wind erosion of indurated or lithified aeolian deposits cannot explain the outward
95 dips observed at Gale unless the topographic depression surrounding the mound (i.e., the moat)
96 seen in Figure 1 was present throughout mound growth. This implies a coupling between mound
97 primary layer orientations, slope winds, and mound relief.
98
99 MODEL
100 To explore this feedback, we aimed to develop the simplest possible model that can account for the
101 structure and stratigraphy of Mars' equatorial sedimentary rock mounds. In one horizontal
102 dimension (x), topographic change dz/dt is given by
103
104 dz/dt = D – E (1)
105
106 where D is an atmospheric source term and E(x,t) is erosion or sediment entrainment rate. Initial
107 model topography (Figure 3) is a basalt (nonerodible) crater/canyon with a flat floor of half-width
108 R and 20° slopes. Although dipping beds in the mound suggest a dominant role for aeolian
109 processes in mound growth, our model does not preclude the possibility of intermittent
110 fluvial/lacustrine deposition, which may have been partly later reworked by aeolian processes. To
111 highlight the role of slope winds in building mounds through erosion and deposition, we initially
112 assume D is constant and uniform (e.g., Niles and Michalski, 2009; Fergason and Christensen,
113 2008; Holt et al., 2010) and focus on E as the driver of wind entrainment and erosion. E typically
114 has a power-law dependence on maximum shear velocity magnitude at the air-sediment interface,
115 U:
116 E = k U α (2)
5
117 where k is an erodibility factor that depends on substrate grainsize and induration/cementation, and
118 α ~ 3-4 for sand transport, soil erosion, and rock abrasion (Kok et al., 2012). We assume that
119 sediments have some cohesive strength, most likely due to processes requiring liquid water (e.g.,
120 damp or cemented sediment, bedrock, crust formation). Shallow diagenetic cementation
121 (McLennan and Grotzinger, 2008), if it occurred, could be driven by snowmelt, rainfall, or fog.
122 Eroded material does not pile up in the moat but is instead removed from the crater, for example
123 through breakdown to easily-mobilized dust-sized particles (Sullivan et al., 2008). We model shear
124 velocity magnitude as
125 (3)
126 which is the sum of a background bed shear velocity Uo and the component of shear velocity due to
127 slope winds. The max|±()| operator returns the maximum of downslope (nighttime) or upslope
128 (daytime) winds, z' is local topography, x and x' are distances from the crater center, and L is a
129 slope-wind correlation length scale that represents the effects of inertia. The slope winds are
130 affected by topography throughout the model domain, but are most sensitive to slopes within L of
131 x.
132
133 RESULTS
134 Model output characteristically produces Gale-like mound structure and stratigraphy (Figure 3
135 shows output for α = 3, D' = 0.4, L = 19km for a Gale-sized crater, the Data Repository shows the
136 results from sensitivity tests). Katabatic winds flowing down the crater walls inhibit sediment layer
137 accumulation both on the crater walls and for an inertial run-out length on the floor that scales with
138 L. Layer accumulation in the quiet crater interior is not inhibited, so layers can be deposited there.
139 Greater wind speeds close to the walls increase sediment erosion and entrainment. The gradient in
6
140 slope-wind shear velocity causes a corresponding gradient in sediment accumulation, which over
141 time defines a moat and a growing mound. Mound aggradation rate does not change significantly
142 upsection, consistent with observations that show no systematic decrease in layer thickness with
143 height (Cadieux et al., 2011; Lewis et al., 2008). Growth does not continue indefinitely; when the
144 relief of the mound becomes comparable to that of the crater walls, slope winds induced by the
145 mound itself become strong enough to erode earlier deposits at the toe of the mound. This erosional
146 front steepens the topography and further strengthens winds, so erosion propagates inward from the
147 edge of the mound, leading to a late-stage net erosional state.
148 This evolution does not require any change in external forcing with time; however,
149 simulating discrete, alternating erosional and depositional events with a constant, short
150 characteristic timescale produces the same model output. Exposure of layering at all elevations on
151 the Gale mound show it has entered the late, erosional stage. The mean dip of all sedimentary
152 layers formed in the radial cut shown in Figure 3a is 4.7°, and erosion progressively destroys the
153 steepest-dipping layers. Exhumed layers are buried to kilometer depths, but relatively briefly,
154 consistent with evidence that clay diagenesis at Gale was minimal (Milliken, 2010). During early
155 mound growth, dz/dt is not much slower than D. If D corresponds to vertical dust settling at rates
156 similar to today, then the lower Gale mound accumulated in 107-8 yr, consistent with the orbital-
157 forcing interpretation of cyclic bedding (Lewis et al., 2010) that suggests that the time represented
158 by the lower Gale mound is a small fraction of Mars’ history.
