the last two meters of exhumation at mount sharp: linking...

The Last Two Meters of Exhumation at Mount Sharp: Linking Rover-scale Geomorphology, Taphonomy, and Winds

For submission to ROSES – MSL Participating Scientist Program 2015 (NNH15ZDA001N-MSLPSP)

1. Table of contents. ..................................................................................................................... 0 2. Scientific/Technical/Management. ......................................................................................... 1 2.1. Summary, Objectives, and Expected Significance. ........................................................ 1 2.2 Scientific Background. ....................................................................................................... 1 2.2.1. Exhumation processes link Mount Sharp geomorphology and climate ....................... 1 2.2.2. Taphonomy can be constrained by rover investigation of exhumation processes ....... 4 2.2.3. Winds’ role in exhumation can be constrained by rover investigation. ........................ 5 2.3 Technical Approach and Methodology. ........................................................................... 6 2.3.1. Stereo-Enabled Slope Analysis of Exhumation Susceptibility. ................................... 7 2.3.2. Mapping Aeolian-Exhumation Potential with Mastcam, REMS, and Modeling ......... 9 2.3.3. Assessing the Role of Late-Stage Weathering in Mount Sharp’s Exhumation ........... 11 2.3.4. Quantifying Exhumation’s Impact on Organic Matter Preservation Potential ........... 12 2.3.5. Assumptions and Caveats ............................................................................................ 14 2.4. Perceived Impact of the Proposed Work ......................................................................... 14 2.5. Relevance of Proposed Work to NASA Goals, MSL Goals, and MSL Mission-level Science Objectives. ........................................................................... 14 2.6. Work Plan. ........................................................................................................................ 15 2.7. Personnel and Qualifications. ........................................................................................... 15 3. References. ............................................................................................................................. 16 4. Biographical Sketch. .............................................................................................................. 28 5. Summary of Work Effort. .................................................................................................... 30 6. Current and Pending Support. ............................................................................................. 31 7. Statements of Commitment and Support. ........................................................................... 32 8. Budget Justification. .............................................................................................................. 34 8.1. Budget Narrative. ........................................................................................................... 34 8.1.1. Budget Analysis to Ensure That Participation in Daily Operations is Sufficiently Supported. ............................................................................................... 34 8.1.2. Facilities and Equipment. ............................................................................................. 37 8.2 Budget Details. ................................................................................................................... 38 9. Data Management Plan. ....................................................................................................... 41

The last two meters of exhumation at Mount Sharp: Linking rover-scale geomorphology, taphonomy, and winds

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2. Scientific/Technical/Management:

2.1. Summary, Objectives, and Expected Significance. In what locations along MSL’s planned traverse have Mount Sharp’s rocks been exhumed over the last 106-109 years sufficiently rapidly to preserve complex organic matter? How has this exhumation been affected by microclimates, and differential rock resistance? To what extent have wind stress, abrader availability, and rare wetting events, controlled the pattern of exhumation? Motivated by the importance of exhumation as a control on taphonomy and as a proxy for climate-driven change, the broad objectives of this proposal are to analyze relief along the planned MSL traverse as a proxy for exhumation processes, determine the pattern of saltation-abrasion potential, and constrain the role of physical abrasion and aqueous processes in exhuming the rocks of Mount Sharp (Aeolis Mons). We will address these objectives through Mastcam, MAHLI and REMS measurements (of topography, rock texture, and winds), supported by modeling, with secondary contributions from DAN. The investigation requires participation in the science team to acquire specific measurements. Specifically, we will use Mastcam [Malin et al. 2010] and MAHLI [Edgett et al. 2012] images, and simple numerical models, to constrain landform degradation and erosional resistance. In parallel, we will use Mastcam image data, models, and REMS [Gómez-Elvira et al. 2012] observations, to carry out landform-scale mapping of saltation-abrasion potential. We will also use Mastcam, REMS, and the PI’s snowmelt model [Kite et al., 2013b], to quantify the possible role of aqueous fluids in exhumation. We shall thus obtain an improved understanding of the exhumation of sedimentary rocks at the <2m length scale and 106-109 yr timescale that are relevant to the preservation of complex organic matter and to geologically recent climate change.

Locations with >2 m of geologically recent (<<109 yr) exhumation have improved potential to preserve complex organic matter [Farley et al. 2014, Grotzinger et al. 2014]. Given the importance of taphonomy to mission goals [MSL Extended Mission Plan 2014], improved understanding of exhumation – and identifying correlates of recent exhumation remotely and on an operational timescale – can enhance the science return from MSL. The proposed research will improve the science team’s ability to understand controls on recent exhumation by integrating data from multiple instruments and promptly comparing them to models. This can enhance the science return from SAM cosmogenic age-dating, by enabling predictions further along the traverse, and for future landing sites. The proposal builds on >8 years of experience by the PI with Mars microclimates, Mars erosion, and late-stage aqueous processes on Mars, and will leverage the PI’s existing funded project to study mountain-terrain effects on exhumation using the Mars meteorological model MRAMS (the PI has previously published papers using MRAMS; Kite et al. 2011a, 2011b) as well as the PI’s published models of Mount Sharp wind erosion and late-stage snowmelt at Mount Sharp [Kite et al. 2013b, 2013c]. Equally important to our proposed scientific investigation is our contribution to the success of the rover’s day-to-day operations, by providing scientists (the PI, a postdoc, and a graduate student) to serve in daily operational roles and assist in the acquisition of specific measurements.

2.2. Scientific Background. 2.2.1. Exhumation processes link Mount Sharp geomorphology and climate. Exhumation is necessary to form the moat that defines Mount Sharp, and to expose the mountain’s outcropping layers. Ongoing exhumation is suggested by Mount Sharp’s low crater density (Fig. 1) [Thomson et al. 2011]. Exhumation requires comminution of rock to fragments,


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Kminek & Bada (2006)Dartnell et al. (2007)GCR only.SCR neglected.500 amu values (dashed) are extrapolated.

Hartmann (2005) crater !uxes.Truncated at 0.13 km crater diameter.Consistent with Sefton-Nash et al. (2014)and Newsom et al. (2015).

Farley et al. (2014)adjusted to equivalent steady exhumation.

