Last edited December 28, 2014

For submission to the Journal of Geophysical Research – Planets

Modeling the Thermal and Physical Evolution of Mount Sharp’s Sedimentary

Rocks, Gale Crater, Mars: Implications for Diagenetic Minerals on the MSL

Curiosity Rover Traverse

Caue S. Borlina1,2, and Bethany L. Ehlmann2,3

1 Department of Atmospheric, Oceanic and Space Sciences, University of Michigan, Ann

Arbor, Michigan, USA

2 Division of Geological and Planetary Sciences, California Institute of Technology,

Pasadena, California, USA

3 Jet Propulsion Laboratory, California Institute of Technology, Pasadena, California,

USA

Corresponding author: caue@umich.edu; 2215 Space Research Building, 2455

Hayward St., Ann Arbor, MI 48109

Keywords: burial diagenesis, sedimentation, erosion, heat flow, Gale Crater, Mars

Science Laboratory (MSL)

1

2

3

4

5

6

7

8

9

10

11

12

13

14

15

16

17

18

19

20

21

22

23

Abstract

Gale Crater, landing site of the NASA Mars Science Laboratory (MSL) mission, has a

unique sedimentary stratigraphy, preserved in a 5 km high mound called Aeolis Mons

(informally known as Mt. Sharp). Understanding the processes of Mt. Sharp

sedimentation, erosion, and secondary mineral formation can reveal important

information regarding past geologic processes, the climate and habitability of early Mars,

and the organic preservation potential of these sedimentary deposits. Two endmember

scenarios for Mt. Sharp's formation are: (1) a complete filling of Gale crater followed by

partial removal of deposited sediments or (2) building of a central deposit with

morphology controlled by slope winds and only incomplete sedimentary fill of Gale

crater. Here we consider depths of burial predicted for both scenarios, coupled with a

thermal model to predict the temperature history of particular sediments presently

exposed at the surface. We compare our results with chemical and mineralogical analyses

by MSL to date and provide sedimentation scenario-dependent predictions of diagenetic

mineralogy in locations between Yellowknife Bay and upper Mt. Sharp. Results show

modeled erosion and deposition rates range from 5-42 μm/yr across different scenarios

and timing scales, consistent with or slightly higher than rates calculated for other

locations on Mars. Mineralogical predictions for locations close to Yellowknife Bay vary

with the choice of a cold or warm early Mars and the filling scenario. For (1),

temperatures experienced by sediments should decrease monotonically over the traverse

and up Mt. Sharp stratigraphy, whereas for (2) maximum temperatures are reached in the

lower units of Mt. Sharp and thereafter decline or hold roughly constant. If surface

temperatures on early Mars were similar to today, (1) predicts temperatures experienced

24

25

26

27

28

29

30

31

32

33

34

35

36

37

38

39

40

41

42

43

44

45

46

by sediments ranging from 50-100°C while for (2) only specificelect scenarios permit

diagenetic fluids (T>0°C) at select locations. In contrast, if surface temperatures on early

Mars were near the melting point of water, diagenetic temperatures were achieved over

the entire Curiosity traverse with temperatures ranging from 100-150°C under scenario

(1) and 0-100°C for variations of scenario (2). Under nearly all scenarios, evidence of

diagenesis at higher temperatures is expected as MSL moves towards Mt. Sharp.

Comparing our predictions with future MSL results on secondary mineral assemblages,

their spatial variation as a function of location on the traverse, and age of any authigenic

phases will constrain the timing and timescale of Mt. Sharp formation, paleosurface

temperature, and the availability and setting of liquid water on early Mars.

47

48

49

50

51

52

53

54

55

56

57

58

1. Introduction

The study of sedimentation processes at specific regions on Mars helps time order

aqueous mineral formation and defines the ancient geologic history of the red planet.

Such analysis can reveal important information about past habitability, presence of water

at or near the surface of the planet, and potential for the long-term preservation of organic

materials. Gale crater (137.4oE, -4.6oN), the landing site of the NASA Mars Science

Laboratory (MSL) mission, has a unique sedimentary stratigraphy, which permits

examining ancient Martian environmental conditions and aqueous alteration. Recent

studies have shown that Gale once hosted a fluvial-lacustrine environment, capable of

supporting life [Grotzinger et al., 2014]. The stratigraphic rock record of the area, and

thus its geologic history, is preserved in a 5 km high mound called Aeolis Mons

(informally, Mt. Sharp) located in Gale crater [Grotzinger et al., 2012] (Figure 1a).

Materials comprising Mt. Sharp have a low thermal inertia and subhorizontally layered

units, which implicate a sedimentary origin [Pelkey et al., 2004; Anderson & Bell, 2010;

Thomson et al., 2011]. More recent work has suggested the lowermost units are cross-

bedded sandstones formed by cementation and lithification of sand dunes [Milliken et al.,

2014]. The lower mound includes distinctive sedimentary beds with hydrated sulfates,

iron oxides and Fe/Mg smectite clay minerals [Millken et al., 2010; Fraeman et al.,

2014]. Spectral signatures associated with the upper mound do not permit unique

identification of secondary mineral phases, though eroded boxwork structures suggest

precipitation of minerals during fluid flow within the sediments [Siebach and Grotzinger,

2014].

59

60

61

62

63

64

65

66

67

68

69

70

71

72

73

74

75

76

77

78

79

80

81

Many hypotheses have been developed to explain Mt Sharp formation, variously

invoking airfall dust or volcanic ash, lag deposits from ice/snow, aeolian and

fluviolacustrine sedimentation [for review, see Anderson & Bell, 2010; Wray, 2013;

LeDeit et al., 2013]. Nevertheless, regardless of the process(es) delivering sediments, two

endmember scenarios describe the time-evolution, i.e., growth and subsequent erosion, of

Mt. Sharp. In Scenario 1, Gale Crater was completely filled with layered sediments then

partially exhumed, leaving a central mound [Malin and Edgett, 2000] (Figure 1b). In

Scenario 2, an aeolian process characterized by slope winds created a wind-topography

feedback enabling growth of a high mound without complete fill of the crater [Kite et al.,

2013] (Figure 1c).