159 Values of L and D on Early Mars are not known, but Gale-like shapes and stratigraphy
160 arise for a wide range of reasonable parameters (Figure DR2). Consistent with observations across
161 Mars, moats are infilled for small R/L, and for the largest R/L multiple mounds can develop within
162 a single crater.
7
163 D could vary on timescales much shorter than the mound growth timescale, for example if
164 orbital cycles pace the availability of liquid water for cementation. To illustrate this, we set D(t) =
165 D(t=0) + D(t=0)cos(nt) where n-1< 166 can be preserved, with the likelihood of unconformities increasing with distance from the mound 167 center. In addition, a late-stage drape crosscuts layers within the mound core at a high angle, and is 168 itself broken up by further erosion (Fig. 2d). Thin mesa units mapped at Gale and more widely on 169 Mars have these characteristics (Malin and Edgett, 2000; Anderson and Bell, 2010). Deposition at 170 a constant long-term-average rate is unrealistic for the entire mound history because the rate of 171 sedimentary rock formation on Mars is close to zero in the modern epoch (Knoll et al., 2008), most 172 likely because atmospheric loss has restricted surface liquid water availability (Andrews-Hanna 173 and Lewis, 2011; Kite et al., 2012). To explore this, we decreased D' over time; this allows winds 174 flowing down the crater rim to expose layers and form a moat even when layers are originally near- 175 horizontal. 176 177 IMPLICATIONS FOR THE “CURIOSITY” ROVER MISSION 178 Slope-wind enhanced erosion and transport is incompatible with a deep-groundwater source for 179 early diagenetic cementation of sedimentary rocks at Gale (e.g., Milliken et al., 2010), because 180 deep-groundwater-limited evaporite deposition would infill moats and produce near-horizontal 181 strata. A water source at or near the mound surface (ice weathering, snowmelt, or rainfall) is 182 predicted instead to explain those observations (e.g., Niles and Michalski, 2009; Kite et al., 2012). 183 Because perennial surface liquid water prevents aeolian erosion, we predict long dry windy periods 184 interspersed by brief wet periods at Gale, similar to observations along the Opportunity rover 185 traverse (Metz et al., 2009). Upon arriving at the mound, MSL can immediately begin to collect 186 observations that will test our model. MSL can confirm a dominantly aeolian origin using 8 187 sedimentology measurements, and constrain present-day winds using its meteorology package, past 188 winds by imaging fossilized bedforms, post-depositional tilting by measuring stream-paleoflow 189 directions, and subsurface dissolution using geochemical measurements. Unconformities, if 190 present, should be oriented away from the center of the present mound. Gale Crater’s geology is 191 diverse, and records many environments including alluvial fans, channels, and possibly lacustrine 192 sediments at the very bottom of the mound. We argue that these deposits are likely reworked by 193 aeolian processes and interbedded with aeolian deposits, necessary conditions for our model to 194 explain the dipping strata and morphology of the mound. If the bulk of the mound did form by 195 slow, perhaps orbitally-paced, aeolian sedimentation, then the preservation potential of organic 196 carbon would be low (e.g. Summons et al., 2011). 197 198 ACKNOWLEDGEMENTS 199 We thank W.E. Dietrich, W.W. Fischer, M. Mischna, A. Spiga, D.J. Stevenson, O. Aharonson, J. 200 Holt, T.C. Brothers, S. Christian, and especially K.E. Stack, for their intellectual contributions. 201 202 REFERENCES CITED 203 Anderson, R.B., and Bell, J.F., 2010, Geologic mapping and characterization of Gale Crater and 204 implications for its potential as a Mars Science Laboratory landing site. Mars 5, p. 76-128. 205 Andrews-Hanna, J.C. and Lewis, K.W., 2011, Early Mars hydrology: 2. Hydrological evolution in 206 the Noachian and Hesperian epochs, J. Geophys. Res. 116, E02007. 207 Andrews-Hanna, J.C., M.T. Zuber, R.E. Arvidson, and S.M. Wiseman, 2010, Early Mars 208 hydrology: Meridiani playa deposits and the sedimentary record of Arabia Terra, J. 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Sedimentary Res. 79, 247-264, doi:10.2110/jsr.2009.033. 