Internal Structure and Evidence for Diagenesis of the. Rocknest Aeolian Deposit, Gale Crater, Mars

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1 Internal Structure and Evidence for Diagenesis of the Rocknest Aeolian Deposit, Gale Crater, Mars Walter Goetz 1, Pierre-Yves Meslin 2, Agnes Cousin 3, Robert Sullivan 4, Martin Fisk 5, Michael A. Velbel 6, Douglas W. Ming 7, Amy McAdam 8, M. Darby Dyar 9, Roger Wiens 3, Jens Frydenvang 10, Morten Bo Madsen 14, Ben Clark 11, David Vaniman 12, Ryan Anderson 13, Diana Blaney 14, Nathan Bridges 15, Samuel M. Clegg 3, Kenneth S. Edgett 16, Cécile Fabre 17, Fred Goesmann 1, John P. Grotzinger 18, Stubbe F. Hviid 19, Gary Kocurek 20, Jéremie Lasue 2, Kevin W. Lewis 21, Nicolas Mangold 22, Sylvestre Maurice 2, Horton E. Newsom 23, Nilton O. Renno 24, Susanne Schröder 2, and the MSL Science Team 1 Max Planck Institute for Solar System Research (MPS), Göttingen, Germany 2 IRAP, CNRS/UPS, Toulouse, France 3 Los Alamos National Laboratories, New Mexico, USA 4 Center for Radiophysics and Space Research, Cornell Univ., Ithaca, New York, USA 5 Univ. of Oregon, Corvallis, Oregon, USA 6 Michigan State Univ., East-Lansing, Michigan, USA 7 NASA Johnson Space Center, Houston, Texas, USA 8 NASA Goddard Space Flight Center, Greenbelt, Maryland, USA 9 Mount Holyoke College, South Hadley, Massachusetts, USA 10 Niels Bohr Institute, Univ. Copenhagen, Denmark 11 SSI, Boulder, Colorado, USA 12 Planetary Science Institute (PSI), Tucson, Arizona, USA 1

2 13 USGS, Flagstaff, Arizona, USA 14 JPL, Pasadena, California, USA 15 APL, Laurel, Maryland, USA 16 MSSS, San Diego, California, USA 17 Université de Lorraine-UHP, Vandœuvre-lès-Nancy, France 18 Div. of Geological and Planetary Sci., Caltech, Pasadena, California, USA 19 DLR, Berlin, Germany 20 Univ. of Texas, Austin, Texas, USA 21 Princeton University, New Jersey, USA 22 Univ. Nantes, France 23 Univ. of New Mexico, Albuquerque, New Mexico, USA 24 Univ. of Michigan, Ann Arbor, Michigan, USA Submitted to Journal of Geophysical Research - Planets Key Words: Mars, Gale Crater, soil, chemical composition, ChemCam/LIBS, MSL One Sentence Summary: The Rocknest aeolian deposit in Gale Crater, Mars, is paved with coarse sand grains and has a thin indurated layer near the top and a subsurface with both albedo and chemical stratification. 2

3 Abstract [Guidelines: max 250 words (currently 249 words), define all abbreviations, avoid reference citations] The Rocknest aeolian sand shadow was investigated by a series of instruments onboard the Curiosity rover during sols Here we focus on the chemical characteristics of the deposit as measured with LIBS (Laser Induced Breakdown Spectroscopy). The top surface of the Rocknest deposit is paved with coarse (millimeter-sized) grains that are embedded in a (sub-)millimeter thick indurated layer. Some grains at the top surface show signs of basaltic weathering. The bonding agent appears to be enriched in iron, but not enriched or depleted in calcium or magnesium, relative to average martian crustal abundances. Accordingly a ferric oxide/oxyhydroxide, likely related to the globally distributed nanophase oxides (npox), is the favored candidate for the bonding agent. Similar features observed in soil profiles at Gusev crater suggest such induration to be wide-spread on Mars. The centimeter-scale subsurface is dominated by fine grains (< 100 μm and largely unresolved in images), but contains coarser sand-sized grains as well. A distinct, ~7 mm thick bright layer is centered at a depth of ~15 mm and extends roughly parallel to the surface. The layer possibly records some intermediate stage during the formation of the deposit. This layer has a slightly enhanced abundance in iron and may be more oxidized than material above and below. In average, the Rocknest deposit material tends to be more mafic than Gusev soils. Coarse sand grains at the top and in the subsurface are more diverse than subsurface fines in terms of chemical composition and associated igneous source areas. 3