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Fig. 1. Exhumation variability has significant implications for the preservation of complex organic matter. MSL’s Extended Mission will encounter terrains with higher crater-obliteration rates than terrain encountered during the Primary Mission. Crater-obliteration curves on Mt. Sharp are consistent with steady exhumation [Smith et al. 2008]. If steady exhumation is occurring (testable with Mastcam; §2.3.1) then enhanced organic-matter preservation potential is expected at Mount Sharp. For details, see §2.2.2 and §2.3.4. Phyllosilicates are mapped at the stratigraphic level of rapid crater obliteration [Thomson et al. 2011].

and surface shear stresses (wind or water) sufficient to export rock fragments from the mountain.1 Landscape-modification rates and processes can be interpreted using crater size-frequency distributions (CSFDs) (Fig. 1). CSFDs for Gale’s moat are well-explained by rare pulses of resurfacing [Newsom et al. 2015], in agreement with Cosmic-Ray Exposure (CRE) ages [Farley et al. 2014], as well as with Mastcam observations indicating slow landscape-averaged lowering rates. By contrast, CSFDs for Mount Sharp show few craters, and are well-fit by a -2 power law exponent, implying steady-state between crater production and crater obliteration for diameters <0.05 km [Opik 1965, Smith et al. 2008, Lewis & Aharonson 2014, Kite et al. 2013a]. In summary, CSFDs suggest that crater-obliteration at Mount Sharp is different from exhumation in Aeolis Palus [Newsom et al. 2015], more variable unit-to-unit, and plausibly much more active.

A physical forcing that is correlated with these observations is the terrain influence on wind. Strong winds occur on steep terrain such as Mount Sharp [Kite et al. 2013c, Vasavada et al. 2012, Tyler & Barnes 2013, Moores et al. 2015] (Fig. 2). Ripples at Gale are in active motion, indicating ongoing saltation and thus the availability of abrasive tools [Silvestro et al. 2013]. Yardangs (straight subparallel crests with a ~4:1 aspect ratio) are seen over Mount Sharp, indicating wind erosion. On Earth, wind exhumation carves canyons and exhumes basins in soft

1 Mass wasting and slope activity also contribute [Anderson & Bell 2010, Dundas & McEwen 2015], but self-limit unless talus blocks are removed by weathering or by abrasion-in-place.


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rocks [Perkins et al. 2015, Rohrmann et al. 2013]. Soft rocks on Mars are aqueously altered, and aqueous alteration is indicated by CRISM spectra of Mount Sharp. These data suggest that exhumation by wind-induced saltation abrasion contributes to the rapid and variable crater-obliteration required to explain Mount Sharp CSFDs.

Variable exhumation at Mount Sharp could correspond to variation in the rate of loose-fragment removal; or alternatively to variations in the rate of comminution due to (1) varying rock resistance (§2.3.1), (2) oversupply/undersupply of saltating sand (§2.3.2) [Sklar & Dietrich 2004], or (3) weathering/erosion changes due to microclimates and/or climate change (§2.3.3). Climate change is particularly important to modulating erosion [Golombek et al. 2006, 2010, 2014]. Both erosion, and weathering, should accelerate under increases in Mars’ atmospheric pressure (p) and pH2O that theory predicts occurred 6×105 – 109 ya. At higher obliquity, pH2O should increase, favoring H2O(i) deposition, hydration-state changes, and possible wetting events [Jakosky et al. 1995, Madeleine et al. 2009]. Wetting would also be increased by higher p in the recent past [Phillips et al. 2011, Barabash et al. 2007, Kite et al. 2013b]. Similarly, aeolian sediment-transport capacity scales as p3 [Armstrong & Leovy 2005, Fenton et al. 2001]. Late-stage wet events may help to account for Mount Sharp’s inverted channels [Milliken et al. 2014]. Because Mount Sharp rocks underwent compaction and cementation [Bridges et al. 2014, Siebach & Grotzinger 2013], their low present-day thermal inertia is consistent with late-stage weathering [Hamilton et al. 2014, Knoll et al. 2008, Arvidson et al. 2010]. Despite long-term net exhumation, it is possible that Mount Sharp is gaining mass under current orbital conditions [Moreau et al. 2014, Kocurek & Lancaster 1999]. The processes responsible for the last 2m of exhumation may be the same as, or differ from, those that produced Mount Sharp’s larger-scale relief (Fig. 3). Rover observations are needed to determine what processes are responsible for spatial and temporal variability in Mount Sharp’s exhumation. For example, rover observations are needed to distinguish small-crater obliteration via diffusive infilling, versus small-crater obliteration via landscape lowering (Fig. 4). These mechanisms are difficult to distinguish from orbit, but only landscape-lowering implies bedrock exhumation (and thus improved organic matter preservation potential).

Fig. 3. Hypotheses for patterns of exhumation, linking exhumation and landscape evolution.

Fig. 2. Steep slopes cause the strongest (annual-max.) wind speeds at Gale crater (color scale: m/s, 1.5 m above ground) [from Kite et al. 2013c].

MT. SHARP EXHUMATION AND RELIEF:Relief scale varies from <10 m (Murray Buttes) to >> 100 m (ascent canyon)

rare pulses/disequilibrium

Craters obliterated by advection (Mastcam, RMI)Non-uniform pedogenesis (Mastcam, DAN) >10x gradients in CRE age (SAM)Saltating sand pervasive (Mastcam, REMS)

Ongoing/Recent Relief Enhancement Relief Degradation via Scarp Retreat Relief Preservation, Patchy Recent Exhumation

Di!usive degradation of craters and scarps (Mastcam)Alteration rinds, pedogenesis (Mastcam, Chemcam, DAN) >10x gradients in CRE age (SAM)Sand accumulates where winds are weak (Mastcam/REMS)Landforms in equilibrium with wind"eld (REMS)

Advection-dominated scarp pro"les (Mastcam)Fallen blocks abraded near scarps (Mastcam)Pedogenesis minor or absent (Mastcam, DAN)Low CRE ages only near scarps (SAM)Non-uniform shear stresses (REMS)

Recent Exhumation

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Farley et al. 2014Borodat et al. 2014

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Grant et al. 2008Fenton et al. 2015

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Fig 4. Length scales of wind-topography interaction along MSL’s traverse: a) Echo dune at 15m high obstacle, “clays” unit. b) Crater 125m across undergoing diffusive infilling, near MSL’s planned traverse. c) Crater 68m across in a rapidly-eroding terrain on Mount Sharp. d) Wind tails in sulfate-cemented sedimentary rock at MAHLI scale (Burns Formation, Sol 392 Opportunity; Fe2O3 obstacles are ~2 mm across). e) Sand concentrated within a small canyon at Mount Sharp (~100m wide, ~30m deep).