251 Milliken, R., 2010, The mineralogy of the four MSL landing sites, 4th MSL Landing Site 252 Workshop, http://marsoweb.nas.nasa.gov/landingsites/msl/workshops/4th_workshop/ 253 talks/4_Milliken_Mineralogy.pdf 254 Milliken, R.E., Grotzinger, J.P., and Thomson, B.J., 2010, Paleoclimate of Mars as captured by the 255 stratigraphic record in Gale Crater. Geophys. Res. Lett. 370, L04201. 256 Murchie, S., et al, 2009, Evidence for the origin of layered deposits in Candor Chasma, Mars, from 257 mineral composition and hydrologic modeling, J. Geophys. 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Res. 114(E13), E02009. 269 Sullivan, R., et al., 2008, Wind-driven particle mobility on Mars: Insights from Mars Exploration 270 Rover observations at “El Dorado” and surroundings at Gusev Crater. J. Geophys. Res., 271 113(E6), E06S07. 272 Summons, R.E., et al., 2011, Preservation of martian organic and environmental records, 273 Astrobiology 11, p.157-181. 274 Thomson, B.J., et al., 2011, Constraints on the origin and evolution of the layered mound in Gale 275 Crater, Mars using Mars Reconnaissance Orbiter data. Icarus 214, p. 413 – 432. 276 Toigo, A.D., Lee, C., Newman, C.E., and Richardson, M.E., 2012, The impact of resolution on the 277 dynamics of the martian global atmosphere: Varying resolution studies with the MarsWRF 278 GCM, Icarus 221, p.276-288. 279 Zabrusky, K., Andrews-Hanna, J.C., and Wiseman, S.M., 2012, Reconstructing the distribution and 280 depositional history of the sedimentary deposits of Arabia Terra, Mars. Icarus 220, p.311-330. 281 12 282 FIGURE CAPTIONS 283 284 Figure 1. Bedding orientation measurements from six locations around the margin of the Gale 285 crater mound. Individual measurements from HiRISE DTMs are marked in red, with the average 286 at each site indicated by the dip symbol. Table DR1 provides a full listing of results. At each 287 location, beds consistently dip away from the center of the mound, consistent with the proposed 288 model. Background elevation data is from the High-Resolution Stereo Camera (HRSC) 289 (http://europlanet.dlr.de/node/index.php?id=380), with superimposed geologic units from Thomson 290 et al. (2011). The MSL landing ellipse is shown in white; landing occurred within ~2 km of the 291 ellipse center. 13 Growth processes Resulting stratigraphy Modi!cation processes Resulting stratigraphy Playa sourced by regional-to-global Di"erential compaction aquifer Lacustrine Flexure under mound load Fluviodeltaic Preferential dissolution Spring mound Landsliding/halotectonics Ice-sheet/ niveoaeolian Lower-crustal #ow processes Aeolian processes (airfall+slope winds) Tectonic doming Observations: 292 293 Figure 2. Comparison of mound growth hypotheses to measurements, for an idealized cross- 294 section of a mound-bearing crater. 14 295 (a) 296 (b) 15 297 (c) (d) 298 299 Figure 3. Simulated sedimentary mound growth and form. Colored lines in (A) correspond to 300 snapshots of the mound surface equally spaced in time (blue being early and red being late), for a 301 radial cut from the crater wall to the crater center. The black line corresponds to the initial 302 topography. D’ is defined as the deposition rate divided by the mean erosion rate on the 303 crater/canyon floor at simulation start. (B) shows mound geometry, where the time step between 304 the dots is half as long as the time step between the lines in (A). I, II, and III highlight stages in the 305 evolution of the mound. (A) and (B) are for steady uniform deposition. Results for time-varying 306 uniform deposition appear very similar at this scale. The maximum model mound radius exceeds 307 its current radius, consistent with observations of a possible mound outlier (Fig. 24 in Anderson 308 and Bell, 2010). (C) shows stratigraphy formed for steady uniform deposition. Note moatward 309 dips. Flank erosion to form the modern deflation surface (gray) tends to remove any 310 unconformities formed near the edge of the mound, while exposing the stratigraphic record of 311 earlier phases for rover inspection. (D) shows stratigraphy resulting from sinusoidally time-varying 16 312 deposition. For visibility, only a small number of oscillations are shown. Color of strata 313 corresponds to deposition rate: blue is high D, which might correspond to wet climates, and red is 314 low D, which might correspond to dry climates (Kite et al., 2012; Andrews-Hanna and Lewis, 315 2011). Note low-angle unconformities, and late-stage flanking unit intersecting the mound core at a 316 high angle. 