The history of Mt. Sharp’s sedimentation and mineralization provides key constraints on

environmental conditions on early Mars, including the availability of liquid water and the

nature of geochemical environments. Understanding sediment deposition and possible

diagenesis is also crucial to establishing the preservation potential through time for

organic carbon of biological or abiotic origin trapped in sedimentary rock strata. The

thermal history of sediments and their exposure to fluids exerts strong control on the

persistence of organic compounds in the sedimentary record (e.g., Harvey et al., 1995;

Lehmann et al., 2002). Initial studies of the diagenesis of Martian sediments pointed out

the apparent ubiquity of the “juvenile” sediments with smectite clays, amorphous silica

and a paucity of evidence for illite, chlorite, quartz and other typical products of

diagenesis, which are common in the terrestrial rock record, thus concluding that

82

83

84

85

86

87

88

89

90

91

92

93

94

95

96

97

98

99

100

101

102

103

104

diagenetic processes on Mars were uncommon, perhaps limited by water availability

[Tosca & Knoll, 2009]. Since then, a growing number of studies have identified clay

minerals such as illite, chlorite, and mixed layer clays that can form from via diagenesis

[Ehlmann et al., 2009, 2011a, 2011b; Milliken & Bish, 2010; Carter et al., 2013]. So far,

minerals identified in Mt. Sharp from orbit do not include these phases. However, in situ

rover data at Yellowknife Bay implies at least early diagenetic reactions to form

mineralized veins, nodules, and filled fractures within the mudstones [Stack et al., 2014;

Siebach et al., 2014], including exchange of interlayer cations in smectite clays or

incipient chloritization [Vaniman et al., 2014; Rampe et al., 2014; Bristow et al., 2014].

Here we model the potential diagenetic history of the sediments comprising Mt. Sharp

and accessible in rock units along Curiosity’s traverse. We couple the two sedimentation

scenarios [Malin and Edgett, 2000; Kite et al., 2013] with a thermal model for ancient

Martian heat flow and timescales for Mt. Sharp sedimentary deposition and erosion

constrained by crater counts. We model temperature variations experienced within the

region between Yellowknife Bay, the base of Mt. Sharp, and the lower unit/upper unit

Mt. Sharp unconformity, and we compare them with select key temperature thresholds

relevant for diagenesis: stability of liquid water (0oC) and the a temperature for efficient

smectite to illite conversion (~40oC). We also analyze the time-temperature integral, an

alternative method for establishing the likelihood and extent of mineral diagenesis as

described by Tosca and Knoll [2009]. The final results are compared to mineralogical and

chemical findings obtained by MSL to date. Potential mineralogical findings across

MSL’s traverse are discussed and correlated with implications for sedimentation

105

106

107

108

109

110

111

112

113

114

115

116

117

118

119

120

121

122

123

124

125

126

127

processes on Mars, timescales, early Mars temperatures, liquid water availability, and the

organic preservation potential of Gale sediments.

2. Methodology

In this section we describe how we defined the pristine Gale basement and final Mt.

Sharp profile, the two different endmember sedimentation scenarios and variations in

parameters used during the modeling, timescales and timing of sedimentation assumed,

the derivation of the thermal model for surface heat flow, and the topographic profile of

the rover traverse that defines the studied region.

2.1 Pristine Gale Basement & Modern Topography

In order to evaluate how Gale Crater changed we needed to first determine the ancient

(starting point) and modern (ending point) topographies of the crater. Gale crater has

been both eroded and filled relative to its original topographic profile. Consequently, a

combination of theoretical constraints and observational constraints from the Mars

Orbiting Laser Altimeter Altitude (MOLA) dataset was used to set the initial conditions

for Gale's crater shape, i.e. its initial topographic profile. Observed crater depth-diameter

relationships on Mars predict a range of initial crater depths for Gale crater (D ~154 km)

ranging from 4.2 km to 5.4 km [Garvin et al., 2003; Boyce and Garbeil, 2007; Robbins

and Hynek, 2007]. Kalynn et al. [2013] empirically have shown Martian central peak

heights of ~1 km for ~100 km craters. Our examination of craters on Mars, better

preserved than Gale, with diameters ranging from 131 km to 155 km (at 16ºW, 43ºS;

36ºW, 36ºS; 45ºE, 42ºN) yielded lower bounds on central peak height ranging from 1.0-

128

129

130

131

132

133

134

135

136

137

138

139

140

141

142

143

144

145

146

147

148

149

150

2.1 km that did not scale in a straightforward way with diameter, and these central peak

heights may be underestimates due to later crater fill. Hence, we set the initial shape of

Gale Crater to be 154 km in diameter and 5 km deep with a central peak height of 1.55

km. In order to have realistic slopes, we scaled the average topographic profile from

Moreux crater (45E, 42N) to fit Gale's parameters.

Modern Gale crater has a highly asymmetric central mound, its cross-sectional profile

varying with azimuth. The average Gale profile shown (Figure 2) was compiled from

multiple roughly NW-SE cross-sections of present-day crater topography and then

averaged after removing local geologic features. The average Gale profile is used for

estimation of an average overburden over the Curiosity rover traverse. Scenarios 2a and

2b were tuned to produce final mounds of width approximately equivalent to the width of

the average profile.

2.2 Mt. Sharp Sedimentation Scenarios

We develop two geological scenarios for the time evolution of Mt. Sharp filling/removal:

(1) a complete filling of the crater followed by partial removal leaving a central mound,

[Malin and Edgett, 2000]; and (2) an aeolian process, where the mound only grew near

the center of the crater with continuous presence of strong mound-flank slope winds

capable of eroding the mound and keeping it away from the sides of the crater [Kite et al.,

2013].

2.2.1 Scenario 1

151

152

153

154

155

156

157

158

159

160

161

162

163

164

165

166

167

168

169

170

171

172

173

Scenario 1 is characterized by a complete fill of the crater to the height of the highest

peak of Mt. Sharp, followed by partial erosion of the crater, leaving the modern shape of

Mt. Sharp as final output. We use an average Gale profile as the final shape (see section

2.1). This scenario is strongly dependent on timescales defined in section 2.3.

Sedimentation and erosion rates are computed linearly based on the defined time period

for erosion/sedimentation processes and necessary burial/erosion height, i.e., deposition

rate is computed as distance from pristine basement to the top of the crater, divided by

deposition time. Erosion rate computed as distance from top of the crater (completely

filled crater) to the average modern profile, divided by erosion time.