2.2.2. Taphonomy can be constrained by rover investigation of exhumation processes. “[L]evels of present-day ionizing radiation measured by [RAD] place severe constraints on preservation [of organic matter]” [MSL Extended Mission Plan 2014]. Preservation of complex organic matter, chiral anomalies, and interpretable 13C anomalies, over Gyr is difficult in the absence of shielding [e.g. Hutchinson 1963, Kminek & Bada 2006, Dartnell et al. 2007, Iglesias-Groth et al. 2011, Pavlov et al. 2012]. Adequate shielding requires >2 m of overburden [Wieler & Graft 2001]. Dividing 2m by typical Mars resurfacing rates implies ≳108 yr exposure [Golombek et al. 2006, Farley et al. 2014]. These exposure ages, combined with measured radiolysis constants, argue against preservation of ancient complex organic matter with molecular mass >150 Da except where exhumation is rapid [Kminek & Bada 2006, Hassler et al. 2014]. In slow-exhumation regions, simple organic molecules might persist – they have been detected by SAM on Mars at levels higher than known instrumental backgrounds [Freissinet et al. 2015] – but are hard to interpret given the high flux of Mars-crossing organic-rich bolides >3.3 Ga [Bottke et al. 2012, Mustard et al. 2013]. In summary, Galactic Cosmic Radiation (GCR) affects the Mars Program’s ability to detect complex organic matter, to discriminate indigenous organic matter from exogenic contaminants, and to diagnose potential biomarkers [Mustard et al. 2013].

Fortunately, three micro-environments along MSL’s traverse have enhanced potential for preserving complex organic matter, because of recent exhumation: (1) zones of enhanced wind erosion; (2) the ejecta of young craters; (3) recently-fallen boulders. A key link between these micro-environments is the wind. Wind erosion drives oversteepening leading to boulder-fall and mass wasting, abrades and removes loose debris including ejecta, and directly exhumes layers at scarps and canyon floors. Therefore, understanding wind erosion patterns is relevant to organic-matter preservation. Organic-matter fractional preservation potential (Ω) at sampling depth increases as exhumation rate (ż) increases. The simplest physically consistent model is (Fig. 1):

where R is GCR dose, a and b are coefficients fit to depth-dependent dose rate calculations [e.g. Dartnell et al. 2007] (adjusted to match RAD surface fluxes [Hassler et al. 2014], and shallow-subsurface hydrogen abundance constrained by DAN), D is depth, k is radiolysis constant (Gy-1)

ab

ed

c


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[e.g. Kminek & Bada 2006], and Mamu is molecular mass (Da). 2 m is the approximate depth below which 40K decay dominates R. The Ω-ż model neglects Solar Energetic Particles (SEP) because SASPAH discards the topmost 1 cm of sample, and neglects the fact that the Pfotzer maximum is slightly below-surface because this has a minor effect on R integrated over a steady exhumation history. R is not very sensitive to p(t) in the range 0-0.05 bar [Dartnell 2008]. The simplicity of the model is justified because it allows improved parameter values (e.g. molecule-specific k) to be easily incorporated as they become available. Fig. 1 uses ż estimated from CSFDs; improved constraints on the pattern of ż require rover measurements (§2.3).

2.2.3. Winds’ role in exhumation can be constrained by rover investigation. REMS data support strong terrain control on winds at Mount Sharp, as expected: long steep slopes should drive a diurnal slope-wind cycle because of Mars’ thin atmosphere [e.g. Rafkin et al. 2003, Kite et al. 2013c, Tyler & Barnes 2013, Moores et al. 2015]. Gomez-Elvira et al. [2014] note “strong correlation with the local topography,” and report afternoon winds consistent with Mount-Sharp sourced slope winds. A basic model with only slope-wind forcing and an erodible mound predicts downslope-oriented yardangs. This basic model is too simple to match data: troughs below -3.8 km elevation are complex and will require rover observations to determine whether trough formation is ongoing, the relative importance of wind erosion, and the rate-limiting process that sets spatial variability of exhumation (Figs. 1-4).

Fig. 5. The last 2 m of exhumation at Mount Sharp probe 106-109 yr timescales that are crucial for taphonomy [Pavlov et al. 2012], include climate changes [MEPAG Goals Document 2012], and significant landscape evolution [Grindrod & Warner 2014, Newsom et al. 2015, Farley et al. 2014]. The proposed investigation will broaden and complement existing MSL Science Team investigations.


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Saltation-abrasion exhumation requires winds strong enough for saltation, rocks weak enough for abrasion, and tools (abrading particles that are not destroyed on impact with the target). When these conditions are met, wind erosion at >1 µm/yr may occur [Sagan 1973, Greeley & Iverson 1982, Bridges et al. 2012, Grindrod & Warner 2014]. Wind erosion may also occur by weathering followed by soil deflation [Shao 2008]. Therefore, the rate-limiting step for saltation-abrasion exhumation may be supply of tools to abrade soft rock (§2.3.1), adequate wind stress to transport particles (§2.3.2), or weathering (§2.3.3). MSL’s payload is well suited to testing the distinct predictions of these scenarios. Specifically, (1) tools-supply limitation predicts that rapid exhumation will be correlated with the supply of saltating sand, which we will map using Mastcam, and that on inspection by MAHLI saltating particles will have a tone, texture, and crushing resistance that are consistent with basaltic sand, as opposed to aggregates or soft grains [Sullivan et al. 2008]. (2) By contrast, rock-resistance limitation predicts high crater density on cliff-forming layers, slow erosion, and the formation of erosion-resistant blocks; this can be tested by using Mastcam to measure scarp profiles, and MAHLI to investigate the texture of cliff-forming vs. slope-forming layers. (3) Wind-stress limitation predicts that maximum wind stresses (as indicated by REMS observations, and modeled using MRAMS and WindNinja) will be most intense in regions of active exhumation. Wind-stress limitation can also be tested by correlation of bedform net migration direction with prevailing strong winds predicted by MRAMS. (4) Finally, liquid water may contribute on 106-109 yr timescales to rock weathering and rock breakdown, and plausibly to geologically recent sediment transport [e.g. Milliken et al. 2014, Martín-Torres et al. 2015]. This predicts that rapid exhumation will be correlated to active/recent formation of soil (disaggregated, in-place material that may be chemically weathered) as determined by Mastcam and Chemcam. All four factors may contribute to varying extents, and anomalies may suggest legacy effects (Fig. 3). Relative exhumation rate can be constrained by CSFDs, scarp profiles, CRE ages, and the presence or absence of weathering/oxidation rinds.

