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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 Traverse
6
7 Caue S. Borlina1,2, and Bethany L. Ehlmann2,3
8
9 1 Department of Atmospheric, Oceanic and Space Sciences, University of Michigan, Ann
10 Arbor, Michigan, USA
11 2 Division of Geological and Planetary Sciences, California Institute of Technology,
12 Pasadena, California, USA
13 3 Jet Propulsion Laboratory, California Institute of Technology, Pasadena, California,
14 USA
15
16 Corresponding author: [email protected]; 2215 Space Research Building, 2455
17 Hayward St., Ann Arbor, MI 48109
18
19 Keywords: burial diagenesis, sedimentation, erosion, heat flow, Gale Crater, Mars
20 Science Laboratory (MSL)
21
22
23 24 Abstract
25 Gale Crater, landing site of the NASA Mars Science Laboratory (MSL) mission, has a
26 unique sedimentary stratigraphy, preserved in a 5 km high mound called Aeolis Mons
27 (informally known as Mt. Sharp). Understanding the processes of Mt. Sharp
28 sedimentation, erosion, and secondary mineral formation can reveal important
29 information regarding past geologic processes, the climate and habitability of early Mars,
30 and the organic preservation potential of these sedimentary deposits. Two endmember
31 scenarios for Mt. Sharp's formation are: (1) a complete filling of Gale crater followed by
32 partial removal of deposited sediments or (2) building of a central deposit with
33 morphology controlled by slope winds and only incomplete sedimentary fill of Gale
34 crater. Here we consider depths of burial predicted for both scenarios, coupled with a
35 thermal model to predict the temperature history of particular sediments presently
36 exposed at the surface. We compare our results with chemical and mineralogical analyses
37 by MSL to date and provide sedimentation scenario-dependent predictions of diagenetic
38 mineralogy in locations between Yellowknife Bay and upper Mt. Sharp. Results show
39 modeled erosion and deposition rates range from 5-42 μm/yr across different scenarios
40 and timing scales, consistent with or slightly higher than rates calculated for other
41 locations on Mars. Mineralogical predictions for locations close to Yellowknife Bay vary
42 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 traverse
44 and up Mt. Sharp stratigraphy, whereas for (2) maximum temperatures are reached in the
45 lower units of Mt. Sharp and thereafter decline or hold roughly constant. If surface
46 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 permit
48 diagenetic fluids (T>0°C) at select locations. In contrast, if surface temperatures on early
49 Mars were near the melting point of water, diagenetic temperatures were achieved over
50 the entire Curiosity traverse with temperatures ranging from 100-150°C under scenario
51 (1) and 0-100°C for variations of scenario (2). Under nearly all scenarios, evidence of
52 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 authigenic
55 phases will constrain the timing and timescale of Mt. Sharp formation, paleosurface
56 temperature, and the availability and setting of liquid water on early Mars.
57
58 59 1. Introduction
60 The study of sedimentation processes at specific regions on Mars helps time order
61 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 water
63 at or near the surface of the planet, and potential for the long-term preservation of organic
64 materials. Gale crater (137.4oE, -4.6oN), the landing site of the NASA Mars Science
65 Laboratory (MSL) mission, has a unique sedimentary stratigraphy, which permits
66 examining ancient Martian environmental conditions and aqueous alteration. Recent
67 studies have shown that Gale once hosted a fluvial-lacustrine environment, capable of
68 supporting life [Grotzinger et al., 2014]. The stratigraphic rock record of the area, and
69 thus its geologic history, is preserved in a 5 km high mound called Aeolis Mons
70 (informally, Mt. Sharp) located in Gale crater [Grotzinger et al., 2012] (Figure 1a).
71
72 Materials comprising Mt. Sharp have a low thermal inertia and subhorizontally layered
73 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 unique
79 identification of secondary mineral phases, though eroded boxwork structures suggest
80 precipitation of minerals during fluid flow within the sediments [Siebach and Grotzinger,
81 2014]. 82
83 Many hypotheses have been developed to explain Mt Sharp formation, variously
84 invoking airfall dust or volcanic ash, lag deposits from ice/snow, aeolian and
85 fluviolacustrine sedimentation [for review, see Anderson & Bell, 2010; Wray, 2013;
86 LeDeit et al., 2013]. Nevertheless, regardless of the process(es) delivering sediments, two
87 endmember scenarios describe the time-evolution, i.e., growth and subsequent erosion, of
88 Mt. Sharp. In Scenario 1, Gale Crater was completely filled with layered sediments then
89 partially exhumed, leaving a central mound [Malin and Edgett, 2000] (Figure 1b). In
90 Scenario 2, an aeolian process characterized by slope winds created a wind-topography
91 feedback enabling growth of a high mound without complete fill of the crater [Kite et al.,
92 2013] (Figure 1c).
