For Submission to the Journal of Geophysical Research Planets

1 Last edited December 28, 2014 2 For submission to the Journal of Geophysical Research – Planets 3 Modeling the Thermal and Physical Evolution of Mount Sharp’s Sedimentary 4 Rocks, Gale Crater, Mars: Implications for Diagenetic Minerals on the MSL 5 Curiosity Rover Traverse67 Caue S. Borlina1,2, and Bethany L. Ehlmann2,389 1 Department of Atmospheric, Oceanic and Space Sciences, University of Michigan, Ann 10 Arbor, Michigan, USA11 2 Division of Geological and Planetary Sciences, California Institute of Technology, 12 Pasadena, California, USA13 3 Jet Propulsion Laboratory, California Institute of Technology, Pasadena, California, 14 USA1516 Corresponding author: [email protected]; 2215 Space Research Building, 2455 17 Hayward St., Ann Arbor, MI 481091819 Keywords: burial diagenesis, sedimentation, erosion, heat flow, Gale Crater, Mars 20 Science Laboratory (MSL)212223 24 Abstract25 Gale Crater, landing site of the NASA Mars Science Laboratory (MSL) mission, has a26 unique sedimentary stratigraphy, preserved in a 5 km high mound called Aeolis Mons27 (informally known as Mt. Sharp). Understanding the processes of Mt. Sharp28 sedimentation, erosion, and secondary mineral formation can reveal important29 information regarding past geologic processes, the climate and habitability of early Mars,30 and the organic preservation potential of these sedimentary deposits. Two endmember31 scenarios for Mt. Sharp's formation are: (1) a complete filling of Gale crater followed by32 partial removal of deposited sediments or (2) building of a central deposit with33 morphology controlled by slope winds and only incomplete sedimentary fill of Gale34 crater. Here we consider depths of burial predicted for both scenarios, coupled with a35 thermal model to predict the temperature history of particular sediments presently36 exposed at the surface. We compare our results with chemical and mineralogical analyses37 by MSL to date and provide sedimentation scenario-dependent predictions of diagenetic38 mineralogy in locations between Yellowknife Bay and upper Mt. Sharp. Results show39 modeled erosion and deposition rates range from 5-42 μm/yr across different scenarios40 and timing scales, consistent with or slightly higher than rates calculated for other41 locations on Mars. Mineralogical predictions for locations close to Yellowknife Bay vary42 with the choice of a cold or warm early Mars and the filling scenario. For (1),43 temperatures experienced by sediments should decrease monotonically over the traverse44 and up Mt. Sharp stratigraphy, whereas for (2) maximum temperatures are reached in the45 lower units of Mt. Sharp and thereafter decline or hold roughly constant. If surface46 temperatures on early Mars were similar to today, (1) predicts temperatures experienced 47 by sediments ranging from 50-100°C while for (2) only specificelect scenarios permit48 diagenetic fluids (T>0°C) at select locations. In contrast, if surface temperatures on early49 Mars were near the melting point of water, diagenetic temperatures were achieved over50 the entire Curiosity traverse with temperatures ranging from 100-150°C under scenario51 (1) and 0-100°C for variations of scenario (2). Under nearly all scenarios, evidence of52 diagenesis at higher temperatures is expected as MSL moves towards Mt. Sharp.53 Comparing our predictions with future MSL results on secondary mineral assemblages,54 their spatial variation as a function of location on the traverse, and age of any authigenic55 phases will constrain the timing and timescale of Mt. Sharp formation, paleosurface56 temperature, and the availability and setting of liquid water on early Mars. 5758 59 1. Introduction60 The study of sedimentation processes at specific regions on Mars helps time order61 aqueous mineral formation and defines the ancient geologic history of the red planet.62 Such analysis can reveal important information about past habitability, presence of water63 at or near the surface of the planet, and potential for the long-term preservation of organic64 materials. Gale crater (137.4oE, -4.6oN), the landing site of the NASA Mars Science65 Laboratory (MSL) mission, has a unique sedimentary stratigraphy, which permits66 examining ancient Martian environmental conditions and aqueous alteration. Recent67 studies have shown that Gale once hosted a fluvial-lacustrine environment, capable of68 supporting life [Grotzinger et al., 2014]. The stratigraphic rock record of the area, and69 thus its geologic history, is preserved in a 5 km high mound called Aeolis Mons70 (informally, Mt. Sharp) located in Gale crater [Grotzinger et al., 2012] (Figure 1a). 7172 Materials comprising Mt. Sharp have a low thermal inertia and subhorizontally layered73 units, which implicate a sedimentary origin [Pelkey et al., 2004; Anderson & Bell, 2010;74 Thomson et al., 2011]. More recent work has suggested the lowermost units are cross-75 bedded sandstones formed by cementation and lithification of sand dunes [Milliken et al.,76 2014]. The lower mound includes distinctive sedimentary beds with hydrated sulfates,77 iron oxides and Fe/Mg smectite clay minerals [Millken et al., 2010; Fraeman et al.,78 2014]. Spectral signatures associated with the upper mound do not permit unique79 identification of secondary mineral phases, though eroded boxwork structures suggest80 precipitation of minerals during fluid flow within the sediments [Siebach and Grotzinger,81 2014]. 