Seventy years of studies enable linking form to process in wind erosion of rock [Blackwelder 1934, McCauley et al. 1977, Ward 1979, Iverson et al. 1982]. Initial grooves focus abrading sand leading to deepening, and channeling increases wind stresses on troughs. Saltating particles usually move <2m from the surface [Bagnold 1936; Kok 2010]. A theoretical basis for yardang abrasion has been established and initial calibrations performed [Wang et al. 2011]. Yardang shapes evolve to minimize global drag. Dust abrasion is unimportant [Laity & Bridges 2009].

2.3 Technical Approach and Methodology. We hypothesize that exhumation rates along the anticipated extended-mission traverse are highly variable (Fig. 1). No single indicator can confirm exhumation rates short of scarce CRE-age measurements, motivating investigation of multiple exhumation-rate proxies. Our investigation consists of four elements, focused on: rock resistance (§2.3.1, ~35% of science effort), aeolian forcing (§2.3.2, ~30% of science effort), and wetting events that reduce rock resistance to aeolian forcing (§2.3.3, ~15% of science effort), leading to updated estimates of the pattern of organic-matter preservation potential (§2.3.4, ~20% of science effort). The balance of science effort between the elements of our investigation will be revisited as knowledge about Mount Sharp exhumation accumulates. Each element of our investigation is enabled by involvement in rover operations, and can be enhanced by specific measurements with the MSL payload. The PI will plan and advocate for these measurements with the members of the appropriate STGs. To optimize usage of the rover resources, we also explain how individual goals can be accomplished without tailored MSL measurements.


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Fig. 6. Steep relief of Mount Sharp allows MSL investigation of exhumation processes. 2.3.1. Stereo-Enabled Slope Analysis of Exhumation Susceptibility. Slope profiles reveal differences in erosion resistance that can assist stratigraphic correlation. Slope profiles can alternatively be used in lithologically homogenous terrains to determine relative ages, as well as the relative importance of scarp retreat (advection) versus diffusive degradation.

Lithologically homogenous: We hypothesize that older scarps are less steep than younger scarps. Mount Sharp has many steep slopes (“scarps”). Scarp degradation is a proxy for relative age, and potentially (after calibration by SAM) for absolute age [Avouac 1993, Pelletier 2006, Burbank & Anderson 2010]. For all scarps with Mastcam stereo products at >5× better-than-HiRISE resolution (<200m range for M34), we will first mask out talus blocks. (The size, and density, of talus blocks, as a function of their distance from the toe of the scarp, will also be measured as part of this masking step). We will then average along-scarp to create an averaged slope profile (with bootstrapped error bar). If multiple stereo products are available we will coregister and average these products to minimize error [Hayes et al. 2011]. If there are no obvious lithological breaks, we shall then fit each slope profile with two free parameters: the diffusivity-time product (κτ, m2), and a dimensionless ratio (Pe) of diffusion to advection u [Pelletier 2008, Perron et al. 2009]. The fit procedure involves solving the equation ∂z/∂t = κ∇2z - u and (following calibration) yields good agreement with CRE for both weakly-indurated and well-indurated materials. We assume slopes are initially at the angle of repose. Slopes initially at the angle of repose can be formed at Mount Sharp by impacts, seismicity, mass wasting, canyon incision, erosion under a past thicker atmosphere, or even by wave action [Palucis et al. 2014]. We shall carry out an end-to-end check of our measurements by comparing HiRISE Digital Terrain Models (DTMs) to Mastcam DTMs of distant tall scarps. In modeling scarp degradation, we will use the full-scarp fitting method which has been shown to be unbiased and less noisy than midpoint-slope methods [Pelletier et al. 2006]. This method has been validated for scarps up to 20m in height, with no evidence for nonlinearity [Pelletier et al. 2006]. If scarp retreat is active [Farley et al. 2014], we expect to find steep slopes and a low percentage coverage of talus. If scarp retreat is inactive, we expect to measure diffusive relaxation of sub-angle-of-repose slopes, with >60% talus coverage marking the past retreat of scarps. Where outliers are large, we will re-assess using a standard terrestrial weathering correction [Anderson & Humphrey 1989] (§2.3.3). We will carry out an analogous procedure on crater rims [Golombek et al. 2014], using initial crater topography from Watters et al. [2015] complemented by a complete crater count from HiRISE on a 100m-wide buffer around the traverse. This will allow us to determine the relative roles of diffusive infilling versus terrain-lowering in crater degradation [Pelletier & Cline 2007].


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We will also determine the κτ of ejected blocks assuming an initial Grady-Kipp distribution of ejected-block fragment sizes [Melosh 1989, Golombek et al. 2010]. We will then compare our κτ and Pe to previous measurements. Diffusive degradation has been used for decades as a proxy for erosion on Earth [Carson & Kirkby 1972] and for Mars [Golombek et al. 2014, Vaz et al. 2014, Fassett & Thomson 2014, Watters et al. 2015], where diffusivities κ ~ 10-6 m2/yr are reported. Lithologically heterogenous: We hypothesize that rock texture at MAHLI scale is correlated to abrasion-resistance inferred from Mastcam slope profiles. Erosion susceptibility (slope-forming versus cliff-forming units) is used for reconnaissance mapping of geologic units in terrestrial fieldwork, and slope-forming and cliff-forming units have been identified at Mount Sharp [e.g. Milliken et al. 2010]. Erosional resistance may, with appropriate caution, be used to map the lateral extent of layers at all stratigraphic scales encountered along the traverse (e.g., fluvial-channel fill) [e.g. DiBiase et al. 2013, Schmidt 1989], supporting geologic analysis. We will empirically determine what factors control erosional resistance at Mount Sharp. Specifically, MAHLI focus merge products will be used to define an empirical erosion-resistance index. The factors to be investigated shall include: MAHLI grain D50 and D95 where measurable, grain sorting, porosity, matrix vs. clast-supported texture, and vein spacing, maximum vein thickness, and S-content from APXS as a proxy for sulfate addition. We will also regress erosion resistance on DAN subsurface H and, separately, subsurface Cl. Identifying the most important factors will constrain exhumation processes. For example, if erosionally weak materials have Cl cements, then that would suggest a role for dissolution (§2.3.3). If coarser-grained materials are more resistant and lags are observed, that would suggest winds are rarely strong enough to mobilize coarse clasts. Yardang formation has a distinctive topographic signature (steep upwind, gentle downwind), and we will take account of wind-stress variations (§2.3.2) in determining erosion resistance. Although rover safety may rule out approaching some cliffs, geologic correlation with more-accessible slopes should be possible because Mt. Sharp’s layers are subhorizontal (Kite et al. 2013c) (e.g. Maria’s PassàStimson/Pahrump). A specific measurement, mid-drive Mastcam imaging to enable long-baseline stereo, can enhance this element of our investigation. The longer stereo baseline allowed by mid-drive imaging can reduce errors for scarps, and craters, of particular interest [Di et al. 2008]. We will plan for and advocate mid-drive imaging where appropriate to support §2.3.1.