2.2.2 Scenario 2

Scenario 2 is an aeolian process where the mound grew close to the center of the crater

with the surrounding topography creating an environment for the presence of strong

mound-flank slope winds capable of eroding the mound. We use the model and code

developed by Kite et al. [2013] to generate the evolution of the topographic profile with

time. Briefly, in Kite et al.’s model a saltation model approximation is used to determine

the balance between the deposition D (set at the beginning of the simulation), and the

erosion E (time varying). The result is the computation of dz/dt (altitude variation over

time) for every time step. The following set of equations define the scenario

dzdt

=D−E , (1)

E=k Uα , (2)

174

175

176

177

178

179

180

181

182

183

184

185

186

187

188

189

190

191

192

193

194

U ( x )=U 0+max [∫x±∞ ∂ z '

∂ x 'exp(−|x−x '|

L )dx ' ] , (3)

where k is an erodibility factor; α is a parameter responsible for sand transport, soil

erosion and rock abrasion on the crater; U is a unitless dimensionless expression related

to the magnitude of the shear velocity (dx’, a mesh unit in Kite et al., is 1i.e. at t = 0 the

basement is represented by a mesh with spacing dx’ of unit value); U0 is the component

of background bed shear velocity; z’ is the height; x is the location within the crater (0 <

x < 154 km, the crater diameter); x’ is the distance half-way between the values of x

starting at x = 1.5 km and ending at x = 153.5 km; and L is a length scale that accounts for

the effects of inertia. A special parameter R/L is used which correlates the width of the

crater wall to the crater size. For small craters we should expect small values of R/L,

around 0.1-1 (for a complete analysis of R/L verify the supplementary material from Kite

et al., [2013]).

Is important to notice that . U and z’ are tiime varying values. Eq. (3) is evaluated for

each time step considering left and right slopes of the winds: for a value of x, we compute

the integral for left slopes (values less than x) and right slopes (values greater than x and

less than 154 km). The operator max[ ] selects the slope with the highest value, which

then permits evaluation of Eq. (2) at each time step. Eq. (1) is also then evaluated in each

timestep, using E from Eq. (2) and a value for D determined from the user-set parameter

D’ (D=D’E0, where E0 is the initial erosion at the start of the simulation)). In Kite et al.

[2013] and our Scenario 2a, D is fixed in the first time step and is constant thereafter. In

our Scenario 2B, D is set initially then declines linearly.

195

196

197

198

199

200

201

202

203

204

205

206

207

208

209

210

211

212

213

214

215

216

We experimented with multiple choices of parameter combinations and choose to

work with two different sets of parameters that yield topographic profiles most similar to

Gale. Parameters used by Kite et al. [2013] and this study are given in Table 1. We name

the two different scenarios as 2a and 2b. Scenario 2a is defined with a constant deposition

rate. The mound grows tall with a relatively constant width over time. Scenario 2b has a

linearly decreasing deposition rate over time, and the mound grows wide then steepens

and narrows. Both scenarios converge on a similar final output though the amount of

sediment overburden as a function of time depends on the intermediate steps. The output

has no time dependency; we implement timescales in the final output of the model by

scaling the output to our specified durations after iterations converged. A special

parameter R/L is introduced at this point, which correlates the width of the crater wall to

the crater size. For small craters we should expect small values of R/L, around 0.1-1 (for

a complete analysis of R/L verify the supplementary material from Kite et al., [2013]).

2.3 Timescales

Crater counts on the ejecta blanket of Gale crater constrain its formation age to Late

Noachian/Early Hesperian, approximately 3.8 to 3.6 Ga, and place an upper limit on the

period of infilling Mt. Sharp sediments [Thomson et al., 2011; Le Deit et al., 2013].

Similarly, an upper limit to the age and extent of lower Mt. Sharp can be obtained using

the superposition relationship of the topographically lower but stratigraphically higher

deposits of Aeolis Palus, which have estimated ages ranging from early Hesperian to

early Amazonian [Thomson et al., 2011; Le Deit et al., 2012; Grant et al., 2012], i.e.,

from ~3.2 to ~3.5 Ga. Thus, most of the formation and erosion of Mt. Sharp to its present

217

218

219

220

221

222

223

224

225

226

227

228

229

230

231

232

233

234

235

236

237

238

239

extent took place during the Hesperian, though processes continued to shape the form of

the mound during the Amazonian.

While surface ages based on crater counts are superb for relative age dating, pinning in

absolute time is challenged by the existence of different chronology models relating the

density of craters with time [e.g., Werner & Tanaka, 2011]. Yet, numerical ages

constraining the start and end of major episodes of Mt. Sharp erosion and deposition are

required to tie burial history to models of the secular cooling of Mars (section 2.4).

Consequently, we examine three different temporal scenarios for the fill and exhumation

of Mt. Sharp: (1) a standard model, (2) a maximum diagenesis model, and (3) a minimum

diagenesis model. For (1), Mt. Sharp formation begins at 3.7 Ga, reaches 5 km in height,

and is then exhumed to reach approximately its present extent by 3.3 Ga. For (2), Gale

crater and Mt. Sharp form early, 3.85 Gyr, and Mt. Sharp is exhumed late, 3.0 Gyr, thus

providing a maximum for heat flow and duration of burial. For (3), Mt. Sharp forms late,

3.6 Gyr and is quickly exhumed by 3.4 Gyr.

2.4 Thermal Model

The thermal model used here defines temperature as a function of depth and time. We

construct it by using the one-dimensional steady-state heat conduction solution that

describes the temperature T as a function of the depth, z, and time, t, as

T ( z , t )=T0+q (t )

kz− ρH ( t )

2 kz2 , (1)

where T0 is mean surface temperature, q(t) is the heat flow as a function of time, k is the

thermal conductivity, ρ is the density and H(t) the heat production as a function of time.

240

241

242

243

244

245

246

247

248

249

250

251

252

253

254

255

256

257

258

259

260

261

We define ρ = 2500 kg/m3, k = 2 W/mC, values typical for average sedimentary rocks on

Earth [Beardsmore and Cull, 2001].