8283 Many hypotheses have been developed to explain Mt Sharp formation, variously84 invoking airfall dust or volcanic ash, lag deposits from ice/snow, aeolian and85 fluviolacustrine sedimentation [for review, see Anderson & Bell, 2010; Wray, 2013;86 LeDeit et al., 2013]. Nevertheless, regardless of the process(es) delivering sediments, two87 endmember scenarios describe the time-evolution, i.e., growth and subsequent erosion, of88 Mt. Sharp. In Scenario 1, Gale Crater was completely filled with layered sediments then89 partially exhumed, leaving a central mound [Malin and Edgett, 2000] (Figure 1b). In90 Scenario 2, an aeolian process characterized by slope winds created a wind-topography91 feedback enabling growth of a high mound without complete fill of the crater [Kite et al.,92 2013] (Figure 1c). 9394 The history of Mt. Sharp’s sedimentation and mineralization provides key constraints on95 environmental conditions on early Mars, including the availability of liquid water and the96 nature of geochemical environments. Understanding sediment deposition and possible97 diagenesis is also crucial to establishing the preservation potential through time for98 organic carbon of biological or abiotic origin trapped in sedimentary rock strata. The99 thermal history of sediments and their exposure to fluids exerts strong control on the100 persistence of organic compounds in the sedimentary record (e.g., Harvey et al., 1995;101 Lehmann et al., 2002). Initial studies of the diagenesis of Martian sediments pointed out102 the apparent ubiquity of the “juvenile” sediments with smectite clays, amorphous silica103 and a paucity of evidence for illite, chlorite, quartz and other typical products of104 diagenesis, which are common in the terrestrial rock record, thus concluding that 105 diagenetic processes on Mars were uncommon, perhaps limited by water availability106 [Tosca & Knoll, 2009]. Since then, a growing number of studies have identified clay107 minerals such as illite, chlorite, and mixed layer clays that can form from via diagenesis108 [Ehlmann et al., 2009, 2011a, 2011b; Milliken & Bish, 2010; Carter et al., 2013]. So far,109 minerals identified in Mt. Sharp from orbit do not include these phases. However, in situ110 rover data at Yellowknife Bay implies at least early diagenetic reactions to form111 mineralized veins, nodules, and filled fractures within the mudstones [Stack et al., 2014;112 Siebach et al., 2014], including exchange of interlayer cations in smectite clays or113 incipient chloritization [Vaniman et al., 2014; Rampe et al., 2014; Bristow et al., 2014]. 114115 Here we model the potential diagenetic history of the sediments comprising Mt. Sharp116 and accessible in rock units along Curiosity’s traverse. We couple the two sedimentation117 scenarios [Malin and Edgett, 2000; Kite et al., 2013] with a thermal model for ancient118 Martian heat flow and timescales for Mt. Sharp sedimentary deposition and erosion119 constrained by crater counts. We model temperature variations experienced within the120 region between Yellowknife Bay, the base of Mt. Sharp, and the lower unit/upper unit121 Mt. Sharp unconformity, and we compare them with select key temperature thresholds122 relevant for diagenesis: stability of liquid water (0oC) and the a temperature for efficient123 smectite to illite conversion (~40oC). We also analyze the time-temperature integral, an124 alternative method for establishing the likelihood and extent of mineral diagenesis as125 described by Tosca and Knoll [2009]. The final results are compared to mineralogical and126 chemical findings obtained by MSL to date. Potential mineralogical findings across127 MSL’s traverse are discussed and correlated with implications for sedimentation 128 processes on Mars, timescales, early Mars temperatures, liquid water availability, and the129 organic preservation potential of Gale sediments. 130131 2. Methodology132 In this section we describe how we defined the pristine Gale basement and final Mt.133 Sharp profile, the two different endmember sedimentation scenarios and variations in134 parameters used during the modeling, timescales and timing of sedimentation assumed,135 the derivation of the thermal model for surface heat flow, and the topographic profile of136 the rover traverse that defines the studied region. 