Wind erosion of layered sediments on Mars: The role of terrain

[Alternative: Slope-enhanced wind erosion of layered sediments on Mars: constraining mesoscale atmospheric interactions with evolving sedimentary landscapes]

Draft/Outline. For submission to the ROSES-2012 Mars Fundamental Research Program (NNH12ZDA001N-MFRP)

1. Table of contents............................................................................................................02. Scientific/Technical/Management................................................................................1 2.1 Executive Summary...............................................................................................1 2.2 Goals of the Proposed Study.................................................................................1 2.3 Relevance to NASA Strategic Goals.....................................................................1 2.4 Scientific Background............................................................................................1 2.4.1. Wind Erosion on Mars..................................................................................3 2.4.2. Slope winds...................................................................................................3 2.4.3. Growth and form of sedimentary mounds....................................................4 2.5 Technical Approach and Methodology................................................................5 2.5.1. Application of the Mars Regional Atmospheric Modeling System..............5 2.5.2. Numerical experiments with idealized craters and canyons.........................6 2.5.3. Consideration of the effect of sedimentary infill (sedimentary mounds).....8 2.5.4. Simulation of slope-eroding winds for geologically realistic terrain............9 2.5.5. Incorporation of slope winds into landscape evolution model...................10 2.5.6. Assumptions and caveats............................................................................11 2.6 Impact of Proposed Work...................................................................................11 2.7 Relevance of Proposed Work..............................................................................12 2.8 Work Plan, Personnel, and Responsibilities......................................................12 2.8.1. Work plan....................................................................................................12 2.8.2. Planned calculations and the parameters explored.....................................13 2.8.3. Personnel, Roles, and Qualifications..........................................................133. References.....................................................................................................................154. Biographical sketches..................................................................................................205. Current and Pending Support....................................................................................256. Statements of Commitment.........................................................................................267. Budget Justification.....................................................................................................27

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2. Scientific/Technical/Management:

2.1 Executive SummarySlope-enhanced wind erosion is the dominant process shaping layered sediments on Mars today, and the sedimentary rock record is predominantly a record of aeolian processes. Downslope-oriented yardangs, crater statistics, and the existence of moats and mounds with layers exposed around their perimeters show that this process is particularly important in equatorial sedimentary rock mounds, such as Valles Marineris and Gale. We propose to utilize mesoscale models to parameterize the effect of diurnal slope winds on wind-erosion rates on Mars, and incorporate this understanding into a landscape evolution model capable of probing atmosphere-landscape coupling over longer timescales. We shall thus obtain:- 1) a parameterization of the effect of terrain on wind erosion at the scale of sedimentary mounds and the craters and canyons that host them; 2) quantitative constraints on the processes and rates of wind erosion on Mars; 3) an understanding of how evolving, erodible topography couples to the terrain-influenced erosive windfield. All 3 of these goals address primary Mars science questions and will substantially improve our physical understanding of atmosphere-surface interaction on both modern and early Mars.

2.2 Goal of the proposed studyThe primary objective of the proposed work is to build a parameterized theory for the role of slope winds in shaping sedimentary accumulations using slope-wind/landscape couplings calculated from mesoscale models. This will involve the following steps:

Step 1. Mesoscale numerical experiments with idealized craters and canyons.Step 2. Additional mesoscale runs with sedimentary infill (sediment mounds and sheets).Step 3. Test theory against geological data in complex terrain, West Candor Chasma.Step 4. Develop and run a landscape evolution model incorporating theory built on mesoscale results.

2.3 Relevance to NASA strategic goalsThis proposal addresses the following Science Area Objective from the 2010 SMD Strategic Plan: “Understand the processes that determine the history and future of habitability of environments on Mars and other solar system bodies.”

2.4 Scientific background

2.4.1. Wind erosion on MarsWind erosion occurs when saltating sand-sized particles strike erodible surfaces [Greeley & Iverson, 1982; Anderson, 1986; Kok et al., 2012]. On Mars, saltating sand sized particles are in active motion [e.g., Bourke et al., 2008; Chojnacki et al., 2011; Silvestro et al., 2011], at rates that predict aeolian erosion of bedrock at 10-50 μm/yr [Bridges et al., 2012a]. Within the last ~0.1 Ma wind has mobilized particles ranging from dust aggregates to hematite granules [Sullivan et al., 2008, Golombek et al., 2010]. Aeolian abrasion of sedimentary rock has occurred within the last roughly 1-10 Ka [Golombek et al., 2010; Figure 1] and is probably ongoing. These recent findings make a compelling case for active aeolian erosion on Mars, and cap four decades of research into Martian aeolian bedforms, saltation and aeolian erosion