The mean surface temperature for early Mars is still an unknown. Here we adopt

two possibilities: T0 = 0oC and T0 = -50oC in order to analyze how different values of T0

can impact the results. The first presumes a warmer early Mars where temperatures

routinely exceed the melting point of water during large portions of the Martian year; the

latter represents the modern-day average equatorial temperature. Finally, q(t) and H(t)

were estimated by curve-fitting to geophysical models for the evolution of heat flow

[Parmentier and Zuber, 2007] and crustal heat production, respectively, through time

[Hahn et al., 2011; their Figure 2]. Results for q(t) and H(t) are shown in Figures 3a and

3b. Parmentier and Zuber [2007] estimated values for q to be ~60mW/m2 at 3.5 Gyr for a

variety of cooling scenarios and 20mW/m2 at present; measurements of modern heat flow

will soon be obtained by the InSight mission.. These values are similar to those

independently predicted by Morschhauser et al. [2011]. Figure 3c shows derived

temperatures as a function of depth for the two different mean surface temperatures and

several time periods.

Gale may have had additional heating from hydrothermal sources such as residual

heat following the Gale-forming impact or local volcanic sources, but we do not include

these in our modeling. Were additional sources of heat present, heat flow would be higher

and temperatures higher. Thus our estimates for temperatures reached are lower bounds.

2.5 Yellowknife Bay to Mt. Sharp: MSL’s Traverse

262

263

264

265

266

267

268

269

270

271

272

273

274

275

276

277

278

279

280

281

282

283

284

After landing, MSL headed toward Yellowknife Bay, a region in the opposite direction of

the expected path toward Mt. Sharp. The rocks at Yellowknife Bay preserve evidence for

a fluvio-lacustrine environment [Grotzinger et al., 2014] and several minerals related to

aqueous alteration, including Mg smectites and hydrated calcium sulfates, were identified

[Vaniman et al., 2014].

Given sections 2.1-2.4 above, we model the sedimentary and thermal history along the

Curiosity traverse, obtained between Yellowknife Bay, the Murray buttes entrance to Mt.

Sharp, and predicted future locations of MSL Curiosity after climbing Mt. Sharp. Figure

4 shows Bradbury Landing, Yellowknife Bay, Hidden Valley and a potential future final

location of MSL. We picked the final future location for modeling to be the

unconformity, marking the boundary between upper and lower Mt Sharp. In order to

locate Yellowknife Bay in our model, we compute the ratio between the distance of the

closest rim to Yellowknife Bay and the distance of the rim to the foothill of Mt. Sharp.

This is necessary because different models output mounds of different widths, and proper

location of the rover relative to the mound is crucial for computation of overburden rates.

The ratio computed here is ~0.93. For the final output of our models we identify

Yellowknife Bay at 0.93 of the distance between the closest rim and the foothill of the

output mound. Yellowknife Bay and Hidden Valley (~ Sol 722) have similar altitudes

and rim distances. The unconformity is ~1000 m higher than Yellowknife Bay: we then

conclude that ~ Sol 722 is represented by the location that is 1000 m higher than the

location set to be Yellowknife Bay (at the 0.93 ratio to the crater). and XX further from

the crater rim toward the interior of the crater. Results for the thermal history are

285

286

287

288

289

290

291

292

293

294

295

296

297

298

299

300

301

302

303

304

305

306

307

subsequently presented as a range of values between Yellowknife Bay and the

unconformity in Mt. Sharp. This range represents MSL’s future traverse range.

2.6 Key diagenetic thresholds

Once deposited, subsequent fluid circulation through sediments can lead to textural

changes as well as alteration of existing minerals and formation of new minerals.

Phyllosilicates, sulfates, iron oxides, and silica phases may form and/or undergo

mineralogic transitions, depending upon the chemistry of diagenetic fluids and

temperature. It is beyond the scope of this work to track all diagenetic transitions [for

review, see Mackenzie, 2005]; we instead focus on reporting modeled temperatures. For

reference, however, we do track three key diagenetic thresholds: the freezing point of

water at 0ºC, the illite to smectite conversion at ~40ºC, and the time-temperature integral.

Smectites are swelling clays that form during the reaction of water with silicates. At

elevated temperatures, however, smectites are no longer the thermodynamically favored

phase, and conversion of smectites to other phyllosilicate phases like illite or chlorite that

lack interlayer water begins. The temperature, the fluid composition, the amount of water

available, and burial duration are dictate resultant mineralogy. Given the presence of

smectite clay, either from in situ formation [Vaniman et al., 2014; Bristow et al., 2014] or

transport from a watershed with smectite clays [Ehlmann & Buz, accepted] in

sedimentary rocks at Gale crater, we track two thresholds for determining whether

smectite would be expected to have converted to another phase: a single temperature

threshold (40ºC) commonly cited for the smectite-illite conversion temperature [CITE]

308

309

310

311

312

313

314

315

316

317

318

319

320

321

322

323

324

325

326

327

328

329

330

and a time-temperature integral with thresholds based on the terrestrial clay mineral rock

record [Tosca & Knoll, 2009]. Tracking the 0ºC threshold also provides a useful

parameterization of water availability for alteration processes (if it is present). Fluids

may, of course, be available at lower temperatures due to freezing point depression from

dissolved salts.

3. Results

In this section we provide results for the scenario modeling erosion/deposition rates,

followed by temperature results for different scenarios and locations. , and Wwe conclude

by presenting the time-temperature integrals for the different models.

3.1 Topographic Evolution and Erosion/Deposition Rates

Figures 5a and 5d show the evolution of topography and overburden values for scenario

1, complete fill and exhumation, under the standard timing model; maximum diagenesis

and minimum diagenesis models have the same overall evolution, albeit with

temperatures achieved at different points in time. Figure 5 also shows the same

information for Sscenarios 2a (Figure 5b and 5e) and 2b (figure 5c and 5f). For the

topography variation the blue line represents the most ancient topography, while the red

line represents the most recent one. In Figure 5, a solid line represents Yellowknife Bay

while a dashed line represents the unconformity described in section 2.5.

331

332

333

334

335

336

337

338

339

340

341

342

343

344

345

346

347

348

349

350

351

352

If the complete filling and exhumation to present-day topography is assumed (Scenario

1), with the time period equally split into an interval of net erosion followed by interval

of net deposition, the average rates of erosion and deposition are 6-27 µm/yr and 8-34

µm/yr, respectively. Assuming a model where strong mound-flank slope winds are

capable of eroding the mound (Scenario 2), erosion and deposition rates were calculated

based on where elevation gradients were either negative (erosion) or positive

(deposition). For Scenario 2a average erosion rates fall between 5-21 µm/yr, while

average deposition rates range from 5-21 µm/yr. For Scenario 2b average erosion rates

are within 7-29 µm/yr while deposition rates range from 8-35 µm/yr. Table 2 summarizes

the obtained results.