Fig. 7. Example of scarp profile extraction, for a lithologically heterogenous target (Sol 935, Opportunity). Break-points in scarp-perpendicular profile (average of 40 profiles, excluding overhang region) correspond to mapped geologic contacts [Squyres et al. 2009, Hayes et al. 2011]. Gray lines show scatter of individual profiles. Slopes (colors) include slopes out of the profile plane.

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2.3.2. Mapping Aeolian-Exhumation Potential with Mastcam, REMS, and modeling. We will produce and maintain a reconnaissance map of exhumation potential that combines a tools-and-cover map (from Mastcam and engineering cameras) with wind-stress maps (from models plus REMS). The tools-and-cover hypothesis predicts that abrasion potential will be maximized where sand cover is neither abundant nor absent, and where wind stress is maximized (Fig. 6). We hypothesize (1) that wind speeds respond to Mount Sharp topography on multiple length scales (2) that exhumation is most rapid where sand is neither abundant nor absent.

The wind-stress map will be at 80m resolution along the traverse, with higher-resolution runs when the rover encounters complex terrain. Because REMS functions with one boom [Gomez-Elvira et al. 2014], we will augment REMS measurements with very-high-resolution models. Modeling down to a horizontal length scale of 80 m shall be carried out using the nonhydrostatic Mars Regional Atmospheric Modeling System (MRAMS) [Michaels et al. 2004, Kite et al. 2011a, Kite et al. 2011b]. MRAMS has been used to support engineering assessment of atmospheric conditions for all Mars landers post-Pathfinder [Vasavada et al. 2012], and has been successfully run at ~100 m horizontal resolution (Fig. 8) [Fenton & Michaels 2010]. The PI has published two lead-authored papers using MRAMS [Kite et al. 2011a; Kite et al. 2011b]. We have already generated an 18-meters-per-pixel CTX DTM and ingested it into MRAMS, using it to drive an 8-nested-grid canyon-resolving model (Fig. 8) for comparison with REMS and Mastcam measurements. Boundary conditions are from the Ames MGCM. We propose to carry out runs for 4 seasonal forcings (Ls = 0°, 90°, 180°, 270°), 2 dust cases (τIR = 0.25 and τIR = 2.5), and at 25.2° and 45° obliquity, for 7 sols per run, in order to sample rock-exhuming conditions for near-modern topography. We will analyze and interpret discrepancies between MRAMS predictions and REMS measurements, but we do not propose to carry out a formal reanalysis.

We shall use MRAMS to predict 95th-percentile wind stresses and prevailing winds, because the strongest gusts are believed to be responsible for wind erosion. In regions of exceptionally complex terrain, we shall use output from single MRAMS pixels, and PDS HiRISE DTMs, to

Fig. 8. Canyon-resolving wind model output (MRAMS). Yellow line corresponds to future traverse (dashed where extrapolated). A: Wind vector field. Arrow size corresponds to wind speed. Range of wind speeds shown is 0-3 m/s. B: Shear velocity (m/s).


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initialize a mass-conserving model, WindNinja [Forthofer et al. 2014], in order to give a qualitative indication of small-scale channeling patterns. WindNinja will allow us to track the effect of yardangs and Periodic Bedrock Ridges on the windfield [Ward & Greeley, 1984] as well as small-scale blocking (sheltering) effects [Kimura & Manins 1988]. We have verified that WindNinja run times with HiRISE topography on single CPUs are typically <45 s.

The tools-and-cover map will bracket the traverse with half-width 100m or the local horizon, whichever is smaller. All points in the map will be classified as either: resolved talus clast (with apparent long dimension measured), unresolved talus clasts, mobile cover, fragmented bedrock, intact bedrock, soil, not determined, or not observed. The tone, apparent thickness, and presence/absence of bedforms will be noted, along with bedform orientation, the resolution of images used to document tools-and-cover, and any evidence for recent mobility. Although the depth of deep sand layers is uncertain, even a thin continuous sand cover will protect underlying rock from erosion (“cover” effect). Nevertheless, we will mitigate sand-depth uncertainty using bedform height (gaps between bedforms) and (where available) wheel scuffs to assess the average thickness of sand. The map will be analyzed by looking for correlations between tools-and-cover and the following variables: local slope, local relief, and stratigraphic units (Figs. 1, 4).

Combining the tools-and-cover map with the REMS-checked map of erosive winds will allow us to test the hypothesis that bedforms switch to a slope-parallel orientation at the location where measured strong winds switch to upslope-downslope, i.e., that current bedform orientation corresponds to the current wind field. Combining the maps will also enable testing the hypothesis that sand accumulates where wind-stress is minimized.

The exhumation-potential map, especially when combined with CSFDs and with crater-degradation and scarp-degradation data (§2.3.1), can help direct attention to likely taphonomic windows. Therefore, the exhumation-potential map could benefit the strategic or even tactical decision making on the mission.