17 317 Data Repository materials. 318 319 1. Methods. 320 321 a. Determination of layer orientations. 1m-resolution stereo terrain models were produced from 322 High-Resolution Imaging Science Experiment (HiRISE) images, using the method of Kirk et al. 323 (2008), and best-fitting planar layer orientations were calculated via linear regression of points 324 along bedding contacts (procedure of Lewis et al., 2006). To confirm that our procedure is 325 measuring layers within the mound, and is not biased by surficial weathering textures nor by the 326 present-day slope, we made measurements around a small reentrant canyon incised into the SW 327 corner of the Gale mound (a DTM illustrating this may be obtained from the authors). Within this 328 canyon, present-day slope dip direction varies through 360°, but as expected the measured layer 329 orientations dip consistently (to the W). 330 331 b. MarsWRF simulations of Gale Crater. MarsWRF (Toigo et al., 2012) is the Mars version of 332 planetWRF (Richardson et al., 2007), an extension of the widely-used Weather Research and 333 Forecasting model. To produce the wind analysis shown in Figure DR1, MarsWRF was run as a 334 global model at 2° resolution, with three increasingly high-resolution domains “nested” over Gale 335 Crater to increase the resolution there to ~4 km. Each nested domain is both driven by its parent 336 domain, and feeds information back to the parent domain, while also responding to surface 337 variations (e.g. topography, albedo) at the higher resolution of the nest. 338 339 c. Assessment of alternative mechanisms for producing outward dips. Few geologic processes 340 can produce primary outward dips of (3±2)° (Figures 1, 2). Spring mounds lack laterally 18 341 continuous marker beds of the >10 km extent observed (Anderson & Bell, 2010). Preferential 342 dissolution, landsliding/halotectonics, post-impact mantle rebound, and lower-crustal flow can lead 343 to postdepositional outward tilting. On Early Mars, isostatic compensation timescales are <<106 yr. 344 In order for postdepositional mantle rebound to produce outward tilts, the mound must have 345 accumulated at implausibly fast rates. Mars’ crust is constrained to be ≲90 km thick at Gale’s 346 location (Nimmo & Stevenson, 2001), so lower-crustal flow beneath 155km-diameter Gale would 347 have a geometry that would relax Gale Crater from the outside in, incompatible with simple 348 outward tilting. Additionally, Gale is incompletely compensated (Konopliv et al., 2011) and 349 postdates dichotomy-boundary faulting, so Gale postdates the era when Mars’ lithosphere was 350 warm enough for crustal flow to relax the dichotomy boundary and cause major deformation (Irwin 351 & Watters, 2010). Any tectonic mechanism for the outward dips would correspond to ~3-4 km of 352 floor uplift of originally horizontal layers. This is comparable to the depth of a fresh crater of this 353 size and inconsistent with the current depth of the southern (mound-free) half of the crater if we 354 make the reasonable approximation that wind cannot quickly erode basalt. Tectonic doming would 355 put the mound's upper surface into extension and produce extensional faults (e.g., p.156 in Melosh, 356 2011), but these are not observed. Preferential dissolution leaves karstic depressions (Hovorka, 357 2000), which are not observed at Gale. Landsliding/halotectonics can produce deformed beds in 358 layered sediments on Earth and Mars (e.g. Metz et al., 2010, Hudec & Jackson 2011). These sites 359 show order-unity strain and contorted bedding, but the layers near the base of the mound show no 360 evidence for large strains at kilometer scale, except for a possible late-stage landslide on mound’s 361 north flank (Anderson & Bell, 2010). 