93
94 The history of Mt. Sharp’s sedimentation and mineralization provides key constraints on
95 environmental conditions on early Mars, including the availability of liquid water and the
96 nature of geochemical environments. Understanding sediment deposition and possible
97 diagenesis is also crucial to establishing the preservation potential through time for
98 organic carbon of biological or abiotic origin trapped in sedimentary rock strata. The
99 thermal history of sediments and their exposure to fluids exerts strong control on the
100 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 out
102 the apparent ubiquity of the “juvenile” sediments with smectite clays, amorphous silica
103 and a paucity of evidence for illite, chlorite, quartz and other typical products of
104 diagenesis, which are common in the terrestrial rock record, thus concluding that 105 diagenetic processes on Mars were uncommon, perhaps limited by water availability
106 [Tosca & Knoll, 2009]. Since then, a growing number of studies have identified clay
107 minerals such as illite, chlorite, and mixed layer clays that can form from via diagenesis
108 [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 situ
110 rover data at Yellowknife Bay implies at least early diagenetic reactions to form
111 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 or
113 incipient chloritization [Vaniman et al., 2014; Rampe et al., 2014; Bristow et al., 2014].
114
115 Here we model the potential diagenetic history of the sediments comprising Mt. Sharp
116 and accessible in rock units along Curiosity’s traverse. We couple the two sedimentation
117 scenarios [Malin and Edgett, 2000; Kite et al., 2013] with a thermal model for ancient
118 Martian heat flow and timescales for Mt. Sharp sedimentary deposition and erosion
119 constrained by crater counts. We model temperature variations experienced within the
120 region between Yellowknife Bay, the base of Mt. Sharp, and the lower unit/upper unit
121 Mt. Sharp unconformity, and we compare them with select key temperature thresholds
122 relevant for diagenesis: stability of liquid water (0oC) and the a temperature for efficient
123 smectite to illite conversion (~40oC). We also analyze the time-temperature integral, an
124 alternative method for establishing the likelihood and extent of mineral diagenesis as
125 described by Tosca and Knoll [2009]. The final results are compared to mineralogical and
126 chemical findings obtained by MSL to date. Potential mineralogical findings across
127 MSL’s traverse are discussed and correlated with implications for sedimentation 128 processes on Mars, timescales, early Mars temperatures, liquid water availability, and the
129 organic preservation potential of Gale sediments.
130
131 2. Methodology
132 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 in
134 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 of
136 the rover traverse that defines the studied region.
137
138 2.1 Pristine Gale Basement & Modern Topography
139 In order to evaluate how Gale Crater changed we needed to first determine the ancient
140 (starting point) and modern (ending point) topographies of the crater. Gale crater has
141 been both eroded and filled relative to its original topographic profile. Consequently, a
142 combination of theoretical constraints and observational constraints from the Mars
143 Orbiting Laser Altimeter Altitude (MOLA) dataset was used to set the initial conditions
144 for Gale's crater shape, i.e. its initial topographic profile. Observed crater depth-diameter
145 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; Robbins
147 and Hynek, 2007]. Kalynn et al. [2013] empirically have shown Martian central peak
148 heights of ~1 km for ~100 km craters. Our examination of craters on Mars, better
149 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 peak
152 heights may be underestimates due to later crater fill. Hence, we set the initial shape of
153 Gale Crater to be 154 km in diameter and 5 km deep with a central peak height of 1.55
154 km. In order to have realistic slopes, we scaled the average topographic profile from
155 Moreux crater (45E, 42N) to fit Gale's parameters.
156
157 Modern Gale crater has a highly asymmetric central mound, its cross-sectional profile
158 varying with azimuth. The average Gale profile shown (Figure 2) was compiled from
159 multiple roughly NW-SE cross-sections of present-day crater topography and then
160 averaged after removing local geologic features. The average Gale profile is used for
161 estimation of an average overburden over the Curiosity rover traverse. Scenarios 2a and
162 2b were tuned to produce final mounds of width approximately equivalent to the width of
163 the average profile.