Models predict that winds on Mount Sharp will be upslope during the day, and downslope during the night. Testing this prediction is crucial to validate wind models, and REMS is capable of carrying out this test. REMS measures wind speed to ±50%, and direction to ±20°, when ASIC temperature exceeds 213K [Gómez-Elvira et al. 2014], but only for a ~180° frontal arc. Therefore, we propose to measure diurnally-reversing winds with a dawn-dusk turn-in-place experiment. This will involve pointing the REMS boom within 30° of downslope during the day to measure anabatic winds, then rotating the rover prior to dusk by ~180° to measure the speed of katabatic winds just after sunset. REMS electronics are isolated, so the experiment will not require keeping the rover powered on after sunset. Experiment location(s) will be chosen to maximize model calibration value, trading off with seasonal variation in ASIC temperatures (Ls=270±45° is best for post-twilight measurements). A similar experiment could also measure wind-shadow effects.

Opportunity results show how joint analysis of rover datasets can illuminate exhumation processes [Arvidson et al. 2015, Grant et al. 2009, Sullivan et al. 2005, Golombek et al. 2010]. Pancam shows how terrain-directed wind erosion exposes bedrock (with strong CRISM aqueous-mineral signatures) downwind of sand traps. MER MI shows the tools are basaltic clasts, and APXS/Mossbauer/RAT show the target rocks are water-altered and soft. Opportunity’s field site is flat, and exhumation is slow [Golombek et al. 2014]. The greater relief and fast exhumation anticipated for Mount Sharp should allow contrast with both Aeolis Palus and Meridiani Planum.


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Rock-abrasion features at Meridiani Planum do not match the orientation of modern ripples, requiring O(106) ya climate change [Fenton et al. 2015, Bridges et al. 2014, Haberle et al. 2003]. Similar discrepancies at Mount Sharp would provide clues to changes 104 km from Meridiani.

2.3.3. Assessing the Role of Late-Stage Weathering in Mount Sharp Exhumation. The wind-refreshed landscape of Mt. Sharp is a natural laboratory for studying soil formation on Mars [Amundson et al. 2008, Arvidson et al. 2010]. Rock-to-soil conversion usually occurs <2m from the surface. The soil production function (dzsoil/ dt = f(zsoil), where zsoil is soil thickness) [e.g. Heimsath et al. 2012, Larsen et al. 2014] is poorly constrained for Mars because disentangling modern production from legacy effects is very difficult unless the landscape is “reset” by rapid exhumation [Cousin et al. 2015]. The rapid exhumation anticipated for Mount Sharp (Fig. 1) suggests that MSL can test Mars soil-production hypotheses. Specifically, an Earth-like “humped” soil production function predicts patches of soil interspersed with exposed bedrock [Heimsath et al. 1997]. By contrast an “extreme desert” soil production function predicts drifts of mobile aeolian material interspersed with bedrock weathering-in-place, and salt-crust accretion competing with aeolian deflation [Ewing et al. 2006]. These predictions (which can be tested by Mastcam) differ greatly in their implications for surface liquid water < 1 Ga [Amundson et al. 2012]. Indeed, <1 Ga liquid water has been suggested for all Mars landing sites including Gale [Martin-Torres et al. 2015, Dundas & McEwen 2015, Knoll et al. 2008, Arvidson et al. 2010, Herkenhoff et al. 2008]. REMS data show Gale moat microclimate marginally permits brines [Martín-Torres et al. 2015], and theory predicts that that Mount Sharp’s complex terrain should favor patchy wetting [Hecht 2002]. Late-stage wetting could aid wind erosion via weathering and via dissolution of indurating salts [Dong et al. 2012, Amundson et al. 2008, Nickling 1984]. If no evidence is found for late-stage wetting, that would suggest >mm exhumation since the last wet event.

We hypothesize that soil formation <1 Ga at Mount Sharp involves both physical and chemical processes. We propose to monitor the landscape along the traverse for signs of rock breakdown and weathering: for example, disaggregated, non-transported material (Mastcam), patchy oxidation rinds (Mastcam/Chemcam), and anomalously high subsurface H concentration (DAN). For parts of the landscape (e.g., slopes and alcoves) where there is reason to suspect past wetting, we will model possible geologically-recent wetting events using the PI’s published seasonal-melting model (Fig. 9) [Kite et al. 2013b]. This model builds upon and extends four decades of community progress in understanding late-stage wetting on Mars [e.g., Toon et al., 1980; Clow, 1987; Carr & Head, 2003; Williams et al. 2009]. The code is computationally inexpensive and so

Fig. 9. Vertical discretization and energy flow in the PI’s seasonal-melting model (Kite et al. 2013b). Solid horizontal lines correspond to nz solid surface layers, dashed horizontal lines correspond to atmospheric layers. Tr is the effective atmospheric radiative temperature, Ta is the atmospheric surface layer temperature, and Ts is the ground surface temperature. Other symbols are explained in the text.


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can be run repeatedly for ~50 Myr of orbital forcing [Laskar et al. 2004, Kite et al. 2015c]. Energy balance for the subsurface layer adjacent to the surface is set by:

and at depth z by the diffusion equation. Here, ρ is density, Cs is specific heat capacity, Δz is layer thickness, T is temperature, k is thermal conductivity, LW↓ is downwelling longwave radiation, sr corrects for Rayleigh-scattering, SW↓ is solar flux, Sfr corresponds to free sensible heat losses, Lfr corresponds to evaporative cooling by free convection, Sfo corresponds to forced sensible heat losses, and Lfo corresponds to additional evaporative cooling by wind. The effect of salt is parameterized as a freezing-point depression. The solid-state greenhouse effect is included. The model is complete and can be applied immediately [Kite et al. 2013b], and we will also leverage refinements that are already funded from an MDAP-2014 grant.

Using our model and Mastcam/Navcam DTMs, we will calculate thermal budgets and snowmelt potential forced by seasonal and obliquity cycles. The general procedure is to calculate the budget of thermal energy and resulting meltwater production, for a snow or ice-covered slope/alcove for all seasons and times-of-day, evaluating the effects of 1) Slope orientation and shadowing; 2) Effects of dust (through albedo control) on the heat budget; 3) Orbital forcing. This will yield the relative likelihood of wetting at points <100m from the traverse at times in the past when snow/ice was present. Melt output can be combined with simple assumptions about soil permeability to predict late-stage runoff.