137138 2.1 Pristine Gale Basement & Modern Topography139 In order to evaluate how Gale Crater changed we needed to first determine the ancient140 (starting point) and modern (ending point) topographies of the crater. Gale crater has141 been both eroded and filled relative to its original topographic profile. Consequently, a142 combination of theoretical constraints and observational constraints from the Mars143 Orbiting Laser Altimeter Altitude (MOLA) dataset was used to set the initial conditions144 for Gale's crater shape, i.e. its initial topographic profile. Observed crater depth-diameter145 relationships on Mars predict a range of initial crater depths for Gale crater (D ~154 km)146 ranging from 4.2 km to 5.4 km [Garvin et al., 2003; Boyce and Garbeil, 2007; Robbins147 and Hynek, 2007]. Kalynn et al. [2013] empirically have shown Martian central peak148 heights of ~1 km for ~100 km craters. Our examination of craters on Mars, better149 preserved than Gale, with diameters ranging from 131 km to 155 km (at 16ºW, 43ºS;150 36ºW, 36ºS; 45ºE, 42ºN) yielded lower bounds on central peak height ranging from 1.0- 151 2.1 km that did not scale in a straightforward way with diameter, and these central peak152 heights may be underestimates due to later crater fill. Hence, we set the initial shape of153 Gale Crater to be 154 km in diameter and 5 km deep with a central peak height of 1.55154 km. In order to have realistic slopes, we scaled the average topographic profile from155 Moreux crater (45E, 42N) to fit Gale's parameters. 156157 Modern Gale crater has a highly asymmetric central mound, its cross-sectional profile158 varying with azimuth. The average Gale profile shown (Figure 2) was compiled from159 multiple roughly NW-SE cross-sections of present-day crater topography and then160 averaged after removing local geologic features. The average Gale profile is used for161 estimation of an average overburden over the Curiosity rover traverse. Scenarios 2a and162 2b were tuned to produce final mounds of width approximately equivalent to the width of163 the average profile. 164 165 2.2 Mt. Sharp Sedimentation Scenarios166 We develop two geological scenarios for the time evolution of Mt. Sharp filling/removal:167 (1) a complete filling of the crater followed by partial removal leaving a central mound,168 [Malin and Edgett, 2000]; and (2) an aeolian process, where the mound only grew near169 the center of the crater with continuous presence of strong mound-flank slope winds170 capable of eroding the mound and keeping it away from the sides of the crater [Kite et al.,171 2013]. 172173 2.2.1 Scenario 1 174 Scenario 1 is characterized by a complete fill of the crater to the height of the highest175 peak of Mt. Sharp, followed by partial erosion of the crater, leaving the modern shape of176 Mt. Sharp as final output. We use an average Gale profile as the final shape (see section177 2.1). This scenario is strongly dependent on timescales defined in section 2.3.178 Sedimentation and erosion rates are computed linearly based on the defined time period179 for erosion/sedimentation processes and necessary burial/erosion height, i.e., deposition180 rate is computed as distance from pristine basement to the top of the crater, divided by181 deposition time. Erosion rate computed as distance from top of the crater (completely182 filled crater) to the average modern profile, divided by erosion time. 183184 2.2.2 Scenario 2185 Scenario 2 is an aeolian process where the mound grew close to the center of the crater186 with the surrounding topography creating an environment for the presence of strong187 mound-flank slope winds capable of eroding the mound. We use the model and code188 developed by Kite et al. [2013] to generate the evolution of the topographic profile with189 time. Briefly, in Kite et al.’s model a saltation model approximation is used to determine190 the balance between the deposition D (set at the beginning of the simulation), and the191 erosion E (time varying). The result is the computation of dz/dt (altitude variation over192 time) for every time step. The following set of equations define the scenario(1) 193(2) 194(3) 195 196 where k is an erodibility factor; α is a parameter responsible for sand transport, soil197 erosion and rock abrasion on the crater; U is a unitless dimensionless expression related198 to the magnitude of the shear velocity (dx’, a mesh unit in Kite et al., is 1i.e. at t = 0 the199 basement is represented by a mesh with spacing dx’ of unit value); U0 is the component200 of background bed shear velocity; z’ is the height; x is the location within the crater (0 <201 x < 154 km, the crater diameter); x’ is the distance half-way between the values of x202 starting at x = 1.5 km and ending at x = 153.5 km; and L is a length scale that accounts for203 the effects of inertia. A special parameter R/L is used which correlates the width of the204 crater wall to the crater size. For small craters we should expect small values of R/L,205 around 0.1-1 (for a complete analysis of R/L verify the supplementary material from Kite206 et al., [2013]). 207208 Is important to notice that . U and z’ are tiime varying values. Eq. (3) is evaluated for209 each time step considering left and right slopes of the winds: for a value of x, we compute210 the integral for left slopes (values less than x) and right slopes (values greater than x and211 less than 154 km). The operator max[ ] selects the slope with the highest value, which212 then permits evaluation of Eq. (2) at each time step. Eq. (1) is also then evaluated in each213 timestep, using E from Eq. (2) and a value for D determined from the user-set parameter214 D’ (D=D’E0, where E0 is the initial erosion at the start of the simulation)). In Kite et al.215 [2013] and our Scenario 2a, D is fixed in the first time step and is constant thereafter. In216 our Scenario 2B, D is set initially then declines linearly. 