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[e.g., Sagan, 1973; Iverson et al., 1975; Ward, 1979; Thomson et al., 2008; de Silva et al. 2010; Montgomery et al., 2012; and many others]. The inability of General Circulation Models (GCMs) to reproduce the observed locations of sand transport [Bridges et al., 2012b] shows that mesoscale winds, not the regional-to-global winds resolved by GCMs, are responsible for saltation. Because of Mars’ thin atmosphere, slope winds dominate the mesoscale circulation within craters and canyons [e.g. Rafkin & Michaels, 2003; Spiga & Forget, 2009], where most sulfate-bearing sedimentary rocks are found. Downslope-oriented yardangs and grooves, crater statistics, and hematite lags show that the sedimentary mounds of the Valles Marineris, Gale, and the Medusae Fossae Formation, as well as layered dust aggregates on the flanks of large volcanoes, are all being actively eroded by slope winds. With rates easily outpacing cratering [Malin et al., 2007], and geologic and model estimates agreeing on the potential for may km of cumulative erosion [Armstrong & Leovy, 2005], wind erosion is the most important process changing mesoscale topography on Mars today and has been a first-order control on landscape evolution in sedimentary terrain. Large-scale exhumation opens the door to tectonic feedbacks, and this has been proposed for the Qaidam Basin on Earth [Kapp et al., 2011] and for Valles Marineris formation [Andrews-Hanna, 2012]. By contrast with the ≥1 μm/yr (up to 10km cumulative) inferred for layered sediments in canyons and craters, aeolian deflation averaged ~0.03 nm/yr over the last ~3.65 Gyr on the basaltic Gusev Plains [Golombek et al. 2006; Greeley et al., 2006]. In addition to the equatorial layered sediments that are the focus of this proposal, slope-enhanced winds define both the large-scale and small-scale topography of the north polar layered deposits (Chasma Boreale and spiral troughs) and circum-polar ice mounds, and played an important role in landscape evolution there [Holt et al., 2010; Smith & Holt, 2010; Conway et al., 2012]. Aeolian processes were also important on early Mars. Most of the observed sedimentary bedforms at the Opportunity and Spirit landing sites are sand dune foresets [Hayes et al., 2011] and aeolianites likely represent a volumetrically significant component of the ancient sedimentary rock record, including within the strata of the Gale mound [Anderson & Bell, 2010]. All known laterally extensive unconformities within the sedimentary record of Mars are consistent with aeolian deflation/erosion surfaces [e.g., Kerber & Head, 2010]. With the exception of the modern deflation surface, regional deflation events inferred from Aeolis-Zephyria and Meridiani predate the Amazonian [Edgett, 2005; Zimbelman & Scheidt, 2012].

Figure 1. Evidence for slope wind erosion of layered sediments on Mars, at a variety of length scales. Left panel: blueberry lag from aeolian erosion surrounds 30cm-diameter block ejected from ~1-10 Ka crater [Golombek et al. 2010]; center panel, yardangs from West Candor Chasma (MOC NA M1301494; 2km across). Right panel: Mesoscale topographic undulations within the Candor canyon sediments (image is ~600km across).

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2.4.2. Slope windsThe early loss of much of Mars’ atmosphere has made Mars a natural laboratory for studying the coupling between terrain and slope-wind erosion. With an atmospheric pressure close to the triple point of water, erosion by liquid water is no longer significant at the mesoscale, so basaltic slopes with many km of relief have persisted near the angle of repose for Gyr. Because of the thin atmosphere and weak greenhouse effect, the surface is close to radiative equilibrium. The combination of a 130K low-latitude diurnal cycle with an atmospheric lapse rate that is smaller than Earth’s (and very poorly coupled to surface temperature) leads to correspondingly strong diurnally-reversing slope winds [e.g., Toyota et al., 2011], and the slope-induced circulation is a forbidding hazard for landings in Valles Marineris [e.g., Kass et al., 2003]. The thin atmosphere also allows for topographic control of the large-scale circulation: in addition to stationary planetary waves and nonmigrating thermal tides, global Hadley-cell asymmetry results from hemispheric-scale slopes [Zalucha

et al., 2010], and slope effects drive regional winds over even very gentle slopes [e.g., Savijarvi & Siili, 1993]. Slope winds are particularly strong within the equatorial craters and canyons that host sedimentary rock mounds, because coriolis effects are weak and relief is high.

Slope wind studies on Earth have validated semi-analytic models of katabatic winds (and drainage flows) that make idealized assumptions about entrainment and topography but also shown their limitations, especially in areas of complex topography where strongly nonlinear effects dominate [e.g. Ellison & Turner, 1959; Manins & Sawford, 1979; Parish & Bromwich, 1987; Horst & Doran, 1986; Papadopoulos et al., 1997; Shapiro & Federovich, 2007, Trachte et al., 2010]. Semi-analytic theories relying on local slope alone do not explain the origin of moats (flat areas with increased erosion). They are inappropriate for equatorial sediment mounds on Mars, and do not provide enough detail for inclusion into a landscape evolution model.

Small-scale topography-windfield coupling produces sand dunes, but mesoscale topography-windfield coupling is not understood in part because slope wind erosion of bedrock is uncommon on Earth. There is no simple theory for slope winds in realistic terrain, and the regular spacing of yardangs also remains unsolved.