3.2 Temperature

The results obtained in this section are based on coupling the topographic evolution of

Mt. Sharp with the thermal model for Mars’ changing geothermal gradient. Figure 6

shows the temperature variation with time for two locations (Yellowknife Bay and Mt.

Sharp), considering three different timing scenarios and three different sedimentary

models. Figures 6a, 6b and 6c show results for the different timing constraints

considering a cold early Mars (-50oC), while Figures 6d, 6e and 6f show the same results

but for a warm early Mars (0oC).

As a general observation, the timing has little effect on the peak temperatures achieved

and overall range of temperatures experienced by the sediments. It does, however,

significantly affect the time-temperature integral, which determines the expected degree

353

354

355

356

357

358

359

360

361

362

363

364

365

366

367

368

369

370

371

372

373

374

375

of diagenesis under Tosca & Knoll [2009]’s suggested approach to evaluating sediment

maturity. Our summary of results below assumes ground waters or ice were present. For

more on this issue, see section 4.2, discussion.

3.2.1 Cold Early Mars

For a cold early Mars, scenario 1 leads to conditions where water would be liquid and

illite could form from smectite everywhere from Yellowknife to Mt. Sharp. Maximum

temperatures reached are ~100ºC and ~70ºC at Yellowknife Bay and Mt. Sharp,

respectively (Figure 6a, 6b and 6c). Scenario 2a does not generate conditions above 0ºC;

there would be no alteration or diagenesis in situ unless driven by salty brines. Scenario

2a implies that swelling clays (if formed by another process and part of the sedimentary

deposit) should be the dominant clay from Yellowknife Bay and Mt. Sharp. Scenario 2b

predicts liquid water at Mt. Sharp that could facilitate diagenetic transitions, though

temperatures above 40ºC are not reached. Under Sscenario 2b, at Yellowknife Bay the

0ºC threshold is not reached, and only freezing point-depressed brines are permitted by

the temperature data.

Using the time-temperature integral as a metric of diagenesis (Figure 7a, 7b and 7c),

Scenario 1 predicts the complete transformation of smectite to illite at Yellowknife Bay

for the standard and maximum diagenesis models and at Mt. Sharp for the maximum

diagenesis model only. In all other Scenario 1 cases, transformation of smectite to illite

occurs, but is partial. Scenario 2b implies partial smectite to illite conversion at Mt.

376

377

378

379

380

381

382

383

384

385

386

387

388

389

390

391

392

393

394

395

396

397

Sharp, but not at Yellowknife Bay. Scenario 2a does not predict that any conversion

process occurs.

3.2.2 Warm Early Mars

For warm early Mars, liquid water that might cause alteration and diagenesis would be

available everywhere between Yellowknife Bay and Mt. Sharp under all scenarios

(Figures 6d, 6e and 6f). Maximum temperatures greater than 100ºC occur at both

Yellowknife Bay and Mt Shap in scenario 1. In both Sscenarios 2a and 2b, temperatures

above 40ºC are reached at Mt. Sharp, while in Sscenario 2b, this threshold is only

exceeded for Mt. Sharp.

Using the time-temperature integral, complete conversion of smectite to illite is predicted

for all scenarios and locations except for 2a at Yellowknife Bay. Surface temperatures at

0o C would imply that any smectites should not have persisted elsewhere from the base to

unconformity of Mt. Sharp. Plots are shown in Figures 7d, 7e and 7f.

4. Discussion

In this section we present a discussion based on the comparison of our results with the

prior work, including MSL Curiosity data, acquired to date. We start by discussing

predicted erosion and deposition rates. We then proceed to describe the current

knowledge regarding the presence of clays throughout MSL’s path, and predictions for

what the rover might encounter under each scenario.

398

399

400

401

402

403

404

405

406

407

408

409

410

411

412

413

414

415

416

417

418

419

420

4.1 Martian Deposition and Erosion Rates

Overall the calculated averages obtained in section 3.1 and summarized in Table 2 fall

near the ranges of 10-100 µm/yr for estimated Martian sedimentary deposition [Lewis &

Aharonson, 2014]. The calculated average ranges moderately exceed estimated erosion

rates of Noachian and Hesperian terrains, 0.7-10 µm/yr, but are at the lower end of

erosion rates from Earth, 2-100µm/yr [Golombek et al., 2006]. Notably, typical Martian

net erosion rates are not compatible with the growth and erosion of Mt. Sharp entirely

during the Hesperian, as suggested by surface ages based on crater counts; at ~1 µm/yr,

the erosion of 4 km of material would take 4 Gyr. Having erosion and deposition rates

falling within a reasonable range derived from the literature confirms the plausibility of

our assumptions and the models described in this work.

It should be noted that deposition/erosion under Scenario 2a and 2b is computed

somewhat differently than for Scenario 1 in which the erosion and deposition and equally

scaled to add and then remove the necessary materials over the specified time period.

While in Scenario 2, a discrete erosion rate is computed in every time step iteration (, the

Kite et al. [2013] model computes deposition rate using a constant value based on the

initial time step that does not change during the simulation, regardless of Mt Sharp form).

For scenario 2a, this assumption is used; however, with our choice of crater wall height, 5

km instead of 10 km in Kite et al. [2013], this leads to a thin central mound. For scenario

2b, we decrease the deposition rate linearly with time. Several behaviors of deposition

rate could be used here. For a distinct endmember, we choose one that yielded results

similar to what we see at Gale, e.g. a mound height and shape that matches modern

421

422

423

424

425

426

427

428

429

430

431

432

433

434

435

436

437

438

439

440

441

442

443

topography and that also steepens with time and retreats back from the wall toward the

center, a sequence originally proposed in Kite et al. [2013]. A step function could be

used here as well: a simulation of volcanic interference while the aeolian process happens

inside the crater. Our analysis with this case showed a final form similar to the constant

deposition case.