362 363 d. Scaling sediment transport. Conservation of sediment (Anderson, 2008) in the atmospheric 364 boundary-layer can be written as: 19 365 dz/dt = D – E = CWs -E 366 Here C is volumetric sediment concentration, Ws is settling velocity, and E is the rate of sediment 367 pick-up from the bed. In aeolian transport of dry sand and alluvial-river transport, induration 368 processes are weak or absent and so the bed has negligible intergrain cohesion. C tends to E/Ws 369 over a saturation length scale that is inversely proportional to Ws (for dz/dt > 0) or E (for dz/dt <0). 370 This scale is typically short, e.g. ~1-20m, for the case of a saltating sand on Earth (Kok et al., 371 2012). Our simplifying assumption that D ≠ fx()and therefore C ≠ fx()implies that this saturation 372 length scale is large compared to the morphodynamic feedback of interest. For the case of net 373 deposition (dz/dt > 0) this could correspond to settling-out of sediment stirred up by dust storms 374 (e.g. Vaughan et al., 2010). These events have characteristic length scales >102 km (Szwast et al., 375 2006), larger than the scale of Gale’s mound and justifying the approximation of uniform D. For 376 the case of net erosion (dz/dt < 0), small E implies a detachment-limited system where sediment 377 has some cohesion. The necessary degree of induration is not large: for example, 6-10 mg/g 378 chloride salt increases the threshold wind stress for saltation by a factor of e (Nickling, 1984). Fluid 379 pressure alone cannot abrade the bed, and the gain in entrained-particle mass from particle impact 380 equals the abrasion susceptibility, ~2 ×10-6 for basalt under modern Mars conditions (Bridges et al., 381 2012) and generally <<1 for cohesive materials, preventing runaway adjustment of C to E/Ws. 382 Detachment-limited erosion is clearly appropriate for slope-wind erosion on modern Mars (because 383 sediment mounds form yardangs and shed boulders, indicating that they are cohesive/indurated), 384 and is probably a better approximation to ancient erosion processes than is transport-limited 385 erosion (given the evidence for ancient near-surface liquid water, shallow diagenesis, and soil 386 crusts; e.g., McLennan & Grotzinger, 2008). 387 20 388 e. Reference parameter choices. Coriolis forces are neglected because almost all sedimentary 389 rock mounds on Mars are equatorial (Kite et al., 2012). Additional numerical diffusivity at the 10-3 390 level is used to stabilize the solution. Analytic and experimental results show that in slope-wind 391 dominated landscapes, the strongest winds occur close to the steepest slopes (Manins & Sarford, 392 1987). L will vary across Mars because of 3D topographic effects, and will vary in time because of 393 changing atmospheric density. Ye et al. (1990) find L ~ 20km for Mars slopes with negligible 394 geostrophic effects, and Equation 49 in Magalhaes & Gierasch (1982) gives L ~ 25 km for Gale- 395 relevant slopes. Simulations of gentle Mars slope winds strongly affected by planetary rotation 396 suggest L ~ 50-100 km (e.g., Savijärvi & Siili, 1993). Entrainment acts as a drag coefficient, ~0.02- 397 0.05 for Gale-relevant slopes (Horst & Doran, 1986, and references therein), suggesting L = 20-50 398 km for a 1km-thick cold boundary layer. Therefore we take L ~ 101-2 km to be reasonable, but with 399 the expectation of significant L/R variability, explored in the next section. 400 401 2. Sensitivity tests: controls on mound growth and form. To confirm that our results do not 402 depend on idiosyncratic parameter choices, we carried out a parameter sweep in α, D’, and R/L 403 (Figure DR2). Weak slope dependence (α = 0.05) is sufficient to produce strata that dip toward the 404 foot of the crater/canyon slope (like a sombrero hat). Similarly weak negative slope dependence (α 405 = -0.05) is sufficient to produce concave-up fill. At low R/L (i.e., small craters) or at low α, D' 406 controls overall mound shape and slope winds are unimportant. When D' is high, layers fill the 407 crater; when D’ is low, layers do not accumulate. When either α or R/L or both are ≳1, slope-wind 408 enhanced erosion and transport dominates the behavior. Thin layered crater floor deposits form at 409 low D', and large