164
165 2.2 Mt. Sharp Sedimentation Scenarios
166 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 near
169 the center of the crater with continuous presence of strong mound-flank slope winds
170 capable of eroding the mound and keeping it away from the sides of the crater [Kite et al.,
171 2013].
172
173 2.2.1 Scenario 1 174 Scenario 1 is characterized by a complete fill of the crater to the height of the highest
175 peak of Mt. Sharp, followed by partial erosion of the crater, leaving the modern shape of
176 Mt. Sharp as final output. We use an average Gale profile as the final shape (see section
177 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 period
179 for erosion/sedimentation processes and necessary burial/erosion height, i.e., deposition
180 rate is computed as distance from pristine basement to the top of the crater, divided by
181 deposition time. Erosion rate computed as distance from top of the crater (completely
182 filled crater) to the average modern profile, divided by erosion time.
183
184 2.2.2 Scenario 2
185 Scenario 2 is an aeolian process where the mound grew close to the center of the crater
186 with the surrounding topography creating an environment for the presence of strong
187 mound-flank slope winds capable of eroding the mound. We use the model and code
188 developed by Kite et al. [2013] to generate the evolution of the topographic profile with
189 time. Briefly, in Kite et al.’s model a saltation model approximation is used to determine
190 the balance between the deposition D (set at the beginning of the simulation), and the
191 erosion E (time varying). The result is the computation of dz/dt (altitude variation over
192 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, soil
197 erosion and rock abrasion on the crater; U is a unitless dimensionless expression related
198 to the magnitude of the shear velocity (dx’, a mesh unit in Kite et al., is 1i.e. at t = 0 the
199 basement is represented by a mesh with spacing dx’ of unit value); U0 is the component
200 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 x
202 starting at x = 1.5 km and ending at x = 153.5 km; and L is a length scale that accounts for
203 the effects of inertia. A special parameter R/L is used which correlates the width of the
204 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 Kite
206 et al., [2013]).
207
208 Is important to notice that . U and z’ are tiime varying values. Eq. (3) is evaluated for
209 each time step considering left and right slopes of the winds: for a value of x, we compute
210 the integral for left slopes (values less than x) and right slopes (values greater than x and
211 less than 154 km). The operator max[ ] selects the slope with the highest value, which
212 then permits evaluation of Eq. (2) at each time step. Eq. (1) is also then evaluated in each
213 timestep, using E from Eq. (2) and a value for D determined from the user-set parameter
214 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. In
216 our Scenario 2B, D is set initially then declines linearly.
217 We experimented with multiple choices of parameter combinations and choose to
218 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 name
220 the two different scenarios as 2a and 2b. Scenario 2a is defined with a constant deposition
221 rate. The mound grows tall with a relatively constant width over time. Scenario 2b has a
222 linearly decreasing deposition rate over time, and the mound grows wide then steepens
223 and narrows. Both scenarios converge on a similar final output though the amount of
224 sediment overburden as a function of time depends on the intermediate steps. The output
225 has no time dependency; we implement timescales in the final output of the model by
226 scaling the output to our specified durations after iterations converged. A special
227 parameter R/L is introduced at this point, which correlates the width of the crater wall to
228 the crater size. For small craters we should expect small values of R/L, around 0.1-1 (for
229 a complete analysis of R/L verify the supplementary material from Kite et al., [2013]).
230
231 2.3 Timescales
232 Crater counts on the ejecta blanket of Gale crater constrain its formation age to Late
233 Noachian/Early Hesperian, approximately 3.8 to 3.6 Ga, and place an upper limit on the
234 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 using
236 the superposition relationship of the topographically lower but stratigraphically higher
237 deposits of Aeolis Palus, which have estimated ages ranging from early Hesperian to
238 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 present
240 extent took place during the Hesperian, though processes continued to shape the form of
241 the mound during the Amazonian. 242
243 While surface ages based on crater counts are superb for relative age dating, pinning in
244 absolute time is challenged by the existence of different chronology models relating the
245 density of craters with time [e.g., Werner & Tanaka, 2011]. Yet, numerical ages
246 constraining the start and end of major episodes of Mt. Sharp erosion and deposition are
247 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 exhumation
249 of Mt. Sharp: (1) a standard model, (2) a maximum diagenesis model, and (3) a minimum
250 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), Gale
252 crater and Mt. Sharp form early, 3.85 Gyr, and Mt. Sharp is exhumed late, 3.0 Gyr, thus
253 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.