217 We experimented with multiple choices of parameter combinations and choose to218 work with two different sets of parameters that yield topographic profiles most similar to 219 Gale. Parameters used by Kite et al. [2013] and this study are given in Table 1. We name220 the two different scenarios as 2a and 2b. Scenario 2a is defined with a constant deposition221 rate. The mound grows tall with a relatively constant width over time. Scenario 2b has a222 linearly decreasing deposition rate over time, and the mound grows wide then steepens223 and narrows. Both scenarios converge on a similar final output though the amount of224 sediment overburden as a function of time depends on the intermediate steps. The output225 has no time dependency; we implement timescales in the final output of the model by226 scaling the output to our specified durations after iterations converged. A special227 parameter R/L is introduced at this point, which correlates the width of the crater wall to228 the crater size. For small craters we should expect small values of R/L, around 0.1-1 (for229 a complete analysis of R/L verify the supplementary material from Kite et al., [2013]). 230231 2.3 Timescales232 Crater counts on the ejecta blanket of Gale crater constrain its formation age to Late233 Noachian/Early Hesperian, approximately 3.8 to 3.6 Ga, and place an upper limit on the234 period of infilling Mt. Sharp sediments [Thomson et al., 2011; Le Deit et al., 2013].235 Similarly, an upper limit to the age and extent of lower Mt. Sharp can be obtained using236 the superposition relationship of the topographically lower but stratigraphically higher237 deposits of Aeolis Palus, which have estimated ages ranging from early Hesperian to238 early Amazonian [Thomson et al., 2011; Le Deit et al., 2012; Grant et al., 2012], i.e.,239 from ~3.2 to ~3.5 Ga. Thus, most of the formation and erosion of Mt. Sharp to its present240 extent took place during the Hesperian, though processes continued to shape the form of241 the mound during the Amazonian. 242243 While surface ages based on crater counts are superb for relative age dating, pinning in244 absolute time is challenged by the existence of different chronology models relating the245 density of craters with time [e.g., Werner & Tanaka, 2011]. Yet, numerical ages246 constraining the start and end of major episodes of Mt. Sharp erosion and deposition are247 required to tie burial history to models of the secular cooling of Mars (section 2.4).248 Consequently, we examine three different temporal scenarios for the fill and exhumation249 of Mt. Sharp: (1) a standard model, (2) a maximum diagenesis model, and (3) a minimum250 diagenesis model. For (1), Mt. Sharp formation begins at 3.7 Ga, reaches 5 km in height,251 and is then exhumed to reach approximately its present extent by 3.3 Ga. For (2), Gale252 crater and Mt. Sharp form early, 3.85 Gyr, and Mt. Sharp is exhumed late, 3.0 Gyr, thus253 providing a maximum for heat flow and duration of burial. For (3), Mt. Sharp forms late,254 3.6 Gyr and is quickly exhumed by 3.4 Gyr. 255256 2.4 Thermal Model257 The thermal model used here defines temperature as a function of depth and time. We258 construct it by using the one-dimensional steady-state heat conduction solution that259 describes the temperature T as a function of the depth, z, and time, t, as(1) 260 where T0 is mean surface temperature, q(t) is the heat flow as a function of time, k is the261 thermal conductivity, ρ is the density and H(t) the heat production as a function of time.262 We define ρ = 2500 kg/m3, k = 2 W/mC, values typical for average sedimentary rocks on263 Earth [Beardsmore and Cull, 2001]. 264 The mean surface temperature for early Mars is still an unknown. Here we adopt o o 265 two possibilities: T0 = 0 C and T0 = -50 C in order to analyze how different values of T0266 can impact the results. The first presumes a warmer early Mars where temperatures267 routinely exceed the melting point of water during large portions of the Martian year; the268 latter represents the modern-day average equatorial temperature. Finally, q(t) and H(t)269 were estimated by curve-fitting to geophysical models for the evolution of heat flow270 [Parmentier and Zuber, 2007] and crustal heat production, respectively, through time271 [Hahn et al., 2011; their Figure 2]. Results for q(t) and H(t) are shown in Figures 3a and272 3b. Parmentier and Zuber [2007] estimated values for q to be ~60mW/m2 at 3.5 Gyr for a273 variety of cooling scenarios and 20mW/m2 at present; measurements of modern heat flow274 will soon be obtained by the InSight mission.. These values are similar to those275 independently predicted by Morschhauser et al. [2011]. Figure 3c shows derived276 temperatures as a function of depth for the two different mean surface temperatures and277 several time periods. 