Figure 2. Example of MRAMS output for slope winds. This is a placeholder figure (use lake_0.001 runs because these include a chasm). Reference: Kite et al., 2011a.

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The proposed work will bring together the existing knowledge base on wind erosion and on slope winds.

Figure 3 . Examples of layered sediments within mounds, surrounded by moats. (Left) In the 30km-diameter Endeavour Crater recently entered by Opportunity, a thin tongue of sedimentary sulfates is three-quarters encircled by a moat. (Center) Radar cross section of Korolev Crater’s ice mound. (Right) Gale Crater (To be added) Valles Marineris chasm and moated mound. Credits: UMSF; ASI/SHARAD/Jack Holt; DLR/ESA.

2.4.3. Growth and form of sedimentary moundsMost of Mars’ sulfate-bearing sedimentary rocks are in the form of intra-crater or intra-canyon mounded deposits surrounded by moats. Moats are typically 15-20 km wide, but with considerable scatter. Most researchers agree that wind erosion is responsible for shaping the moats and mounds, but identifying the physical mechanism(s) that explain the growth and form of sedimentary mounds and moats has been challenging because physically motivated parameterizations of the coupling between terrain and slope-wind erosion are lacking. It is this gap that the proposed work will fill. One hypothesis, partly motivated by global-groundwater model for the sedimentary-rock water source, is that sedimentary layers completely filled each crater/canyon at least to the summit of the present-day mound deposit, followed by erosion to scoop out the moats and expose earlier deposited layers [Malin & Edgett, 2000; Andrews-Hanna, 2012]. An alternative hypothesis, motivated by the snowmelt model for the sedimentary-rock water source [Kite et al., in review, arXiv:1205.6226], is that sedimentary layer deposition and wind erosion occurred simultaneously, so that the moats were always present [Kite et al., in review, arXiv:1205.6840]. Concurrent sedimentary layer depositin and slope-wind erosion can explain the outward-dipping layer orientations and lack of illitization/chloritization of smectites near the base of the mound. Thus, understanding how sedimentary rocks are eroded today is relevant to understanding the water source for sedimentary rock formation. Several processes may contribute to slope-wind erosion:- (i) breakdown of sedimentary layers to wind-transportable fragments by processes associated with chemical transformations, such as weathering and/or volume changes associated with hydration state changes [e.g., Vaniman & Chipera, 2007]; (ii) physical degradation of hillslopes by mass wasting, followed by aeolian removal of talus to maintain steep slopes and allow continued mass wasting; (iii) aeolian erosion of weakly salt-cemented sediments [Shao,

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2000, Ch. 9]; (iv) aeolian abrasion of bedrock. These processes range from transport-limited to detachment-limited, and predict correspondingly different shear-stress dependencies, threshholds for erosion, and mesoscale mound morphologies.

We propose a focused study of wind erosion of sedimentary rocks (including the lower unit of the Gale mound), but our work is also relevant to slope-wind erosion of layered dust aggregates of the flanks of volcanoes [Spiga & Lewis, 2010, Bridges et al., 2010], and of the Medusae Fossae Formation (which is of unknown origin, but is tentatively correlated to the upper unit of the Gale mound; Zimbelman & Scheidt, 2012).

2.5 Proposed work

2.5.1. Description of the MRAMS (Mars Regional Atmospheric Modeling System).

We will use MRAMS, which is derived from the terrestrial RAMS code [Pielke et al. 1992] and has been adapted to Mars problems by Rafkin and Michaels.  MRAMS was used for entry, descent and landing simulations for the Mars Exploration Rovers, Mars Phoenix, and Mars Science Laboratory [Rafkin et al. 2001; Michaels and Rafkin, 2008], and has also been used in LES mode to study aeolian processes including dust lifting [Fenton & Michaels, 2010]. Because of our focus on dynamics, the aerosol microphysics capabilities of MRAMS [e.g. Michaels 2006] will not be used:- instead, dust will be specified using a simple, fixed Conrath-nu profile, and water ice will be zeroed out, leading to a several-fold improvement in speed. Water vapor will be included only as a passive, noncondensible tracer, and initial and boundary conditions will be chosen self-consistently so that water vapor is never saturated. Consistent with theory for saltation on Mars [Kok et al., 2012], we assume that the number of saltating grains at a given time is insufficient to modify the wind profile within the surface layer.

Vertical resolution will be varied from 2.3 km at the top of the model to 3 m near the ground. Horizontal resolution will be adjusted to be the lesser of 3km or 1.5% of the width of the feature being simulated (crater or canyon). Output will be sampled every 60s in order to capture short-lived wind events. Calculations shall be carried out on the CITerra/Fram cluster at Caltech. CPU requirements are set by the longest allowable timestep, which decreases with increasing simulated relief. From experience gained during a prior collaboration [Kite et al. 2011a, 2011b], we expect to spend ≤1 CPU month for idealized-terrain runs and ~2 CPU months for Valles Marineris simulations. Fram’s capabilities easily satisfy all our computing requirements.