255
256 2.4 Thermal Model
257 The thermal model used here defines temperature as a function of depth and time. We
258 construct it by using the one-dimensional steady-state heat conduction solution that
259 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 the
261 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 on
263 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 T0
266 can impact the results. The first presumes a warmer early Mars where temperatures
267 routinely exceed the melting point of water during large portions of the Martian year; the
268 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 flow
270 [Parmentier and Zuber, 2007] and crustal heat production, respectively, through time
271 [Hahn et al., 2011; their Figure 2]. Results for q(t) and H(t) are shown in Figures 3a and
272 3b. Parmentier and Zuber [2007] estimated values for q to be ~60mW/m2 at 3.5 Gyr for a
273 variety of cooling scenarios and 20mW/m2 at present; measurements of modern heat flow
274 will soon be obtained by the InSight mission.. These values are similar to those
275 independently predicted by Morschhauser et al. [2011]. Figure 3c shows derived
276 temperatures as a function of depth for the two different mean surface temperatures and
277 several time periods.
278
279 Gale may have had additional heating from hydrothermal sources such as residual
280 heat following the Gale-forming impact or local volcanic sources, but we do not include
281 these in our modeling. Were additional sources of heat present, heat flow would be higher
282 and temperatures higher. Thus our estimates for temperatures reached are lower bounds.
283
284 2.5 Yellowknife Bay to Mt. Sharp: MSL’s Traverse
285 After landing, MSL headed toward Yellowknife Bay, a region in the opposite direction of
286 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 to
288 aqueous alteration, including Mg smectites and hydrated calcium sulfates, were identified
289 [Vaniman et al., 2014].
290
291 Given sections 2.1-2.4 above, we model the sedimentary and thermal history along the
292 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. Figure
294 4 shows Bradbury Landing, Yellowknife Bay, Hidden Valley and a potential future final
295 location of MSL. We picked the final future location for modeling to be the
296 unconformity, marking the boundary between upper and lower Mt Sharp. In order to
297 locate Yellowknife Bay in our model, we compute the ratio between the distance of the
298 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 proper
300 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 identify
302 Yellowknife Bay at 0.93 of the distance between the closest rim and the foothill of the
303 output mound. Yellowknife Bay and Hidden Valley (~ Sol 722) have similar altitudes
304 and rim distances. The unconformity is ~1000 m higher than Yellowknife Bay: we then
305 conclude that ~ Sol 722 is represented by the location that is 1000 m higher than the
306 location set to be Yellowknife Bay (at the 0.93 ratio to the crater). and XX further from
307 the crater rim toward the interior of the crater. Results for the thermal history are
308 subsequently presented as a range of values between Yellowknife Bay and the
309 unconformity in Mt. Sharp. This range represents MSL’s future traverse range. 310
311 2.6 Key diagenetic thresholds
312 Once deposited, subsequent fluid circulation through sediments can lead to textural
313 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 undergo
315 mineralogic transitions, depending upon the chemistry of diagenetic fluids and
316 temperature. It is beyond the scope of this work to track all diagenetic transitions [for
317 review, see Mackenzie, 2005]; we instead focus on reporting modeled temperatures. For
318 reference, however, we do track three key diagenetic thresholds: the freezing point of
319 water at 0ºC, the illite to smectite conversion at ~40ºC, and the time-temperature integral.