278279 Gale may have had additional heating from hydrothermal sources such as residual280 heat following the Gale-forming impact or local volcanic sources, but we do not include281 these in our modeling. Were additional sources of heat present, heat flow would be higher282 and temperatures higher. Thus our estimates for temperatures reached are lower bounds.283284 2.5 Yellowknife Bay to Mt. Sharp: MSL’s Traverse285 After landing, MSL headed toward Yellowknife Bay, a region in the opposite direction of286 the expected path toward Mt. Sharp. The rocks at Yellowknife Bay preserve evidence for 287 a fluvio-lacustrine environment [Grotzinger et al., 2014] and several minerals related to288 aqueous alteration, including Mg smectites and hydrated calcium sulfates, were identified289 [Vaniman et al., 2014]. 290291 Given sections 2.1-2.4 above, we model the sedimentary and thermal history along the292 Curiosity traverse, obtained between Yellowknife Bay, the Murray buttes entrance to Mt.293 Sharp, and predicted future locations of MSL Curiosity after climbing Mt. Sharp. Figure294 4 shows Bradbury Landing, Yellowknife Bay, Hidden Valley and a potential future final295 location of MSL. We picked the final future location for modeling to be the296 unconformity, marking the boundary between upper and lower Mt Sharp. In order to297 locate Yellowknife Bay in our model, we compute the ratio between the distance of the298 closest rim to Yellowknife Bay and the distance of the rim to the foothill of Mt. Sharp.299 This is necessary because different models output mounds of different widths, and proper300 location of the rover relative to the mound is crucial for computation of overburden rates.301 The ratio computed here is ~0.93. For the final output of our models we identify302 Yellowknife Bay at 0.93 of the distance between the closest rim and the foothill of the303 output mound. Yellowknife Bay and Hidden Valley (~ Sol 722) have similar altitudes304 and rim distances. The unconformity is ~1000 m higher than Yellowknife Bay: we then305 conclude that ~ Sol 722 is represented by the location that is 1000 m higher than the306 location set to be Yellowknife Bay (at the 0.93 ratio to the crater). and XX further from307 the crater rim toward the interior of the crater. Results for the thermal history are308 subsequently presented as a range of values between Yellowknife Bay and the309 unconformity in Mt. Sharp. This range represents MSL’s future traverse range. 310311 2.6 Key diagenetic thresholds312 Once deposited, subsequent fluid circulation through sediments can lead to textural313 changes as well as alteration of existing minerals and formation of new minerals.314 Phyllosilicates, sulfates, iron oxides, and silica phases may form and/or undergo315 mineralogic transitions, depending upon the chemistry of diagenetic fluids and316 temperature. It is beyond the scope of this work to track all diagenetic transitions [for317 review, see Mackenzie, 2005]; we instead focus on reporting modeled temperatures. For318 reference, however, we do track three key diagenetic thresholds: the freezing point of319 water at 0ºC, the illite to smectite conversion at ~40ºC, and the time-temperature integral.320321 Smectites are swelling clays that form during the reaction of water with silicates. At322 elevated temperatures, however, smectites are no longer the thermodynamically favored323 phase, and conversion of smectites to other phyllosilicate phases like illite or chlorite that324 lack interlayer water begins. The temperature, the fluid composition, the amount of water325 available, and burial duration are dictate resultant mineralogy. Given the presence of326 smectite clay, either from in situ formation [Vaniman et al., 2014; Bristow et al., 2014] or327 transport from a watershed with smectite clays [Ehlmann & Buz, accepted] in328 sedimentary rocks at Gale crater, we track two thresholds for determining whether329 smectite would be expected to have converted to another phase: a single temperature330 threshold (40ºC) commonly cited for the smectite-illite conversion temperature [CITE]331 and a time-temperature integral with thresholds based on the terrestrial clay mineral rock332 record [Tosca & Knoll, 2009]. Tracking the 0ºC threshold also provides a useful 333 parameterization of water availability for alteration processes (if it is present). Fluids334 may, of course, be available at lower temperatures due to freezing point depression from335 dissolved salts. 336337338 3. Results339 In this section we provide results for the scenario modeling erosion/deposition rates,340 followed by temperature results for different scenarios and locations. , and Wwe conclude341 by presenting the time-temperature integrals for the different models. 342343 3.1 Topographic Evolution and Erosion/Deposition Rates344 Figures 5a and 5d show the evolution of topography and overburden values for scenario345 1, complete fill and exhumation, under the standard timing model; maximum diagenesis346 and minimum diagenesis models have the same overall evolution, albeit with347 temperatures achieved at different points in time. Figure 5 also shows the same348 information for Sscenarios 2a (Figure 5b and 5e) and 2b (figure 5c and 5f). For the349 topography variation the blue line represents the most ancient topography, while the red350 line represents the most recent one. In Figure 5, a solid line represents Yellowknife Bay351 while a dashed line represents the unconformity described in section 2.5. 