Participation in the rover team is needed for §2.3.3 because the rover will encounter signs of soil formation on an operational timescale. For example, consider a detection of soil by ChemCam or Mastcam. With slope information from DTMs, and REMS-constrained thermal inertia, the computationally inexpensive snowmelt tool can then be used to assess the relative likelihood that soil formation involved late-stage wetting. If consensus is reached that the rover should investigate further, then MAHLI, APXS, ChemCam, and/or wheel-scuffing can measure vertical trends within soil and test pedogenetic processes [Amundson et al. 2008, Arvidson et al. 2010]. 2.3.4. Quantifying Exhumation’s Impact on Organic Matter Preservation Potential. We hypothesize that geomorphic analysis, assisted by wind modeling, can be used (on operational timescales) to identify sites with low Cosmic-Ray Exposure (CRE) ages.

Our approach emphasizes exhumation processes and relative rates, but predicting absolute rates is necessary for comparison with CRE ages, and to close the circle with taphonomy. Therefore, the final element of our investigation is a tool that maps out organic-matter preservation potential given an inferred exhumation history. Exhumation history is inferred from Crater Size-Frequency Distributions (CSFDs) validated by Mastcam (§2.3.1), from maps of saltation-abrasion potential (§2.3.2), from scarp freshness (§2.3.1), from liquid-water availability calculations constrained by orbital history (§2.3.3), and potentially from CRE measurements extrapolated to rocks that have quantitatively similar erosion resistance (§2.3.1). Organic-matter preservation potential is then calculated by fitting basic equations to experimental radiolysis data (Ω-ż model, §2.2.2) [Kminek & Bada 2006, Iglesias-Groth et al. 2011]. Exhumation-associated radiolysis calculations will contribute to Team assessment of the chances of finding a “taphonomic window” [Summons et


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al. 2011, Mustard et al. 2013].2 Radiolysis is potentially important and radiation exposure can be straightforwardly related to exhumation history, so our work may assist drill site selection.

As an example, we apply our tool to Cumberland using the Farley et al. [2014] CRE age and assuming scarp-retreat exhumation. Our tool predicts negligible organic matter >200 Da but partial (>10-4) fractional preservation at 113 Da (Fig. 1). This is consistent with SAM detection of chlorobenzene (~113 Da) at 150-300 ppbw, and nondetection of organic matter >200 Da [Freissinet et al. 2015]. In applying the equations from §2.2.2, we initially assume a density of 2 Mg/m3, but this could be adjusted as low as 1.2-1.4 Mg/m3 depending on local geology and evidence for weathering [Herkenhoff et al. 2008].

We will adjust predictions using DAN-reported H content. H content is important, because ~10 wt% H2O-equivalent H cuts complex-organic-matter preservation significantly [Dartnell et al. 2007, Gerakines & Hudson 2012]. H contents as high as 5 wt% H2O-equivalent have already been measured by DAN [Mitrofanov et al. 2014], and hydrated minerals are reported by CRISM at Mount Sharp. We assume that the H content as a function of depth below the surface is constant during exhumation. The self-shielding and topographic shielding factors for GCR shall be calculated using PDS HiRISE DTMs and the ArcGIS Viewshed tool, assuming a 10-mbar atmosphere [Codilean 2006, Phillips et al. 2011]. The topographic shielding factor for impactors (needed to correct CSFDs) will be calculated using the Williams code for impactor-atmosphere interaction [Kite et al. 2014, Williams et al. 2014]. Because the Ω-ż model is simple, it is easy to add further refinements, for example to the fluxes of impactors or of GCR [Hartmann 2005, Daubar et al. 2013, Hassler 2014]. It is also straightforward to specify exhumation rates that vary with time (e.g., scarp retreat, passage of a sand dune). Finally, we will add a Pfotzer maximum. SAM CRE ages will permit end-to-end checking of our workflow: for example, SAM should find CRE ages <107 yr in the ascent canyon (Fig. 1).

Applying this tool requires the PI’s involvement in the team to help plan for exhumation-history-constraining measurements. For example, a key parameter linking CSFDs to exhumation rate is target-material strength [Kite et al. 2014, Dundas et al. 2010]. Strength uncertainty can introduce impactor-size errors of up to a factor of 2 and thus (assuming a -2 CSFD power-law exponent) an exhumation-rate error of up to a factor of 4. To mitigate this uncertainty, we will help to plan Mastcam and/or RMI images of the interiors and ejecta of each crater encountered along the traverse, and use them to determine the strength of the impacted materials (rocks vs. regolith). The strength of planetary surfaces has been determined using crater-interior and ejecta morphology for five decades [Oberbeck & Quaide 1967]; because of its robustness this analysis was a key step in the validation of InSight’s landing site [Warner & Golombek 2014]. We will extrapolate to craters formed in materials with similar strength as determined using the workflow in §2.3.1. To relate rock-mass strength to impact-crater size, we shall use the Holsapple scaling [Holsapple 1993, Holssaple & Housen 2007, as implemented in Kite et al. 2014]. Finally, we will update exhumation rates using the improved crater-retention ages.

2 The “taphonomic window” can be closed earlier in the preservation chain if a suitable habitable environment was not present, by early-diagenetic degradation [Berner 1980], by oxidizing fluids [Fraeman et al., 2013], or by thermal degradation [Borlina, Ehlmann & Kite, submitted ms.]. CRISM shows clays at a range of stratigraphic levels, so the presence/absence of clays need not be dispositive for organic taphonomy.


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2.3.5 Assumptions and caveats. Variability in crater-obliteration processes make CSFDs problematic for dating areas <103 km2

[Warner et al. 2015, Daubar et al. 2013, McEwen et al. 2005, Hartmann 2007], but because the focus of our investigation is variability in crater-obliteration processes, crater-dating errors are not a concern for this investigation. Refinements in the absolute flux of impactors will accumulate in the next few years [e.g. Daubar et al. 2013] and when applied to CSFDs will provide an independent source of absolute-rate calibration (alongside CRE ages). The multi-element nature of our investigation mitigates the uncertainty in which process will dominate by allowing flexibility. For example, the dawn-dusk turn-in-place measurement plan assumes a major role for winds and meteorology in exhumation [Kite et al. 2013c], however if evidence accumulates that aqueous processes have predominated in the last 2 m of exhumation then we will emphasize the late-stage wetting investigation for which we are well prepared (as demonstrated by §2.3.3, Kite et al. 2015c, and Kite et al. 2013b).