4.2 Mineralogical Implications from Ground Measurements and Modeling

4.2.1 Yellowknife Bay

Vaniman et al. [2014] identified trioctahedral smectites at Yellowknife Bay in the

samples John Klein and Cumberland. No illites were positively recognized. Interestingly,

XRD patterns indicate that the smectite interlayers in John Klein are collapsed, while

those in Cumberland are held open, perhaps by metal-hydroxides [Bristow et al., in

press]. This suggests an additional episode(s) of fluid interaction with the smectite clays

after formation. This is consistent with the overall history of the Yellowknife Bay

sedimentary rock which shows, raised ridges, nodules, and calcium sulfate veins inferred

to result from post-depositional processes [Grotzinger et al., 2014; Siebach et al., 2014;

Stack et al., 2014].

Our analysis of models for the temperatures experienced by the Yellowknife Bay

sediments indicates either Yellowknife Bay was either never buried by >1 km of fill and

mean Hesperian Mars surface temperatures were substantially below zero or water was

unavailable during most of Yellowknife Bay’s burial. Any scenario where Yellowknife

Bay is buried under 5 km of fill (Scenario 1) leads to conversion of smectite to illite if

444

445

446

447

448

449

450

451

452

453

454

455

456

457

458

459

460

461

462

463

464

465

466

water is available. The conversion is complete in all except the minimum diagenesis, T surf

= 50 C case. Scenarios where the crater is partially filled (Scenarios 2a, 2b; up to XX km)

do not predict any smectite to illite conversion if average mean surface temperatures were

substantially below 0ºC because, even with burial, subsurface temperatures do not exceed

0ºC. In a warmer early Mars, where Tsurf ≥ 0ºC, partial to complete conversion of smectite

to illite would be expected at Yellowknife Bay, even with partial burial, so long as water

is available.

Thus, if the deposition of the Yellowknife Bay sediments pre-dates Mt. Sharp formation,

a cold early Mars and slope wind model with relatively little burial is implicated.

However, the stratigraphic relationships between Yellowknife Bay sediments and other

units are ambiguous, and the deposit might instead represent a late-stage unit from Peace

Vallis alluvial activity emplaced atop materials left behind during Mt. Sharp’s retreat.

Continued analysis of orbital and in situ data to understand contact relationships is

crucial. Moreover, these data highlight the importance of analysis of the mineralogy of

Mt. Sharp sediments where the stratigraphic relationships are clear to determine the

environmental history of Gale crater deposits.

4.2.2. Implications for Diagenesis along the Traverse and Lower Mt. Sharp

As Curiosity continues its trek toward Mt Sharp, continued collection of mineralogic data

will test whether paleotemperatures were higher or lower in Mt. Sharp compared to

Yellowknife Bay and whether the smectite to illite conversion took place. Figure 8

provides maximum temperatures experienced sedimentary rocks at locations between

467

468

469

470

471

472

473

474

475

476

477

478

479

480

481

482

483

484

485

486

487

488

489

Yellowknife Bay and Mt. Sharp, considering different scenarios and timing scales.

Scenario 1 has a maximum temperature at every location that decreases from

Yellowknife Bay to Mt. Sharp. Scenario 2a has an increasing maximum temperature

across MSL’s traverse, and scenario 2b has a peak maximum temperature located

somewhere between Yellowknife Bay the Mt. Sharp unconformity, near an elevation of

2.6 km. These distinctive patterns provide crucial, testable constraints for the history of

sedimentary deposition because these patterns of temperature increase would be recorded

in the sediments, if there were waters within Gale crater during the Hesperian. If Gale

crater was completely filled then exhumed, diagenesis should decrease with distance up

Mt. Sharp, whereas incomplete fill under a slope wind model scenario would show

increased diagenesis toward the center part of the mound. If temperature can also be

discerned from diagenetic mineral assemblages, these constrain Hesperian surface

temperatures, and thus paleoclimate, at Gale crater.

5. Conclusions

Understanding the conditions that led to sediment deposition, erosion, and secondary

mineral formation in Gale crater advances our understanding of the evolution of

environmental conditions on ancient Mars. We developed a coupled sedimentation-

thermal model that traced temperature as a function of time for two different scenarios: 1)

complete fill of Gale crater followed by partial erosion and 2) partial fill dictated by slope

winds as described by Kite et al., [2013]. Determining maximum temperatures of

sediments and computation of time-temperature integrals provides a set of distinctive

predictions for the extent of diagenesis, testable by what we are seeing today by the MSL

490

491

492

493

494

495

496

497

498

499

500

501

502

503

504

505

506

507

508

509

510

511

512

mission. Our models have erosion and deposition rates within expected Martian

deposition and erosion values. We find that in the presence of water during Mt. Sharp

formation and erosion, the smectite to illite conversion should have occurred at all levels

in lower Mt. Sharp if Gale crater had been completely filled. Moreover, either

Yellowknife Bay was never buried by >1km of overburden, or there was insufficient

water available for diagenesis when it was buried, due to either aridity or Hesperian mean

annual surface temperatures well below zero. Complete fill of Gale crater means the

rover should observe decreasing diagenesis as it climbs Mt. Sharp, whereas a slope wind

model [Kite et al., 2013] would predict increasing diagenesis as the rover climbs lower

Mt. Sharp. Characterization of diagenetic mineral assemblages observed by Mt. Sharp as

well as their spatial pattern along the rover traverse thus collectively can reveal the

paleoenvironmental and sedimentary histories of Gale crater’s Mt. Sharp.

Acknowlegements

We thank Edwin Kite for sharing his Matlab code for the wind slope model for

generation of central mounds of discrete widths. This work was partially funded by an

MSL Participating Scientist grant to B.L.E. The Caltech Summer Undergraduate

Research Fellowship program provided programmatic support to C.B.

References

Anderson, R. C., and J. F. Bell III (2010), Geologic mapping and characterization of Gale

Crater and implications for its potential as a Mars Science Laboratory landing

site, Mars, 5, 76–128, doi:10.1555/mars.2010.0004.

513

514

515

516

517

518

519

520

521

522

523

524

525

526

527

528

529

530

531

532

533

534

Beardsmore, G. R., & Cull, J. P. (2001). Crustal heat flow: A guide to measurement and

modelling Cambridge University Press.