Periodic boundary conditions in the horizontal dimensions will be employed, and an absorbing (“sponge”) upper boundary condition will be used. MRAMS shall be initialized with no motion and spun up until output from successive sols has converged; we expect this will require 2-4 simulated sols (longer for high-pressure runs). Other initial and boundary conditions will be varied, as specified below.

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Figure 4. South-to-north vertical cross-section across Valles Marineris as simulated by MRAMS. Wind speed is shaded, temperature (K) is contoured. Upslope winds are noted along the canyon walls, and compensating subsidence is evidence in the center of the canyon. This compensating flow is poorly represented in hydraulic models.

2.5.2. Step 1: Numerical experiments with idealized craters and canyons.Initially we will use idealized topography to simulate a diurnal cycle of slope winds. We hypothesize that the strongest winds occur near the bottom of the slope and in a ‘runout zone’ on the floor. The existence and width of this runout zone is crucial because it can enhance erosion and/or prevent accumulation, defining the observed moats.

Input to the model will be as follows. Idealized topography for craters will be axisymmetric, using typical depth:diameter ratio for mound-hosting craters on Mars with 3 km depth, a 1km high rim, 20° rim and wall slope and a flat floor. There shall be a central peak summitting at the elevation of surrounding plains and with 20° slopes. Idealized topography for canyons will have prismatic symmetry: a 5km deep trough with no rim, 20° wall slopes and a flat floor. As appropriate for equatorial layered sediments, there will be no Coriolis force in the horizontal plane, so canyon orientation is unimportant. Diurnal solar forcing will be constant and equinoctial (0.50 sols of sunlight) at Mars’ mean distance from the Sun, but local time will be tracked in an E-W sense in order to permit a diurnal thermal tide. The surrounding terrain will be flat, and the distance to the edge of the model domain shall be no less than 3000 km including lower-resolution nest grids. Thermal inertia retrievals near strong terrain are unreliable because of slope winds [Spiga et al., 2011]; we will adopt a uniform thermal inertia of 230 kieffers and uniform albedo 0.23 (corresponding to thermophysical class C of Putzig et al., 2005). We assume uniform aerodynamic roughness 10-2 m, intermediate between roughnesses calculated for the MPF and PHX sites [Hébrard et al., 2012]. The atmosphere shall be initialized with a Meridiani-like vertical thermal profile with a surface pressure of 6 mbar.

Output will consist of shear velocity (magnitude and direction) at all surface points. From this we will derive maps of the mean, maximum and skewness (gustiness) of the windfield. We will convolve this with existing theories for wind erosion and sediment transport to calculate

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the erosion potential at each point as a function of the cohesion/abrasion susceptibility of the substrate. The theories employed [Shao, 2001; Kok, 2012] will be appropriate for (i) transport limited sand, (ii) soil, (iii) weakly salt-cemented soil, (iv) detachment-limited bedrock abrasion. We will measure the runout distance/correlation length scale for each model run. Finally, we will store relative humidity and dew point.

Figure 5. Rationale for length scales chosen for the idealized model runs. Blue dots correspond to nonpolar crater data, red squares correspond to canyon data, and green dots correspond to polar ice mound data. Vertical dashed line correspond to model runs discussed in the text. Gray vertical lines show range of uncertainty in largest-mound width Valles Marineris/circum-Chryse mounds. Blue dot to left of “G” corresponds to Gale Crater. Craters smaller than 10km were measured using CTX or HiRISE. All other craters, canyons and mounds were measured using the THEMIS global day IR mosaic on a MOLA base. Width is defined as polygon area divided by the longest straight-line length that can be contained within that polygon. Normalized mound width does not keep pace with increasing container size, a trend that we will investigate using the results from Step 2.

The following parameters will be varied. First, we will carry out a set of runs varying width:- craters 20 km, 40 km, 80 km (reference crater), 160 km, and 320 km diameter; and canyons 60 km, 120 km (reference canyon) and 240 km wide. These dimensions are of particular interest, based on our survey of mound-hosting craters and canyons on Mars (Figure 5). We will also carry out runs for 2 km crater diameter and 10km canyon (=valley) width, to look for interesting behavior at small length scale as observed for Meteor Crater, Arizona [Kiefer & Zhong, 2011]. Mars has lost CO2 over time, so we will vary atmospheric for our reference crater and reference canyon. From the reference case at 6 mbar, we will model 24, 96 and finally 384 mbar (Earthlike atmospheric density). This will also allow us to investigate the transition from Earthlike to Marslike katabatic winds, and testing the prediction that a thick early atmosphere would have had higher rates of wind erosion [Armstrong & Leovy, 2005]. Because of the longer thermal time constant of denser atmospheres, these runs will require proportionately longer spin-up time. We will investigate the effect of wall slope by carrying out runs at 5° and 30° slope for the reference cases. To determine the effect of dust on slope winds, we will carry out 1 run for the reference crater case under high-dust (τIR ~3) conditions, maintaining a Conrath-ν profile in the vertical. To investigate katabatic-anabatic asymmetries, we will carry out one crater run with reversed topography. This geologically unrealistic run will allow us to separate the effect of divergent flow (daytime upslope) from day-night asymmetry for the crater case. In order to link to the general circulation (and the effects of changing orbital forcing), we will test the effect of a synoptic background wind field of 5 m/s imposed at the boundary (perpendicular to the canyon), and seperately to a Coriolis force (f-plane approximation) appropriate for 50° latitude (the approximate latitude of Galle and