320
321 Smectites are swelling clays that form during the reaction of water with silicates. At
322 elevated temperatures, however, smectites are no longer the thermodynamically favored
323 phase, and conversion of smectites to other phyllosilicate phases like illite or chlorite that
324 lack interlayer water begins. The temperature, the fluid composition, the amount of water
325 available, and burial duration are dictate resultant mineralogy. Given the presence of
326 smectite clay, either from in situ formation [Vaniman et al., 2014; Bristow et al., 2014] or
327 transport from a watershed with smectite clays [Ehlmann & Buz, accepted] in
328 sedimentary rocks at Gale crater, we track two thresholds for determining whether
329 smectite would be expected to have converted to another phase: a single temperature
330 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 rock
332 record [Tosca & Knoll, 2009]. Tracking the 0ºC threshold also provides a useful 333 parameterization of water availability for alteration processes (if it is present). Fluids
334 may, of course, be available at lower temperatures due to freezing point depression from
335 dissolved salts.
336
337
338 3. Results
339 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 conclude
341 by presenting the time-temperature integrals for the different models.
342
343 3.1 Topographic Evolution and Erosion/Deposition Rates
344 Figures 5a and 5d show the evolution of topography and overburden values for scenario
345 1, complete fill and exhumation, under the standard timing model; maximum diagenesis
346 and minimum diagenesis models have the same overall evolution, albeit with
347 temperatures achieved at different points in time. Figure 5 also shows the same
348 information for Sscenarios 2a (Figure 5b and 5e) and 2b (figure 5c and 5f). For the
349 topography variation the blue line represents the most ancient topography, while the red
350 line represents the most recent one. In Figure 5, a solid line represents Yellowknife Bay
351 while a dashed line represents the unconformity described in section 2.5.
352
353 If the complete filling and exhumation to present-day topography is assumed (Scenario
354 1), with the time period equally split into an interval of net erosion followed by interval
355 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 are
357 capable of eroding the mound (Scenario 2), erosion and deposition rates were calculated
358 based on where elevation gradients were either negative (erosion) or positive
359 (deposition). For Scenario 2a average erosion rates fall between 5-21 µm/yr, while
360 average deposition rates range from 5-21 µm/yr. For Scenario 2b average erosion rates
361 are within 7-29 µm/yr while deposition rates range from 8-35 µm/yr. Table 2 summarizes
362 the obtained results.
363
364 3.2 Temperature
365 The results obtained in this section are based on coupling the topographic evolution of
366 Mt. Sharp with the thermal model for Mars’ changing geothermal gradient. Figure 6
367 shows the temperature variation with time for two locations (Yellowknife Bay and Mt.
368 Sharp), considering three different timing scenarios and three different sedimentary
369 models. Figures 6a, 6b and 6c show results for the different timing constraints
370 considering a cold early Mars (-50oC), while Figures 6d, 6e and 6f show the same results
371 but for a warm early Mars (0oC).
372
373 As a general observation, the timing has little effect on the peak temperatures achieved
374 and overall range of temperatures experienced by the sediments. It does, however,
375 significantly affect the time-temperature integral, which determines the expected degree
376 of diagenesis under Tosca & Knoll [2009]’s suggested approach to evaluating sediment
377 maturity. Our summary of results below assumes ground waters or ice were present. For
378 more on this issue, see section 4.2, discussion. 379
380 3.2.1 Cold Early Mars
381 For a cold early Mars, scenario 1 leads to conditions where water would be liquid and
382 illite could form from smectite everywhere from Yellowknife to Mt. Sharp. Maximum
383 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. Scenario
386 2a implies that swelling clays (if formed by another process and part of the sedimentary
387 deposit) should be the dominant clay from Yellowknife Bay and Mt. Sharp. Scenario 2b
388 predicts liquid water at Mt. Sharp that could facilitate diagenetic transitions, though
389 temperatures above 40ºC are not reached. Under Sscenario 2b, at Yellowknife Bay the
390 0ºC threshold is not reached, and only freezing point-depressed brines are permitted by
391 the temperature data.
392
393 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 Bay
395 for the standard and maximum diagenesis models and at Mt. Sharp for the maximum
396 diagenesis model only. In all other Scenario 1 cases, transformation of smectite to illite
397 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 conversion
399 process occurs.
400
401 3.2.2 Warm Early Mars 402 For warm early Mars, liquid water that might cause alteration and diagenesis would be
403 available everywhere between Yellowknife Bay and Mt. Sharp under all scenarios
404 (Figures 6d, 6e and 6f). Maximum temperatures greater than 100ºC occur at both
405 Yellowknife Bay and Mt Shap in scenario 1. In both Sscenarios 2a and 2b, temperatures
406 above 40ºC are reached at Mt. Sharp, while in Sscenario 2b, this threshold is only
407 exceeded for Mt. Sharp.