mounds at high D'. If L is approximated as being constant across the planet, then 410 R/L is proportional to crater/canyon size. There is net aggradation everywhere for small R/L, 411 although a small moat can form as a result of relatively low net aggradation near the crater wall. 21 412 For larger R/L, moats form, and for the largest craters/canyons, multiple mounds can form 413 eventually because slope winds break up the deposits. This is consistent with data, which suggest a 414 maximum length scale for mounds (Figure DR3). Small exhumed craters in Meridiani show 415 concentric layering consistent with concave-up dips. Larger craters across Meridiani, together with 416 the north polar ice mounds, show a simple single mound. Gale and Nicholson Craters, together 417 with the smaller Valles Marineris chasmata, show a single mound with an undulating top. The 418 largest canyon system on Mars (Ophir-Candor-Melas) shows multiple mounds per canyon. Gale- 419 like mounds (with erosion both at the toe and the summit) are most likely for high R/L, high α, and 420 intermediate D' (high enough for some accumulation, but not so high as to fill the crater) (Figure 421 DR2). 422 Uo is set to zero in Figure 3. Sensitivity tests show that for a given D', varying Uo has little 423 effect on the pattern of erosion because spatial variations are still controlled by slope winds. 424 Equation (3) implies the approximation E ~ max(U)α ~ ∑Uα, which is true as α à ∞. To check 425 that this approximation does not affect conclusions for α = 3-4 (Kok et al., 2012), we ran a α α 426 parameter sweep with E ~ (U+ + U- ). For nominal parameters (Figure 3), this leads to only minor 427 changes in mound structure and stratigraphy (e.g., 6% reduction in mound height and <1% in 428 mound width at late time). For the parameter sweep as a whole, the change leads to a slight 429 widening of the regions where the mound does not nucleate or overspills the crater (changing the 430 outcome of 7 out of the 117 cases shown in Figure DR2). The approximation would be further 431 supported if (as is likely) there is a threshold U below which erosion does not occur. If MSL shows 432 that persistent snow or ice was needed as a water source for layer cementation (Niles & Michalski, 433 2009; Kite et al., 2012), then additional terms will be required to track humidity and the drying 434 effect of föhn winds (e.g. Madeleine et al., 2012). 22 435 These sensitivity tests suggest that mounds are a generic outcome of steady uniform 436 deposition modified by slope-wind enhanced erosion and transport for reasonable Early Mars 437 parameter values. 438 439 Data Repository References 440 Anderson, R.S., 2008, The Little Book of Geomorphology: Exercising the Principle of 441 Conservation, http://instaar.colorado.edu/~andersrs/The_little_book_010708_web.pdf 442 Horst, T. W., & Doran, J. C., 1986, Nocturnal drainage flow on simple slopes. Boundary-Layer 443 Meteorol. 34, p. 263-286. 444 Hovorka, S. D., 2000, Understanding the processes of salt dissolution and subsidence in sinkholes 445 and unusual subsidence over solution mined caverns and salt and potash mines, Technical 446 Session: Solution Mining Research Institute Fall Meeting, p. 12–23, downloaded from 447 http://www.beg.utexas.edu/environqlty/pdfs/hovorka-salt.pdf 448 Hudec, M.R., & Jackson, M.P.A., 2011, The salt mine : a digital atlas of salt tectonics. Austin, Tex: 449 Jackson School of Geosciences, University of Texas at Austin. 450 Irwin, R. P., III, and T. R. Watters, 2010, Geology of the Martian crustal dichotomy boundary, J. 451 Geophys. Res. 115, E11006, doi:10.1029/2010JE003658. 452 Konopliv, A.S. et al., 2011, Mars high resolution gravity fields from MRO, Mars seasonal gravity, 453 and other dynamical parameters, Icarus 211, p. 401-428. 454 Madeleine, J.-B., Head, J. W., Spiga, A., Dickson, J. L., & Forget, F., 2012, A study of ice 455 accumulation and stability in Martian craters under past orbital conditions using the LMD 456 mesoscale model, Lunar and Planet. Sci. Conf. 43, abstract no. 1664. 457 Magalhaes, J., & Gierasch, P., 1982, A model of Martian slope winds: Implications for eolian 458 transport, J. Geophys. Res. 87, p. 9975-9984. 