2.4. Perceived Impact of the proposed work. With an improved understanding of exhumation patterns and processes, point measurements of CRE age can be connected to areal measurements of CSFDs, allowing comparison and cross-checking of these two independent exhumation-age measurement techniques. Because the Mars 2020 and ExoMars rovers lack a CRE-age dating capability, and because CSFDs can be measured from orbit, using MSL to improve our understanding of rover-measurable controls on exhumation may help field-site selection and traverse planning for future rovers. Therefore, this investigation will enhance the science return from cosmogenic isotope age-dating. In combination with analysis of the redox and pressure-temperature history of Mt. Sharp materials, our investigation may identify “taphonomic windows” along MSL’s traverse that are suitable for the preservation of complex organic matter. The 106-109 yr interval that is relevant for taphonomy has seen many climate changes on Mars [MEPAG Goals Document, 2012], and because exhumation patterns are sensitive to modest changes in climate (e.g. p, pH2O), exhumation processes are also a probe of environmental transitions on Mars. The stereo analysis workflow outlined in §2.3.1 could be applied to targets of opportunity, such as perched boulders (which constrain paleoseismicity) [e.g., Brune et al. 2006, Anderson et al. 2011, Haddad et al. 2012]. Sedimentary-rock erosion is a major unsolved puzzle all over Mars, not just at Gale [Malin & Edgett 2000, Grindrod 2014]. Recent soil-formation (§2.3.3) has comparative planetology implications [Dietrich & Perron 2006]. If our investigation finds that both physical and chemical processes contribute to the last 2m of Mount Sharp exhumation, this will mirror the mounds’ >3 Ga construction, which involved both aeolian and aqueous processes [Anderson & Bell 2010, Kite et al. 2013c, Milliken et al. 2014].

2.5. Relevance of proposed work to NASA goals, MSL Goals, and MSL Mission-‐Level Science Objectives. Because erosional exhumation is a probe of taphonomy and climate change, our work addresses MSL’s overall science goal “to assess the present and past habitability of [Mt. Sharp]” [MSL Extended Mission Plan, 2014]. Our work will address MSL Science Objectives #1 (“characterize geologic features, contributing to deciphering geological history and the processes that have modified rocks and regolith”) and #8 (“characterize […] basic meteorology”), and assist MSL Science Objective #9 (identify and quantitatively assess taphonomic windows). Geomorphology can provide evidence for climate change, so our work is relevant to Objective #10. Our work is complementary to the research already being conducted by MSL Science Team members (Fig. 5).


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2.6. Work plan. Milestones & Accomplishments Deliverables*

Yea

r 1

• Participate in training activities (3 days × 2 sessions × 3 participants). • Service in daily operational roles. Attendance of Team meetings. • Carry out and analyze MRAMS runs. • Construct detailed plans for, target, & analyze Camera measurements. • Create weathering-potential and exhumation-potential maps.

✓ Trained personnel. ✓ Reconnaissance exhumation-potential map. ✓ LPSC presentation on: Model predictions and comparison to initial Mt. Sharp data.

Yea

r 2

• Service in daily operational roles. Attendance of meetings. • Identify suitable target,conduct measurements,data-model comparison. • Construct detailed plans for, target, & analyze Camera measurements. • Update and refine weathering- and exhumation-potential maps. • Plan and execute wind experiment. Compare results to MRAMS.

✓ LPSC presentation on: Empirical erosion-resistance index. ✓ GRL-length paper on: Radiolysis for a range of exhumation scenarios. ✓ Improved exhumation-potential map.

Yea

r 3

• Participate in training activities (3 days × 2 sessions × 1 participant). • Service in daily operational roles. Attendance of Team meetings. • Construct detailed plans for, target, and analyze measurements. • Update and refine weathering- and exhumation-potential maps. • Analyze and interpret results of data-model comparisons. • Detailed data analysis.

✓ LPSC presentation on: Process implications of exhumation mapping. ✓ JGR-length paper on: Meteorology (from REMS and models) and sand distribution à saltation-abrasion potential and tests of canyon-forming exhumation scenarios.

Yea

r 4

• Service in daily operational roles. Attendance of Team meetings. • Construct detailed plans for, target, and analyze measurements. • Continue data analysis and write-up. • Update and refine weathering- and exhumation-potential maps.

✓ LPSC presentation: Test of the active-canyon-exhumation hypothesis. ✓ JGR-length paper on: Controls on exhumation processes and relative rates.

*We will lead-author papers and presentations as approved by the MSL team, as well as provide internal presentations / contributions to larger Team papers.

2.7. Personnel and Qualifications. (For FTE information, see §8, Budget Justification). The Principal Investigator (Edwin Kite) is an Assistant Professor at the University of Chicago. In the past 30 months the PI has lead-authored 6 Mount-Sharp-related peer-reviewed publications ranging from basic theory to detailed data analysis: the first climate model to identify physical processes that make Mount Sharp a hemispheric optimum for habitability [Kite et al., 2013b]; constraining the duration of Mars’ wet era using CSFDs [Kite et al., 2013a]; implementation of a novel sedimentary-rock proxy for Mars atmospheric evolution [Kite et al., 2014]; allostratigraphy of a near-Gale sedimentary basin using orbiter data [Kite et al., 2015a]; stratigraphic logging of the dimensions of fluvial deposits near Gale using high-resolution Mars DTMs [Kite et al., 2015b]; and the first paper to integrate geology and physical modelling to suggest a scenario for the origin and history of Mount Sharp [Kite et al., 2013c]. The PI has also co-authored papers published as the result of collaborations with current members of the MSL science team, including relating runoff evidence to climate constraints (Mangold et al., 2012; Kite 2nd author, wrote the runoff-production model and contributed to geological analysis), and analyzing Mount Sharp diagenesis (Borlina, Ehlmann & Kite, submitted; Kite wrote the wind-erosion model and contributed to the geologic analysis). The PI will supervise a Graduate Student at UChicago, who will carry out a substantial portion of the geologic mapping and analysis, and take part in running and analyzing the simulations. A Postdoctoral Researcher at UChicago will contribute to the geological analysis, lead the weathering analysis, and contribute to running and analyzing the simulations (especially the weathering model). While all three investigators are expected to participate in all tasks, they will have larger or smaller roles in each. The PI, a postdoc, and a grad student will each be trained to take part in daily operations at ≥50% level and will be available to actively and regularly take part in daily rover science operations (50% PI time, 50% postdoc time, 67% graduate student time). In addition, a Planetary GIS/Data Specialist, at UChicago, will support GIS/data integration for all tasks.


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the last two meters of exhumation at mount sharp: linking...

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