408
409 Using the time-temperature integral, complete conversion of smectite to illite is predicted
410 for all scenarios and locations except for 2a at Yellowknife Bay. Surface temperatures at
411 0 o C would imply that any smectites should not have persisted elsewhere from the base to
412 unconformity of Mt. Sharp. Plots are shown in Figures 7d, 7e and 7f.
413
414 4. Discussion
415 In this section we present a discussion based on the comparison of our results with the
416 prior work, including MSL Curiosity data, acquired to date. We start by discussing
417 predicted erosion and deposition rates. We then proceed to describe the current
418 knowledge regarding the presence of clays throughout MSL’s path, and predictions for
419 what the rover might encounter under each scenario.
420
421 4.1 Martian Deposition and Erosion Rates
422 Overall the calculated averages obtained in section 3.1 and summarized in Table 2 fall
423 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 of
426 erosion rates from Earth, 2-100µm/yr [Golombek et al., 2006]. Notably, typical Martian
427 net erosion rates are not compatible with the growth and erosion of Mt. Sharp entirely
428 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 rates
430 falling within a reasonable range derived from the literature confirms the plausibility of
431 our assumptions and the models described in this work.
432
433 It should be noted that deposition/erosion under Scenario 2a and 2b is computed
434 somewhat differently than for Scenario 1 in which the erosion and deposition and equally
435 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 (, the
437 Kite et al. [2013] model computes deposition rate using a constant value based on the
438 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, 5
440 km instead of 10 km in Kite et al. [2013], this leads to a thin central mound. For scenario
441 2b, we decrease the deposition rate linearly with time. Several behaviors of deposition
442 rate could be used here. For a distinct endmember, we choose one that yielded results
443 similar to what we see at Gale, e.g. a mound height and shape that matches modern
444 topography and that also steepens with time and retreats back from the wall toward the
445 center, a sequence originally proposed in Kite et al. [2013]. A step function could be
446 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 constant
448 deposition case.
449
450 4.2 Mineralogical Implications from Ground Measurements and Modeling
451 4.2.1 Yellowknife Bay
452 Vaniman et al. [2014] identified trioctahedral smectites at Yellowknife Bay in the
453 samples John Klein and Cumberland. No illites were positively recognized. Interestingly,
454 XRD patterns indicate that the smectite interlayers in John Klein are collapsed, while
455 those in Cumberland are held open, perhaps by metal-hydroxides [Bristow et al., in
456 press]. This suggests an additional episode(s) of fluid interaction with the smectite clays
457 after formation. This is consistent with the overall history of the Yellowknife Bay
458 sedimentary rock which shows, raised ridges, nodules, and calcium sulfate veins inferred
459 to result from post-depositional processes [Grotzinger et al., 2014; Siebach et al., 2014;
460 Stack et al., 2014].
461
462 Our analysis of models for the temperatures experienced by the Yellowknife Bay
463 sediments indicates either Yellowknife Bay was either never buried by >1 km of fill and
464 mean Hesperian Mars surface temperatures were substantially below zero or water was
465 unavailable during most of Yellowknife Bay’s burial. Any scenario where Yellowknife
466 Bay is buried under 5 km of fill (Scenario 1) leads to conversion of smectite to illite if
467 water is available. The conversion is complete in all except the minimum diagenesis, Tsurf
468 = 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 exceed
471 0ºC. In a warmer early Mars, where Tsurf ≥ 0ºC, partial to complete conversion of smectite
472 to illite would be expected at Yellowknife Bay, even with partial burial, so long as water
473 is available.
474
475 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 other
478 units are ambiguous, and the deposit might instead represent a late-stage unit from Peace
479 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 is
481 crucial. Moreover, these data highlight the importance of analysis of the mineralogy of
482 Mt. Sharp sediments where the stratigraphic relationships are clear to determine the
483 environmental history of Gale crater deposits.