Boyce, J. M., & Garbeil, H. (2007). Geometric relationships of pristine martian complex

impact craters, and their implications to mars geologic history. Geophysical

Research Letters, 34(16)

Bristow, T.F. et al., The origin and implications of clay minerals from Yellowknife Bay,

Gale crater, Mars. American Mineralogist, in press

Carter, J., F. Poulet, J.-P. Bibring, N. Mangold, and S. Murchie (2013), Hydrous minerals

on Mars as seen by the CRISM and OMEGA imaging spectrometers: Updated global

view, J. Geophys. Res. Planets, 118, doi: 10.1029/2012JE004145.

Ehlmann, B.L. and J. Buz. Mineralogy and Fluvial History of the Watersheds of Gale,

Knobel, and Sharp craters: A Regional Context for MSL Curiosity’s Exploration,

Geophysical Research Letters, accepted

Ehlmann, B. L., et al. (2009), Identification of hydrated silicate minerals on Mars using

MRO-CRISM: Geologic context near Nili Fossae and implications for aqueous

alteration, J. Geophys. Res., 114, E00D08, doi:10.1029/2009JE003339.

Ehlmann, B. L., J. F. Mustard, S. L. Murchie, J.-P. Bibring, A. Meunier, A. A. Fraeman,

and Y. Langevin (2011a), Subsurface water and clay mineral formation during the

early history of Mars, Nature, 479, 53–60, doi:10.1038/nature10582.

535

536

537

538

539

540

541

542

543

544

545

546

547

548

549

550

551

552

553

Ehlmann, B. L., J. F.Mustard, R. N. Clark, G.A. Swayze, and S. L.Murchie (2011b),

Evidence for low-grade metamorphism, hydrothermal alteration, and diagenesis on

mars from phyllosilicate mineral assemblages, Clays Clay Miner., 590(4), 359–377,

doi:10.1346/CCMN.2011.0590402.

Fraeman, A., et al. (2013). A hematite-bearing layer in gale crater, mars: Mapping and

implications for past aqueous conditions. Geology, 41(10), 1103-1106.

Garvin, J., Sakimoto, S., & Frawley, J. (2003). Craters on mars: Global geometric

properties from gridded MOLA topography. Sixth International Conference on

Mars,  , 1 3277.

Golombek, M., et al. (2006). Erosion rates at the mars exploration rover landing sites and

long‐term climate change on mars. Journal of Geophysical Research: Planets

(1991–2012), 111(E12)

Grant, J. A., et al. (2011). The science process for selecting the landing site for the 2011

mars science laboratory. Planetary and Space Science, 59(11), 1114-1127.

Grotzinger, J. P., et al. (2012). Mars science laboratory mission and science

investigation. Space Science Reviews, 170(1-4), 5-56.

Grotzinger, J. P., Sumner, D. Y., Kah, L. C., Stack, K., Gupta, S., Edgar, L. et al. (2014).

A habitable fluvio-lacustrine environment at Yellowknife Bay, Gale Crater,

mars. Science (New York, N.Y.), 343(6169), 1242777. doi:10.1126/science.1242777.

554

555

556

557

558

559

560

561

562

563

564

565

566

567

568

569

570

571

572

Harvey H. R., J.H. Tuttle, J.T. Bell (1995) Kinetics of phytoplankton decay during

simulated sedimentation: Changes in biochemical composition and microbial activity

under oxic and anoxic conditions. Geochim. Cosmochim. Acta 59(16), 3367–3377.

Kalynn, J., Johnson, C. L., Osinski, G. R., & Barnouin, O. (2013). Topographic

characterization of lunar complex craters. Geophysical Research Letters, 40(1), 38-

42.

Kite, E. S., Lewis, K. W., Lamb, M. P., Newman, C. E., & Richardson, M. I. (2013).

Growth and form of the mound in gale crater, mars: Slope wind enhanced erosion

and transport. Geology, 41(5), 543-546.

Le Deit, L., E. Hauber, F. Fueten, M. Pondrelli, A. P. Rossi, and R. Jaumann (2013),

Sequence of infilling events in Gale Crater, Mars: Results from morphology,

stratigraphy, and mineralogy, J. Geophys. Res. Planets, 118, 2439–2473,

doi:10.1002/2012JE004322.

Lehmann, M.F., S.M. Bernasconi, Barbieri, J.A. McKenzie (2002), Preservation of

organic matter and alteration of its carbon and nitrogen isotope composition during

simulated and in situ early sedimentary diagenesis. Geochimica et Cosmochimica

Acta, 66(20), 3573-3584.Lewis, K. W., & Aharonson, O. (2014). Occurrence and

origin of rhythmic sedimentary rocks on mars. Journal of Geophysical Research:

Planets, 119, 1432–1457, doi:10.1002/2013JE004404.

Mackenzie, F. T., ed. (2005). Sediments, Diagenesis, and Sedimentary Rocks: Treatise

on Geochemistry, Second Edition, Volume 7, Elsevier, Oxford, UK, 446 pp.

573

574

575

576

577

578

579

580

581

582

583

584

585

586

587

588

589

590

591

592

593

Malin, M. C., & Edgett, K. S. (2000). Sedimentary rocks of early

mars. Science, 290(5498), 1927-1937.

Milliken, R. E., and D. L. Bish (2010), Sources and sinks of clay minerals on Mars.

Philos. Mag., 90, 2293–2308, doi:10.1080/14786430903575132.

Milliken, R. E., J. P. Grotzinger, and B. J. Thomson (2010), Paleoclimate of Mars as

captured by the stratigraphic record in Gale Crater, Geophys. Res. Lett., 37, L04201,

doi:10.1029/2009GL041870.

Milliken, R. E., R. C. Ewing, W. W. Fischer, and J. Hurowitz (2014), Wind-blown

sandstones cemented by sulfate and clay minerals in Gale Crater, Mars, Geophys.

Res. Lett., 41, 1149–1154, doi:10.1002/2013GL059097.

Morschhauser, A., Grott, M., & Breuer, D. (2011). Crustal recycling, mantle dehydration,

and the thermal evolution of mars.  Icarus, 212(2), 541-558.

Pelkey, S.M., B.M., Jakosky, P.R. Christensen (2004) Surficial properties in Gale Crater,

Mars, from Mars Odyssey THEMIS data. Icarus, 167, 244-270.

Rampe, Am Min

Robbins, S. J., & Hynek, B. M. (2012). A new global database of mars impact craters≥ 1

km: 1. database creation, properties, and parameters. Journal of Geophysical

Research: Planets (1991–2012), 117(E5)

594

595

596

597

598

599

600

601

602

603

604

605

606

607

608

609

610

611

Siebach, K. L., and J. P. Grotzinger (2014), Volumetric estimates of ancient water on

Mount Sharp based on boxwork deposits, Gale Crater, Mars, J. Geophys. Res.