352353 If the complete filling and exhumation to present-day topography is assumed (Scenario354 1), with the time period equally split into an interval of net erosion followed by interval355 of net deposition, the average rates of erosion and deposition are 6-27 µm/yr and 8-34 356 µm/yr, respectively. Assuming a model where strong mound-flank slope winds are357 capable of eroding the mound (Scenario 2), erosion and deposition rates were calculated358 based on where elevation gradients were either negative (erosion) or positive359 (deposition). For Scenario 2a average erosion rates fall between 5-21 µm/yr, while360 average deposition rates range from 5-21 µm/yr. For Scenario 2b average erosion rates361 are within 7-29 µm/yr while deposition rates range from 8-35 µm/yr. Table 2 summarizes362 the obtained results. 363 364 3.2 Temperature365 The results obtained in this section are based on coupling the topographic evolution of366 Mt. Sharp with the thermal model for Mars’ changing geothermal gradient. Figure 6367 shows the temperature variation with time for two locations (Yellowknife Bay and Mt.368 Sharp), considering three different timing scenarios and three different sedimentary369 models. Figures 6a, 6b and 6c show results for the different timing constraints370 considering a cold early Mars (-50oC), while Figures 6d, 6e and 6f show the same results371 but for a warm early Mars (0oC). 372373 As a general observation, the timing has little effect on the peak temperatures achieved374 and overall range of temperatures experienced by the sediments. It does, however,375 significantly affect the time-temperature integral, which determines the expected degree376 of diagenesis under Tosca & Knoll [2009]’s suggested approach to evaluating sediment377 maturity. Our summary of results below assumes ground waters or ice were present. For378 more on this issue, see section 4.2, discussion. 379380 3.2.1 Cold Early Mars381 For a cold early Mars, scenario 1 leads to conditions where water would be liquid and382 illite could form from smectite everywhere from Yellowknife to Mt. Sharp. Maximum383 temperatures reached are ~100ºC and ~70ºC at Yellowknife Bay and Mt. Sharp,384 respectively (Figure 6a, 6b and 6c). Scenario 2a does not generate conditions above 0ºC;385 there would be no alteration or diagenesis in situ unless driven by salty brines. Scenario386 2a implies that swelling clays (if formed by another process and part of the sedimentary387 deposit) should be the dominant clay from Yellowknife Bay and Mt. Sharp. Scenario 2b388 predicts liquid water at Mt. Sharp that could facilitate diagenetic transitions, though389 temperatures above 40ºC are not reached. Under Sscenario 2b, at Yellowknife Bay the390 0ºC threshold is not reached, and only freezing point-depressed brines are permitted by391 the temperature data. 392393 Using the time-temperature integral as a metric of diagenesis (Figure 7a, 7b and 7c),394 Scenario 1 predicts the complete transformation of smectite to illite at Yellowknife Bay395 for the standard and maximum diagenesis models and at Mt. Sharp for the maximum396 diagenesis model only. In all other Scenario 1 cases, transformation of smectite to illite397 occurs, but is partial. Scenario 2b implies partial smectite to illite conversion at Mt.398 Sharp, but not at Yellowknife Bay. Scenario 2a does not predict that any conversion399 process occurs. 400401 3.2.2 Warm Early Mars 402 For warm early Mars, liquid water that might cause alteration and diagenesis would be403 available everywhere between Yellowknife Bay and Mt. Sharp under all scenarios404 (Figures 6d, 6e and 6f). Maximum temperatures greater than 100ºC occur at both405 Yellowknife Bay and Mt Shap in scenario 1. In both Sscenarios 2a and 2b, temperatures406 above 40ºC are reached at Mt. Sharp, while in Sscenario 2b, this threshold is only407 exceeded for Mt. Sharp. 408409 Using the time-temperature integral, complete conversion of smectite to illite is predicted410 for all scenarios and locations except for 2a at Yellowknife Bay. Surface temperatures at411 0 o C would imply that any smectites should not have persisted elsewhere from the base to412 unconformity of Mt. Sharp. Plots are shown in Figures 7d, 7e and 7f. 413414 4. Discussion415 In this section we present a discussion based on the comparison of our results with the416 prior work, including MSL Curiosity data, acquired to date. We start by discussing417 predicted erosion and deposition rates. We then proceed to describe the current418 knowledge regarding the presence of clays throughout MSL’s path, and predictions for419 what the rover might encounter under each scenario. 