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Spallanzani, which contain the highest-latitude sedimentary mounds on Mars). A significant advantage of using smooth, idealized topography is that it allows for a larger timestep, and so more runs for the same computer power. In total, we propose 27 runs for Step 1.Figure 6. Sketch of the geometries to be investigated in Steps 1 and 2.

2.5.3. Step 2: Consideration of the effect of sedimentary infill (sedimentary mounds) We hypothesize that the presence of a sedimentary mound intensifies wind erosion potential within the crater/canyon.

In this step we introduce sedimentary infill to the containers (craters/canyons) modeled previously. Mars sedimentary rock mounds steepen near their toe (avoiding landslides and obvious mass wasting), so we will adopt a mound profile of the form z ∝ sqrt(x) [Before submission, need to be able to pick a simple mound profile and justify it with measurements. sqrt(x) is a placeholder.] Output will be the same as for the previous runs. In total, we propose ~50 runs for Task 2, as follows:

Change mound height. [Details, tbd, including choice of “reference” mound (Gale-like) ]Change mound width. [Details, tbd including choice of “reference” mound]Change mound slope exponent [Details, tbd].Change container width. [Detais, tbd]

Stepwise erosion of nearly-filled container. We will initialize the “reference” crater and canyon with. We will define an erosion rate map using the with a threshold u* of 90% of the maximum wind speed experienced anywhere in the model domain during the first run using a cohesive-soil wind erosion law for the sedimentary infill and zero erosion elsewhere. The erosion rate map will be used to construct an eroded topography differing from the previous cycle by at most 1/5 of the original thickness of sedimentary infill, and the mesoscale model rerun on this altered topography. We will repeat this cycle 7 times for each container type.

As in Task 1, we will carry out a run with synoptic u = 5 m/s.

Asymmetric mound placement (1 simulation).

Icelike material properties. Taking account of reports of slope-wind erosion being important in the north polar ice mounds within craters, we will carry out 1 crater simulation with TI = 2000 for the mound.

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Corrugated sheet: In these experiments we will corrugate the entire landscape (inside and outside the canyon) with small and large amplitude topographic oscillations oriented perpendicular to the canyon long axis. We will vary both amplitude and wavelength. The goal here is to seek a preferred wavelength for slope-wind erosion on Mars that might correspond to the observed mode in moat width (15-20km).

Using results from Tasks 1 and 2, we will determine the explanatory power of the following topographic parameters in explaining mesoscale patterns of maximum shear stress across high-relief Martian landscapes: local slope, drainage area, vertical and horizontal distance from furthest ridge, vertical and horizontal distance from nearest ridge, potential energy in drainage area assuming uniform thickness of drainage flow, all of the above criteria but with inverted topography (for upslope winds), and additional parameters, both jointly and separately. We will then use these results together with an information criterion to select a parameterization with an appropriate number of independent variables (some of the variables listed are not independent). Finally, we will compare the results to predictions from hydraulic theory, and analyze the residuals. Alongside this empirical approach (essentially multivariate regression), we will attempt to develop a physical theory to account for the numerical output.

2.5.4. Step 3: Simulation of slope-eroding winds for geologically realistic terrain.For a transect across layered sediments in West Candor Chasma, we hypothesize that the strongest wind stresses occur in terrain that has undergone the highest rates of geologically recent erosion.

The floor of West Candor Chasma is a ~3x104 km2 sedimentary outcrop almost devoid of impact craters, indicating high rates of erosion [Malin et al., 2007]. We will simulate winds at West Candor Chasma using 4-5 nested grids forced at the outermost (hemispheric) nest by the NASA Ames Mars GCM. To constrain long-term wind erosion, we require four-season 24-hour output, climatological dust for each season, and no water cycle. Mesoscale output generated for mission support purposes does not satisfy these criteria. There is no evidence for strong control of orbital forcing on synoptic equatorial winds over the last 5 Mya, which is sufficient for determining the deflation rate recorded by the relative density of the small craters. We will also calculate predicted yardang orientations (from km-scale shear stress victors), pattern of erosion at mesoscale (10s-100s km), and rates of erosion. We will employ the full wind speed history to sedimentary transport and erosion for the detachment limited case [e.g., Wang et al., 2011], making the simplifying approximation dz/dt ≈ ke (u* – u*c)α

with α = 3 or 4 [Kok et al., 2012]. This full-history calculation will allow us to determine whether nighttime winds, daytime winds, or both are important for wind erosion, and also to determine whether erosion rate can be approximated as a function of the maximum wind speed only. We will also carry out 1 control experiment at perihelion season using zero wind at the outermost model boundaries in order to isolate the contribution of the general circulation to wind erosion. (Total: 5 experiments for Task 3).