Planets, 119, 189–198, doi:10.1002/2013JE004508.

Siebach, K. L., and J. P. Grotzinger (2014), Volumetric estimates of ancient water on

Mount Sharp based on boxwork deposits, Gale Crater, Mars, J. Geophys. Res.

Planets, 119, 189–198, doi:10.1002/2013JE004508.

Stack, K. M., et al. (2014), Diagenetic origin of nodules in the Sheepbed member,

Yellowknife Bay formation, Gale crater, Mars, J. Geophys. Res. Planets, 119,

1637–1664, doi:10.1002/2014JE004617.

Thomson, B., et al. (2011). Constraints on the origin and evolution of the layered mound

in Gale crater, Mars using Mars Reconnaissance Orbiter data.  Icarus,214(2), 413-

432.

Tosca, N. J., & Knoll, A. H. (2009). Juvenile chemical sediments and the long term

persistence of water at the surface of mars. Earth and Planetary Science

Letters, 286(3), 379-386.

Vaniman, D. T., Bish, D. L., Ming, D. W., Bristow, T. F., Morris, R. V., Blake, D. F. et

al. (2014). Mineralogy of a mudstone at yellowknife bay, gale crater, mars. Science

(New York, N.Y.), 343(6169), 1243480. doi:10.1126/science.1243480 [doi]

Wray, J. J. (2012), Gale crater: The Mars Science Laboratory/Curiosity Rover Landing

Site, Int. J. Astrobiol., doi:10.1017/S1473550412000328.

612

613

614

615

616

617

618

619

620

621

622

623

624

625

626

627

628

629

630

631

Werner, S., & Tanaka, K. (2011). Redefinition of the crater-density and absolute-age

boundaries for the chronostratigraphic system of mars.  Icarus, 215(2), 603-607.

Tables

Timing Scenario

Deposition start (Ga)

Erosion End (Ga)

Calc. Avg. Net Erosion/Deposition

Rate (µm/yr)

Erosion/Deposition Rates Used (µm/yr)

Scen. 1 Scen. 2a Scen. 2b(1) Standard

model 3.7 3.3 13.5 / 16.9 8 / 42 11/11 15/17

(2) Maximum diagenesis 3.85 3.0 6.4 / 7.9 6 / 8 5/5 7/8

(3) Minimum diagenesis 3.6 3.4 27.0 / 33.7 27 / 34 21/22 29/35

Table 2. Scenarios for the timing of Mt. Sharp formation and consequent inferred rates of erosion/deposition under different sedimentation scenarios

Parameters Kite et al. (2013) Scenario 2a Scenario 2bα 3 3 3

D’ 5Linear variation from D’0 = 2.47

to D’ = 00.4

k 0.005 0.01 0.01U0 0 0 0

R/L 2.49 2.49 2.4

Table 1. Parameters comparing the model used by Kite et al. (2013) and the ones used here.

632

633

634

635

637

638

639

640

Figure Captions

Figure 1. (A) Overview of 154-km diameter Gale crater (137.4oE, -4.6oN). The star

indicates MSL’s Bradbury landing site. Aeolis Mons (Mt. Sharp) is located on center of

the figure. Two scenarios for Mt. Sharp’s formation are analyzed in this paper: (B)

complete fill of the crater to the rim, followed by partial exhumation; (C) the Kite et al.,

[2013] model of wind slopes creating feedback with the sides of the crater, enabling

mound formation in the center with only incomplete fill.

Figure 2. Ancient Gale profile (solid black line) is compared with the modern (dashed

gray line) and average (solid gray line) Gale profiles. The ancient Gale profile represents

the initial state of the model for every scenario. Here we try to explain how the ancient

Gale profile turned out to be the modern Gale profile.

Figure 3. Depth to temperature relation considering two different early Mars

temperatures (0oC and -50oC) as well as four period of time (Noachian/Hesperian, Early

Amazonian, Late Amazonian and Modern).

Figure 4. Bradbury Landing, Yellowknife Bay (YKB), Hidden Valley and a potential

future MSL location (an unconformity at Mt. Sharp) are identified in the figure. The

change in altitude from Yellowknife Bay to the unconformity is of 1000 m. One should

also notice that the distance from Yellowknife Bay to the Hidden Valley, longitudinally

Timing Scenario

Deposition start (Ga)

Erosion End (Ga)

Calc. Avg. Net Erosion/Deposition

Rate (µm/yr)

Erosion/Deposition Rates Used (µm/yr)

Scen. 1 Scen. 2a Scen. 2b(1) Standard

model 3.7 3.3 13.5 / 16.9 8 / 42 11/11 15/17

(2) Maximum diagenesis 3.85 3.0 6.4 / 7.9 6 / 8 5/5 7/8

(3) Minimum diagenesis 3.6 3.4 27.0 / 33.7 27 / 34 21/22 29/35

Table 2. Scenarios for the timing of Mt. Sharp formation and consequent inferred rates of erosion/deposition under different sedimentation scenarios

641

642

643

644

645

646

647

648

649

650

651

652

653

654

655

656

657

658

659

660

661

662

speaking, is small when compared to the distance from Yellowknife Bay to the

unconformity.

Figure 5. Topography variation and overburden rates for scenario 1 (Figures 5a and 5d),

scenario 2a (Figures 5b and 5e) and scenario 2b (Figures5c and 5f) are presented here.

Figure 6. Temperature as a function of timing scenarios for sedimentary models 1, 2a

and 2b considering an early mean surface temperature of -50oC and 0oC. Solid lines

represent location at Yellowknife Bay; dashed lines represent location at Mt. Sharp.

Figure 7. Time-temperature integral as a function of timing scenarios for sedimentary

models 1, 2a and 2b considering an early mean surface temperature of -50oC and 0oC.

Solid lines represent location at Yellowknife Bay; dashed lines represent location at Mt.

Sharp.

Figure 8. Maximum and minimum temperatures experienced by MSL’s traverse

considering different sedimentary scenarios and timescales for cold and warm early Mars.

663

664

665

666

667

668

669

670

671

672

673

674

675

676

677

678

679

680

681

682

683

684