23 459 Manins, P. C., & Sawford, B. L.,1987, A model of katabatic winds, J. Atmos. Sci. 36, 619-630. 460 Metz, J., Grotzinger, J., Okubo, C., & Milliken, R., 2010, Thin-skinned deformation of sedimentary 461 rocks in Valles Marineris, Mars, J. Geophys. Res. 115, E11004. 462 Melosh, H.J., 2011, Planetary Surface Processes, Cambridge University Press. 463 Nickling, W.G., 1984, The stabilizing role of bonding agents on the entrainment of sediment by 464 wind. Sedimentology 31, 111-117. doi: 10.1111/j.1365-3091.1984.tb00726.x. 465 Nimmo, F., and Stevenson, D.J. 2001, Estimates of Martian crustal thickness from viscous 466 relaxation of topography, J. Geophys. Res. 106, 5085-5098, doi:10.1029/2000JE001331. 467 Richardson, M.I., Toigo, A.D., and Newman, C.E., 2007, PlanetWRF: A general purpose, local to 468 global numerical model for planetary atmospheric and climate dynamics, J. Geophys. Res. 112, 469 E09001. 470 Szwast, M., Richardson, M. and Vasavada, A., 2006, Surface dust redistribution on Mars as 471 observed by the Mars Global Surveyor and Viking orbiters. J. Geophys. Res. 111, E11008. 472 Savijärvi, H., and Siili, T., 1993, The Martian slope winds and the nocturnal PBL jet. J. Atmos. Sci. 473 50, p. 77-88. 474 Vaughan, A.F., et al., 2010. Pancam and Microscopic Imager observations of dust on the Spirit 475 Rover: Cleaning events, spectral properties, and aggregates, Mars 5, p. 129-145. 476 Ye, Z.J., Segal, M., & Pielke, R.A., 1990, A comparative study of daytime thermally induced 477 upslope flow on Mars and Earth. J. Atmos. Sci. 47, p. 612-628. 24 478 Data Repository Table 1: Layer orientation measurements Lat Lon Z (m) Dip (°) Dip Az (°) HiRISE Image ID -5.022347 138.394900 -3263.1 3.53 30.68 PSP_008437_1750 -5.023877 138.392660 -3201.9 2.52 62.01 PSP_008437_1750 -5.015876 138.386310 -3216.9 2.1 94.68 PSP_008437_1750 -5.015508 138.387020 -3201.4 7.31 41.72 PSP_008437_1750 -4.998358 138.391680 -3554.2 2.06 54.81 PSP_008437_1750 -5.003035 138.387800 -3429.9 0.43 -21.29 PSP_008437_1750 -5.004517 138.379910 -3425.8 5.04 89.54 PSP_008437_1750 -5.004179 138.379580 -3434.5 3.79 70.24 PSP_008437_1750 -4.997492 138.392530 -3583.8 4.65 51.98 PSP_008437_1750 -5.012374 138.396710 -3421.5 4.14 47.28 PSP_008437_1750 -5.031424 138.395590 -3290.1 4.07 40.92 PSP_008437_1750 -5.030106 138.396180 -3308.2 2.55 76.18 PSP_008437_1750 -5.03171 138.393740 -3260.1 3.24 43.31 PSP_008437_1750 -5.035189 138.392050 -3176.3 2.07 75.25 PSP_008437_1750 -5.035062 138.391700 -3173.3 2.21 86.77 PSP_008437_1750 -4.685812 137.494850 -4098.5 3.34 148.1 ESP_023957_1755 -4.684778 137.491970 -4103.8 3.98 174.47 ESP_023957_1755 -4.689331 137.480520 -4101.3 6.06 132.82 ESP_023957_1755 -4.659387 137.533150 -4120.3 6.48 114.03 ESP_023957_1755 -4.65886 137.537400 -4108.2 0.5 13.56 ESP_023957_1755 -4.662119 137.535520 -4073.1 3.9 -149.05 ESP_023957_1755 -4.664102 137.525670 -4127.1 7.16 130.19 ESP_023957_1755 -4.663656 137.524870 -4138.3 4.83 152.05 ESP_023957_1755 -4.665528 137.526530 -4101.2 4.59 -153.08 ESP_023957_1755 -4.676802 137.506150 -4083.2 2.32 83.97 ESP_023957_1755 -4.672137 137.510280 -4128 3.24 91.62 ESP_023957_1755 -4.871501 137.270980 -3849.6 1.09 10.19 PSP_001488_1750 -4.871837 137.266710 -3857.7 5.19 85.97 PSP_001488_1750 -4.872691 137.270730 -3833.2 2.61 -51.44 PSP_001488_1750 -4.917952 137.284340 -3513.9 6.78 136.55 PSP_001488_1750 -4.831027 137.330630 -3768.9 2.16 140.09 PSP_001488_1750 -4.828565 137.330360 -3792.5 2.65 143.84 PSP_001488_1750 -4.846642 137.303380 -3802.8 4.38 124.94 PSP_001488_1750 -4.845724 137.303070 -3815.7 4.51 111.27 PSP_001488_1750 -4.845501 137.304420 -3799.2 2.34 79.11 PSP_001488_1750 -4.863885 137.332420 -3507.1 2.04 132.56 PSP_001488_1750 -4.93696 137.311640 -3278.4 3.74 129.01 PSP_001488_1750 -4.938936 137.309840 -3287.6 4.46 119.79 PSP_001488_1750 -4.920126 137.322160 -3290.9 1.79 134.69 PSP_001488_1750 -4.922859 137.317020 -3306.6 4.2 160.7 PSP_001488_1750 -4.901912 137.338220 -3265.6 4.87 -174.83 PSP_001488_1750 25 -4.892401 137.332800 -3389.9 6.71 117.93 PSP_001488_1750 -4.863151 137.342100 -3464.4 5.66 105.35 PSP_001488_1750 -4.779928 137.409690 -3656.9 3.69 167.09 PSP_009149_1750 -4.777029 137.405330 -3736.8 2.13 168.14 PSP_009149_1750 -4.75303 137.438670 -3810.7 6.07 128.67 PSP_009149_1750 -4.752131 137.438100 -3823.7 5.92 123.62 PSP_009149_1750 -5.348423 137.227120 -2745.9 2.68 154.47 ESP_012907_1745 -5.348098 137.227470 -2757.1 2.32 144.88 ESP_012907_1745 -5.341016 137.209820 -2891.2 2.58 135.34 ESP_012907_1745 -5.337968 137.210930 -2796.8 4.54 151.03 ESP_012907_1745 -5.377012 137.207730 -2879.1 1.63 165.41 ESP_012907_1745 -5.384663 137.190610 -3005.3 4.73 156.7 ESP_012907_1745 -5.413138 137.185680 -2848.5 4.57 133.72 ESP_012907_1745 -5.40772 137.197850 -2779.3 4.05 158.01 ESP_012907_1745 -5.392686 137.208330 -2756.1 10.83 163.21 ESP_012907_1745 -5.371398 137.174520 -3359.7 6.02 159.19 ESP_012907_1745 -5.342561 137.176890 -3130.9 6.76 160.11 ESP_012907_1745 -5.458482 137.183800 -2752.8 2.49 142.76 ESP_012907_1745 -5.459221 137.188840 -2674.5 3.58 162.7 ESP_012907_1745 -5.460307 137.188440 -2696.5 3.97 166.94 ESP_012907_1745 -5.392535 137.196770 -2848.2 2.48 170.27 ESP_012907_1745 -5.390379 137.195930 -2898.2 3.04 156.57 ESP_012907_1745 -5.390219 137.191070 -2988.8 2.6 144.9 ESP_012907_1745 -5.411481 137.194250 -2811.9 2.09 146.02 ESP_012907_1745 -5.623904 138.328080 -2971.2 1.74 -43.56 ESP_014186_1745 -5.627848 138.337550 -3000.2 2.17 -71.76 ESP_014186_1745 -5.598681 138.339300 -2856.2 1.26 116.18 ESP_014186_1745 -5.584488 138.338620 -2705.9 2.25 -69.68 ESP_014186_1745 -5.584654 138.354940 -2710 2.45 -82.66 ESP_014186_1745 -5.605716 138.360870 -2907.7 0.3 -175.97 ESP_014186_1745 -5.570108 138.366460 -2674.6 5.23 -61.61 ESP_014186_1745 -5.572424 138.360000 -2589.6 2.17 -86.77 ESP_014186_1745 -5.588566 138.326250 -2685.2 2.6 -27.67 ESP_014186_1745 -5.580666 138.333270 -2681.5 1.76 -81.46 ESP_014186_1745 -5.534321 138.325470 -2424.8 4.35 -117.38 ESP_014186_1745 -5.542813 138.327860 -2454.8 3.37 -104.06 ESP_014186_1745 -5.573589 138.289520 -2658.6 2.2 -115.72 ESP_014186_1745 -5.566658 138.288200 -2644.4 1.15 74.46 ESP_014186_1745 -5.581102 138.309950 -2667.3 1.06 -65.97 ESP_014186_1745 479 480 481 26 482 Data Repository Figure Captions Maximum wind speed -4.0 -4.5 -5.0 Latitude (deg N) -5.5 -6.0 -6.5 136.5 137.0 137.5 138.0 138.5 Longitude (deg E) 4.8664 8.0060 11.146 14.285 17.425 483 3.2966 6.4362 9.5758 12.715 15.855 18.995 484 Figure DR1. Annual maximum wind speed (m/s) within Gale Crater from MarsWRF simulations, 485 showing that the strongest winds within the crater are associated with steep slopes. Black 486 topography contours are spaced at 500m intervals. The winds are extrapolated to 1.5m above the 487 surface using boundary layer similarity theory (the lowest model layer is at ~9m above the 488 surface). 27 489 490 Figure DR2. Overall growth and form of sedimentary mounds – results from a model parameter 491 sweep varying R/L and D', with fixed α = 3. Black square corresponds to the results shown in more 492 detail in Figure 3. Symbols correspond to the overall results:– no net accumulation of sediment 493 anywhere (blue open circles); sediment overtops crater/canyon (red filled circles); mound forms 494 and remains within crater (green symbols). Green filled circles correspond to outcomes where 495 layers are exposed at both the toe and the summit of mound, similar to Gale. 28 1 0.8 0.6 0.4 G 0.2 Nonpolar crater mound Canyon mound Mound width/container width Polar ice mound 0 0 1 2 3 10 10 10 10 496 Container width (km) 497 Figure DR3. Width of largest mound does not keep pace with increasing crater/canyon width, 498 suggesting a length threshold beyond which slope winds break up mounds. Blue dots correspond to 499 nonpolar crater data, red squares correspond to canyon data, and green dots correspond to polar ice 500 mound data. Gray vertical lines show range of uncertainty in largest-mound width for Valles 501 Marineris canyons. Blue dot adjacent to “G” corresponds to Gale Crater. Craters smaller than 10km 502 were measured using Context Camera (CTX) or HiRISE images. All other craters, canyons and 503 mounds were measured using the Thermal Emission Imaging System (THEMIS) global day 504 infrared mosaic on a Mars Orbiter Laser Altimeter (MOLA) base. Width is defined as polygon area 505 divided by the longest straight-line length that can be contained within that polygon. 29