Each of these predictions is testable using existing CTX and HiRISE images of West Candor Chasma that are available in the PDS. Yardang orientations will be plotted where visible and the residuals relative to prediction calculated on a ½ degree grid across the chasm. For the part

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of the model domain corresponding to sedimentary rock, we will convert from relative to absolute erosion rates using the mechanical properties of kieserite and epsomite that have recently been measured [Grindrod et al., 2010] to set abrasion susceptibility. When erosion rates are high, small-crater frequency is inversely proportional to erosion rate [e.g., Smith et al., 2008]. Crater size-frequency distributions will be collated using a CTX basemap and available HiRISE. The crater density in West Candor Chasma is low, so we will bin craters in order of the predicted erosion rate at that crater’s location which will allow us to compare the trend and magnitude of aeolian erosion appropriately. Extremely resistant (caprock) surfaces will be excluded.

2.5.5. Step 4: Incorporation of slope winds into landscape evolution model.We hypothesize that slope wind/landscape feedbacks played a significant role in the current shape of moats and mounds on Mars.

In this final step we will embed either a parameterization or a physical theory that links topography to diurnal-slope-wind erosion as an erosion rule in a landscape evolution model.Landscape evolution models trade decreased detail for the ability to model processes over geological time. For example, liquid water is the principal erosive fluid on Earth. River erosion is controlled by intermittent floods and is heterogenous in detail because of varying

substrate erodibility, but for orogen-scale fluvial

Figure 7. Preliminary work using landscape evolution model, showing simulated sedimentary mound growth and form for one example of a hypothetical idealized atmosphere-topography feedback. Colored lines correspond to snapshots of the mound surface equally spaced in time (blue being early and red being late). Black line corresponds to the initial nonerodible “container” topography. Topographic change is the balance of an atmospheric source term and wind erosion. In this hypothetical idealized case, the atmospheric source term is uniform in space and constant in time [e.g., Michalski & Niles, 2012] and the stratigraphy and geomorphology therefore results solely from slope-wind/terrain coupling, which is parameterized using an exponential kernel. See Kite et al. [arXiv:1205.6840] for details of the coupling equation; the proposed work will supersede this parameterization. Inset plots show (top right) the resulting stratigraphy at late time and (bottom right) dip measurements from HiRISE DTMs at the Gale mound. Note that slope-wind enhanced erosion characteristically produces outward (moatward) dips, consistent with observations. Given that this is an idealized model, the similarity to the stratigraphy and layer orientations within the Gale mound is intriguing [Anderson & Bell, 2010; Thomson et al., 2011; Kite et al., arxiv: 1205.6840].

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landscape evolution models a simple streampower law is used. Similarly, it is impossible to run a mesoscale model for 3 Gyr, so appropriate averaging is required. We have developed a computationally inexpensive landscape evolution model for this purpose. The model simulates landscape evolution in one horizontal dimension (radius or width) (Figure 7).

Following embedding of the erosion rule, we will carry out parameter sweeps. Because of the minimal computational cost, we can carry out a large number of sweeps, as follows:- (i) nonerodible container initially overfilled with erodible material; (ii) nonerodible container initially half-filled with erodible material; (iii) coupled growth and wind erosion using uniform deposition ; (iv) transport, not just wind erosion; (v) competing aeolian and fluvial processes (using Howard, 2007, for the fluvial processes). Each of these parameter sweeps will consist of a wide range of crater and canyon dimensions, synoptic wind speeds, and wall slope angles.

The landscape evolution model will allow us to leverage the relatively small (<100) number of mesoscale model runs to explore a much wider range of parameter space. Thus we can determine if moats and moats are generic outcomes of slope wind erosion on Mars (supporting our hypothesis) or alternatively if special circumstances are required to for moats and mounds with slope winds (disfavoring our hypothesis).

The purpose of this model is to clear the way for the study of 2-way mesoscale atmosphere geomorphology feedbacks on Mars. For example, the landscape evolution model will generate a large number of physically motivated predictions for the hypothesis that slope wind/terrain coupling played a significant role in the layer orientations and stratigraphy within sedimentary mounds on Mars (parameter sweep iii above), which can be tested from orbit (with HiRISE stereo DTMs), and by MSL as it ascends the Gale mound.

2.5.6 Assumptions and caveatsIt is worth emphasizing the limitations and assumptions of these modeling methods. First, we assume that armoring (lag formation) processes do not vary strongly acorss the model domain at the mesoscale. This means that the model is inapplicable to plains traversed by the Opportunity rover, where both slopes (<<1°) and erosion rates (~1 nm/yr) are small relative to the crater and canyon sites that we focus on. Second, we do not track tools responsible for erosion in detail (there is no bookkeeping for abrading clasts). Because our ultimate goal is to develop a landscape evolution model, we assume that over long timescale tools are available.