420421 4.1 Martian Deposition and Erosion Rates422 Overall the calculated averages obtained in section 3.1 and summarized in Table 2 fall423 near the ranges of 10-100 µm/yr for estimated Martian sedimentary deposition [Lewis &424 Aharonson, 2014]. The calculated average ranges moderately exceed estimated erosion 425 rates of Noachian and Hesperian terrains, 0.7-10 µm/yr, but are at the lower end of426 erosion rates from Earth, 2-100µm/yr [Golombek et al., 2006]. Notably, typical Martian427 net erosion rates are not compatible with the growth and erosion of Mt. Sharp entirely428 during the Hesperian, as suggested by surface ages based on crater counts; at ~1 µm/yr,429 the erosion of 4 km of material would take 4 Gyr. Having erosion and deposition rates430 falling within a reasonable range derived from the literature confirms the plausibility of431 our assumptions and the models described in this work.432433 It should be noted that deposition/erosion under Scenario 2a and 2b is computed434 somewhat differently than for Scenario 1 in which the erosion and deposition and equally435 scaled to add and then remove the necessary materials over the specified time period.436 While in Scenario 2, a discrete erosion rate is computed in every time step iteration (, the437 Kite et al. [2013] model computes deposition rate using a constant value based on the438 initial time step that does not change during the simulation, regardless of Mt Sharp form).439 For scenario 2a, this assumption is used; however, with our choice of crater wall height, 5440 km instead of 10 km in Kite et al. [2013], this leads to a thin central mound. For scenario441 2b, we decrease the deposition rate linearly with time. Several behaviors of deposition442 rate could be used here. For a distinct endmember, we choose one that yielded results443 similar to what we see at Gale, e.g. a mound height and shape that matches modern444 topography and that also steepens with time and retreats back from the wall toward the445 center, a sequence originally proposed in Kite et al. [2013]. A step function could be446 used here as well: a simulation of volcanic interference while the aeolian process happens 447 inside the crater. Our analysis with this case showed a final form similar to the constant448 deposition case. 449450 4.2 Mineralogical Implications from Ground Measurements and Modeling 451 4.2.1 Yellowknife Bay452 Vaniman et al. [2014] identified trioctahedral smectites at Yellowknife Bay in the453 samples John Klein and Cumberland. No illites were positively recognized. Interestingly,454 XRD patterns indicate that the smectite interlayers in John Klein are collapsed, while455 those in Cumberland are held open, perhaps by metal-hydroxides [Bristow et al., in456 press]. This suggests an additional episode(s) of fluid interaction with the smectite clays457 after formation. This is consistent with the overall history of the Yellowknife Bay458 sedimentary rock which shows, raised ridges, nodules, and calcium sulfate veins inferred459 to result from post-depositional processes [Grotzinger et al., 2014; Siebach et al., 2014;460 Stack et al., 2014]. 461462 Our analysis of models for the temperatures experienced by the Yellowknife Bay463 sediments indicates either Yellowknife Bay was either never buried by >1 km of fill and464 mean Hesperian Mars surface temperatures were substantially below zero or water was465 unavailable during most of Yellowknife Bay’s burial. Any scenario where Yellowknife466 Bay is buried under 5 km of fill (Scenario 1) leads to conversion of smectite to illite if467 water is available. The conversion is complete in all except the minimum diagenesis, Tsurf468 = 50 C case. Scenarios where the crater is partially filled (Scenarios 2a, 2b; up to XX km)469 do not predict any smectite to illite conversion if average mean surface temperatures were 470 substantially below 0ºC because, even with burial, subsurface temperatures do not exceed471 0ºC. In a warmer early Mars, where Tsurf ≥ 0ºC, partial to complete conversion of smectite472 to illite would be expected at Yellowknife Bay, even with partial burial, so long as water473 is available.474475 Thus, if the deposition of the Yellowknife Bay sediments pre-dates Mt. Sharp formation,476 a cold early Mars and slope wind model with relatively little burial is implicated.477 However, the stratigraphic relationships between Yellowknife Bay sediments and other478 units are ambiguous, and the deposit might instead represent a late-stage unit from Peace479 Vallis alluvial activity emplaced atop materials left behind during Mt. Sharp’s retreat.480 Continued analysis of orbital and in situ data to understand contact relationships is481 crucial. Moreover, these data highlight the importance of analysis of the mineralogy of482 Mt. Sharp sediments where the stratigraphic relationships are clear to determine the483 environmental history of Gale crater deposits.484485 4.2.2. Implications for Diagenesis along the Traverse and Lower Mt. Sharp 486 As Curiosity continues its trek toward Mt Sharp, continued collection of mineralogic data487 will test whether paleotemperatures were higher or lower in Mt. Sharp compared to488 Yellowknife Bay and whether the smectite to illite conversion took place. Figure 8489 provides maximum temperatures experienced sedimentary rocks at locations between490 Yellowknife Bay and Mt. Sharp, considering different scenarios and timing scales.491 Scenario 1 has a maximum temperature at every location that decreases from492 Yellowknife Bay to Mt. Sharp. Scenario 2a has an increasing maximum temperature 493 across MSL’s traverse, and scenario 2b has a peak maximum temperature located494 somewhere between Yellowknife Bay the Mt. Sharp unconformity, near an elevation of495 2.6 km. These distinctive patterns provide crucial, testable constraints for the history of496 sedimentary deposition because these patterns of temperature increase would be recorded497 in the sediments, if there were waters within Gale crater during the Hesperian. If Gale498 crater was completely filled then exhumed, diagenesis should decrease with distance up499 Mt. Sharp, whereas incomplete fill under a slope wind model scenario would show500 increased diagenesis toward the center part of the mound. If temperature can also be501 discerned from diagenetic mineral assemblages, these constrain Hesperian surface502 temperatures, and thus paleoclimate, at Gale crater. 