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1 Growth and form of the mound in Crater, : 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 provide archives of climate and habitability on Mars. Gale Crater, the landing

13 site for the (MSL), hosts a 5 km high sedimentary mound (Mt. Sharp /

14 Aeolis Mons). Hypotheses for mound formation include evaporitic, lacustrine, fluviodeltaic, and

15 , 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 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 surface, and possibly other .

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 . 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 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 -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

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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; 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 are meters-to-decameters thick, laterally continuous, have horizontal-to-

90 draping layer orientations, and display 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 (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

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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 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.

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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 “” 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 , 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

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187 sedimentology measurements, and constrain present-day winds using its meteorology package, past

188 winds by imaging fossilized bedforms, post-depositional tilting by measuring -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

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259 weathering in massive ice deposits, Nature Geosci. 2, p. 215-220.

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280 depositional history of the sedimentary deposits of Arabia Terra, Mars. Icarus 220, p.311-330.

281

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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

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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.

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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

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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’ 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 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 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 . 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, 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 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

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 (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 ( 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 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