2.6 Impact of proposed workCurrent and Decadal-Survey-recommended missions. MSL is a mission to a layered sediment mound which is undergoing wind erosion which exposes layers, most of which are consistent with atmospherically transported sediments [Anderson & Bell, 2010]. The proposed work is relevant to understanding sand dunes, geomorphology and sedimentology at Gale Crater. Our landscape evolution modeling is also relevant to Mars Sample Return. Wind exhumation determines the depth of burial and pressure-temperature-time history of exposed samples, which is key to diagenesis including thermal alteration of organic matter.

Scientific priorities. The proposed work addresses two of the key questions defined by a recent review paper by members of the Mars sedimentary geology community [Grotzinger et al.,

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2011]: “How Did Source-to-Sink Sediment Transport Systems Evolve on Mars?”, and “In What Ways Did Martian Sedimentary Rocks Become Modified after Their Deposition?” For example, the Medusae Fossae Formation shows yardangs and mesoscale undulations which may result from the interaction of slope winds with the synoptic flow. The proposed work on slope-wind erosion is relevant (to be expanded) to the origin of sedrocks/stratigraphy on an early mars where wind erosion important, the origin of sand in dunes, the origin(s) of Valles Marineris [Andrews-Hanna, 2012], the growth and form of the ice mounds, the formation of hematite lags (e.g., Weitz et al., in press, sequence stratigraphy, and the possible universal length scale of moat width.

In theory, one-to-one mapping between a landscape and a pattern of erosion implies that the model can be inverted for past landscapes, as has recently been demonstrated for channel networks on Earth [Abrams et al., 2009]. In detail, this is not possible for wind erosion on Mars because of umodelled diffusive geologic processes, and especially because the presence of essentially nonerodible material (basalt) leads to degeneracies when the flow of time is reversed. Nevertheless, with care and only for sediment-dominated landscapes such as the MFF, it may be possible to place some constraints on past landscapes.

2.7 Relevance of proposed workOur proposed work will advance knowledge about the effect of the Martian atmosphere on the surface, the effect of terrain on the atmospheric circulation, and links to the growth and form of ancient layered sediments. Our proposal thus fits within the scope of the Mars Fundamental Research Program (MFRP). Specifically, it is highly relevant to Goals III.A.6 and Goal III.A.2 outlined in the Mars Exploration Program Analysis Group (MEPAG) Science Goals Document, and also addresses issues raised in Goals II.A.4 and III.A.3.

Goal III.A.6. is to “Characterize surface-atmosphere interactions on Mars” and Goal IIIA.2. is to “evaluate volcanic, fluvial/laucustrine, hydrothermal, and polar erosion and sedimentation processes that modified the Martian landscape over time.” Our proposal addresses a fundamental atmosphere-surface interaction (wind-induced topographic change, and the feedback on the windfield) that has probably operated throughout Mars history.

Goal II.A.4. is to “Search for microclimates,” including “areas of significant change in surface accumulations of volatiles or dust.” Areas of strong slope winds are microclimates [Spiga et al. 2011], and wind erosion leads to significant change in surface accumulations of sand and dust.

Goal III.A.3. is to “Constrain the absolute ages of major Martian crustal geologic processes.” For the first time, we will carry out regional crater count for West Candor Chasma, leading to absolute constraints on the timing of resurfacing by wind erosion (with the premise that directly-imaged small fresh crater size-frequency distribution can be extrapolated into the recent past). These are complementary to the site-specific measurements made by Opportunity [Golombek et al., 2010].

2.8 Plan of work, personnel, and responsibilities2.8.1 Work plan

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Activities/milestones DeliverablesYear 1 Compile MRAMS on CITerra/Fram. Carry out

idealized-topography runs.• Complete analysis of idealized-topography mesoscale runs.Analyze output to derive parameterization for terrain-induced erosion• Incorporate terrain-induced wind erosion parameterization into landscape evolution model.

✓ LPSC presentation on: Slope winds on idealized topography.✓ Short GRL-length manuscript on: Slope winds on idealized topography and patterns of wind erosion.

Year 2 Carry out realistic-topography runs.•Complete analysis of realistic-terrain mesoscale runs

•Complete parameter sweeps with landscape evolution model.

✓ Detailed manuscript on: Slope enhanced wind erosion on Mars, including realistic terrain.✓ Short GRL-length manuscript on slope-wind erosion/landscape evolution coupling (may be expanded into a detailed manuscript, depending on results).

2.8.2 Planned Calculations and the Parameters ExploredCalculation Mesoscale model runs Landscape evolution model

runsNumber of Calculations ~60 three-dimensional

simulationsTotal: ~500,000 CPU-hours

~ 104 landscape evolution model runsTotal: ≤104 CPU-hours

Parameters Explored (and Sensitivity Tests)

crater/canyon width, pressure, wall slope, mound width, mound height, mound slope exponent, (dust loading, synoptic wind, Coriolis force).