503504 5. Conclusions505 Understanding the conditions that led to sediment deposition, erosion, and secondary506 mineral formation in Gale crater advances our understanding of the evolution of507 environmental conditions on ancient Mars. We developed a coupled sedimentation-508 thermal model that traced temperature as a function of time for two different scenarios: 1)509 complete fill of Gale crater followed by partial erosion and 2) partial fill dictated by slope510 winds as described by Kite et al., [2013]. Determining maximum temperatures of511 sediments and computation of time-temperature integrals provides a set of distinctive512 predictions for the extent of diagenesis, testable by what we are seeing today by the MSL513 mission. Our models have erosion and deposition rates within expected Martian514 deposition and erosion values. We find that in the presence of water during Mt. Sharp515 formation and erosion, the smectite to illite conversion should have occurred at all levels 516 in lower Mt. Sharp if Gale crater had been completely filled. Moreover, either517 Yellowknife Bay was never buried by >1km of overburden, or there was insufficient518 water available for diagenesis when it was buried, due to either aridity or Hesperian mean519 annual surface temperatures well below zero. Complete fill of Gale crater means the520 rover should observe decreasing diagenesis as it climbs Mt. Sharp, whereas a slope wind521 model [Kite et al., 2013] would predict increasing diagenesis as the rover climbs lower522 Mt. Sharp. Characterization of diagenetic mineral assemblages observed by Mt. Sharp as523 well as their spatial pattern along the rover traverse thus collectively can reveal the524 paleoenvironmental and sedimentary histories of Gale crater’s Mt. Sharp. 525526 Acknowlegements527 We thank Edwin Kite for sharing his Matlab code for the wind slope model for528 generation of central mounds of discrete widths. This work was partially funded by an529 MSL Participating Scientist grant to B.L.E. The Caltech Summer Undergraduate530 Research Fellowship program provided programmatic support to C.B. 531 References532 Anderson, R. C., and J. F. Bell III (2010), Geologic mapping and characterization of Gale533 Crater and implications for its potential as a Mars Science Laboratory landing 534 site, Mars, 5, 76–128, doi:10.1555/mars.2010.0004.535 Beardsmore, G. R., & Cull, J. P. 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Sharp’s formation are analyzed in this paper: (B)645 complete fill of the crater to the rim, followed by partial exhumation; (C) the Kite et al.,646 [2013] model of wind slopes creating feedback with the sides of the crater, enabling647 mound formation in the center with only incomplete fill.648649 Figure 2. Ancient Gale profile (solid black line) is compared with the modern (dashed650 gray line) and average (solid gray line) Gale profiles. The ancient Gale profile represents651 the initial state of the model for every scenario. Here we try to explain how the ancient652 Gale profile turned out to be the modern Gale profile. 653654 Figure 3. Depth to temperature relation considering two different early Mars655 temperatures (0oC and -50oC) as well as four period of time (Noachian/Hesperian, Early656 Amazonian, Late Amazonian and Modern).657658659 Figure 4. Bradbury Landing, Yellowknife Bay (YKB), Hidden Valley and a potential660 future MSL location (an unconformity at Mt. Sharp) are identified in the figure. The661 change in altitude from Yellowknife Bay to the unconformity is of 1000 m. One should662 also notice that the distance from Yellowknife Bay to the Hidden Valley, longitudinally663 speaking, is small when compared to the distance from Yellowknife Bay to the664 unconformity. 665666 Figure 5. Topography variation and overburden rates for scenario 1 (Figures 5a and 5d),667 scenario 2a (Figures 5b and 5e) and scenario 2b (Figures5c and 5f) are presented here. 668669 Figure 6. Temperature as a function of timing scenarios for sedimentary models 1, 2a670 and 2b considering an early mean surface temperature of -50oC and 0oC. Solid lines671 represent location at Yellowknife Bay; dashed lines represent location at Mt. Sharp. 672673 Figure 7. Time-temperature integral as a function of timing scenarios for sedimentary 674 models 1, 2a and 2b considering an early mean surface temperature of -50oC and 0oC. 675 Solid lines represent location at Yellowknife Bay; dashed lines represent location at Mt. 676 Sharp.677678 Figure 8. Maximum and minimum temperatures experienced by MSL’s traverse679 considering different sedimentary scenarios and timescales for cold and warm early Mars.680681682683684685

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