484
485 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 data
487 will test whether paleotemperatures were higher or lower in Mt. Sharp compared to
488 Yellowknife Bay and whether the smectite to illite conversion took place. Figure 8
489 provides maximum temperatures experienced sedimentary rocks at locations between
490 Yellowknife Bay and Mt. Sharp, considering different scenarios and timing scales.
491 Scenario 1 has a maximum temperature at every location that decreases from
492 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 located
494 somewhere between Yellowknife Bay the Mt. Sharp unconformity, near an elevation of
495 2.6 km. These distinctive patterns provide crucial, testable constraints for the history of
496 sedimentary deposition because these patterns of temperature increase would be recorded
497 in the sediments, if there were waters within Gale crater during the Hesperian. If Gale
498 crater was completely filled then exhumed, diagenesis should decrease with distance up
499 Mt. Sharp, whereas incomplete fill under a slope wind model scenario would show
500 increased diagenesis toward the center part of the mound. If temperature can also be
501 discerned from diagenetic mineral assemblages, these constrain Hesperian surface
502 temperatures, and thus paleoclimate, at Gale crater.
503
504 5. Conclusions
505 Understanding the conditions that led to sediment deposition, erosion, and secondary
506 mineral formation in Gale crater advances our understanding of the evolution of
507 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 slope
510 winds as described by Kite et al., [2013]. Determining maximum temperatures of
511 sediments and computation of time-temperature integrals provides a set of distinctive
512 predictions for the extent of diagenesis, testable by what we are seeing today by the MSL
513 mission. Our models have erosion and deposition rates within expected Martian
514 deposition and erosion values. We find that in the presence of water during Mt. Sharp
515 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, either
517 Yellowknife Bay was never buried by >1km of overburden, or there was insufficient
518 water available for diagenesis when it was buried, due to either aridity or Hesperian mean
519 annual surface temperatures well below zero. Complete fill of Gale crater means the
520 rover should observe decreasing diagenesis as it climbs Mt. Sharp, whereas a slope wind
521 model [Kite et al., 2013] would predict increasing diagenesis as the rover climbs lower
522 Mt. Sharp. Characterization of diagenetic mineral assemblages observed by Mt. Sharp as
523 well as their spatial pattern along the rover traverse thus collectively can reveal the
524 paleoenvironmental and sedimentary histories of Gale crater’s Mt. Sharp.
525
526 Acknowlegements
527 We thank Edwin Kite for sharing his Matlab code for the wind slope model for
528 generation of central mounds of discrete widths. This work was partially funded by an
529 MSL Participating Scientist grant to B.L.E. The Caltech Summer Undergraduate
530 Research Fellowship program provided programmatic support to C.B.
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635 Tables
636
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640
641 Figure Captions 642 Figure 1. (A) Overview of 154-km diameter Gale crater (137.4oE, -4.6oN). The star
643 indicates MSL’s Bradbury landing site. Aeolis Mons (Mt. Sharp) is located on center of
644 the figure. Two scenarios for Mt. 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, enabling
647 mound formation in the center with only incomplete fill.
648
649 Figure 2. Ancient Gale profile (solid black line) is compared with the modern (dashed
650 gray line) and average (solid gray line) Gale profiles. The ancient Gale profile represents
651 the initial state of the model for every scenario. Here we try to explain how the ancient
652 Gale profile turned out to be the modern Gale profile.
653
654 Figure 3. Depth to temperature relation considering two different early Mars
655 temperatures (0oC and -50oC) as well as four period of time (Noachian/Hesperian, Early
656 Amazonian, Late Amazonian and Modern).
657
658
659 Figure 4. Bradbury Landing, Yellowknife Bay (YKB), Hidden Valley and a potential
660 future MSL location (an unconformity at Mt. Sharp) are identified in the figure. The
661 change in altitude from Yellowknife Bay to the unconformity is of 1000 m. One should
662 also notice that the distance from Yellowknife Bay to the Hidden Valley, longitudinally
663 speaking, is small when compared to the distance from Yellowknife Bay to the
664 unconformity. 665
666 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.
668
669 Figure 6. Temperature as a function of timing scenarios for sedimentary models 1, 2a
670 and 2b considering an early mean surface temperature of -50oC and 0oC. Solid lines
671 represent location at Yellowknife Bay; dashed lines represent location at Mt. Sharp.
672
673 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.
677
678 Figure 8. Maximum and minimum temperatures experienced by MSL’s traverse
679 considering different sedimentary scenarios and timescales for cold and warm early Mars.
680
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685