2.8.3 Personnel and QualificationsCaltech: PI Michael Lamb will be responsible for the overall direction of the effort and the use of funds. Lamb has written 33 papers on sediment transport, erosion and geomorphology. He will assist Kite with calculating erosion rates from wind stress histories, and with incorporating slope winds into the landscape evolution model. Undergraduates will also help with the proposed research through the Caltech Summer Undergraduate Research Fellowship.

Co-I/Science PI Edwin Kite will be responsible for the scientific direction of the work. Kite has submitted 16 planetary science papers to peer-reviewed journals during four years of grad school at UC Berkeley and six months as a prize fellow at Caltech (10 lead author, 2 second author, 1 third author), including two papers from a previous collaboration using MRAMS with Rafkin and Michaels [Kite et al., 2011a, 2011b]. Kite will run and analyzing MRAMS, parameterize the MRAMS output, and develop, run, and analyze results from the landscape evolution model

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SwRI: Co-I Tim Michaels and collaborator Scot Rafkin built the mesoscale model, MRAMS, that has been used by NASA for Entry, Descent and Landing calculations for the MERs, Phoenix, and MSL. Co-I Michaels will be responsible for supplying the most recent stable version of the MRAMS code, supporting Kite’s setup of MRAMS on CITerra/Fram, and assisting with the definition of boundary conditions for the katabatic-wind numerical experiments including supplying GCM boundary conditions where this is appropriate. Both Co-I Michaels and collaborator Rafkin will assist in the overall assessment of the mesoscale results including the development of the wind-erosion parameterization for use in the landscape evolution model.

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4. Biographical Sketches Lamb (2 page limit)

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Kite (2 page limit as Science PI/Co-I)

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Michaels (1 page limit)

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Rafkin (1 page limit)

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5. Current and pending support

Lamb Current and Pending Support

Kite Current and Pending SupportTitle: O.K. Earl Postdoctoral FellowshipSponsoring: Caltech prize fellowship. Contact Marcia Hudson, 626-395-6111, marcia@gps.caltech.eduPeriod: January 9, 2012 – January 9, 2014Annual effort: Full-time fellowship (12.0 months)

Michaels Current and Pending Support (required)

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6. Statements of commitmentRequired from all parties: Lamb, Kite, Michaels, Rafkin

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7. Budget justification7.1. Budget narrativeThis proposal requests funds for direct labor, travel and publication expenses for a two-year study to develop and apply physical models for wind erosion and landscape evolution of sedimentary rocks on Mars. The Institutional PI (Michael Lamb) is budgeted for ($5,000 in months) months per year. The Science PI/Co-Investigator (Edwin Kite) is budgeted for 9 months per year. The Co-Investigator (Timothy Michaels) is budgeted for 1 month per year. These budgeted amounts are commensurate with the level of effort to be put into the proposed work; for further details, please see the table below. Funds are not requested for the Collaborator. While the team consists of several members, each brings an area of (complementary) expertise necessary for achieveing the goals of this proposal.

Funds are requested for the Science PI/Co-Investigator to attend the annual LPSC meeting and present our research results (~$800 e.a.). We also request support for the Science PI/Co-Investigator to spend three days at SwRI (or the Co-Investigator to spend three days at Caltech; TBD) in order to facilitate the integration of the different modeling results (one visit; ~$800).

Funds are requested to cover the publication changes for one publication per year. This type of publication is the most direct way for the knowledge gained from this work to be disseminated to the scientific community. We anticipate our publication costs will exceed the budgeted amount, and extra publication costs will be covered by PI Lamb’s startup funds.

7.1.1. Personnel and work efforts.Proposal role Year 1 Year 2

Michael Lamb Institutional PI $5,000 in months $5,000 in monthsEdwin Kite Science PI/Co-

Investigator9.0 mo 9.0 mo

Timothy Michaels Co-Investigator 1.0 mo 1.0 mo

7.1.2. Facilities and equipment.The proposed work entails extensive model development and will require significant CPU-time. The more computer-intensive (mesoscale) models will be carried out on the CITerra/Fram cluster at Caltech. From our earlier collaboration [Kite et al. 2011a, 2011b], we expect ≤1 CPU month per idealized-terrain simulations and ~2 CPU months per Valles Marineris simulations. Fram’s 3768 CPUs and 500 TB storage are easily enough for our needs. Caltech’s in-house charge for CITerra/Fram use is $0.012/CPU-hour for <400K CPU-hours/quarter and $0.006/CPU-hour for any CPU hours above this level. (To verify, contact systems administrator Naveed Ansari: naveed@caltech.edu). CITerra/Fram’s CPUs are 12/core and core RAM limitations preclude running more than 1 MRAMS instance simultaneously on any given core. Therefore we request $6000 over two years for computing costs (~60 model runs are required, ~1 core-month per model run). We also have access to a network of ~10 Linux workstations at Caltech, which is sufficient for the less computer-intensive landscape evolution models and which we can use without charge.

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