4 1. Introduction The Mars Science Laboratory (MSL) rover Curiosity landed on the northwestern floor of Gale crater (Edgett and Malin, 2000; Anderson and Bell, 2010; Milliken et al., 2010; Wray, 2013) in early August 2012 and spent over 40 sols (sol ) at a sand shadow deposit in the Rocknest area, about 400 m east of the landing site (named Bradbury Landing, Grotzinger et al., 2013). Rocknest is a cm thick stratigraphic unit of local bedrock (Grotzinger et al., 2013), some 15 m in elevation below Bradbury Landing. The Rocknest sand deposit is one of several sand deposits within a "nest" of diverse rocks (Blaney et al., 2013) that are either outcrops of the Rocknest stratigraphic unit or float rocks derived from that unit. In the following these outcrops and float rocks will be termed "Rocknest rocks". The Rocknest deposit (also referred to as "Rocknest") was chosen for extensive instrument testing, including many first-time activities such as soil sampling and measurements by the analytical laboratories in the body of the rover. By virtue of its aeolian origin the deposit was expected to most likely display global chemical and mineralogical characteristics similar to those of comparable deposits examined at other landing sites (Viking Landers, Mars Pathfinder, Mars Exploration Rovers (MER), Phoenix Mars Lander). Several papers have already addressed the Rocknest sand deposit as part of Curiosity's first 100 sols (Grotzinger, Bish et al, Blake et al, Meslin et al., and Leshin et al., 2013). The present paper emphasizes the morphological and chemical properties of the Rocknest deposit as inferred from imaging and data from the Chemistry and Camera (ChemCam) instrument (Maurice et al., 2012; Wiens et al., 2012) which utilizes Laser-Induced Breakdown Spectroscopy (LIBS). The Rocknest deposit has a distinct subsurface layering providing clues to its formation and incipient diagenetic processes. We address the internal structure of the deposit from a chemical point of view. 4

5 ChemCam and the color cameras (rover mast cameras Mastcam-34 and Mastcam-100, and the Mars Hand Lens Imager, MAHLI, Edgett et al., 2012) are the only MSL instruments that provide information on in-place bedform features with significant spatial resolution. The cameras guide the investigation while ChemCam provides chemical constraints on different horizons in the subsurface. Finally, the Alpha Particle X-ray Spectrometer (APXS, Campbell et al., 2012), the Sample Analysis at Mars (SAM, Mahaffy et al., 2012), and the Chemistry and Mineralogy instrument (CheMin, Blake et al., 2012) provide bulk chemical and mineralogical data. The Rocknest deposit is oriented north-south, is about 5 m long, ~1 to 1.5 m wide, and at the crest is up to 20 cm high (height with respect to surface on which it was deposited) (Figure 1). Five troughs were scooped, respectively, on sols 61, 66, 69, 74, and 93, on the east-facing flank of the deposit (Figure 1). Local slopes are around 15. Only trough #2 was investigated by ChemCam. Figures provide imaging context for the ChemCam observations, and summarize information about physical properties (color, grain size, texture, cohesion, induration) and subsurface layering. The remainder of the paper discusses ChemCam data obtained on Rocknest deposit soils: six ChemCam experiments were performed on trough #2 (Schmutz-2, Epworth, Epworth-2, Kenyon, Epworth-3, Kenyon-high-albedo) and a seventh ChemCam experiment (Portage) investigated soil in the wheel scuff (Figure 1). Table 1 summarizes the characteristics of the ChemCam experiments. For a sense of scale in Figures , the MSL scoop is 45 mm wide, and created a trough of about the same width at all five scooping locations. Figure 2.1 illustrates surface and subsurface properties at scoop trough #1. An obvious feature is the disparity in average grain size from millimeter-sized top-surface grains to unresolved grains (<100 μm) in the subsurface (Minitti et al., 2013). Another feature is a bright band located further down along the scoop entrance plane (Goetz et al., 2013; Minitti et al., 2013). The trough floor is littered with clods of 5

6 cohesive soil and fragments of indurated crust. Some soil clods appear fairly heterogeneous (Figure 2.1a). Many of the angular fragments (including those on the trough floor) still carry the coarse surface armor, indicating that these originated at the surface. The bright band is seen on the near side of the trough as well as more clearly at the trough s left end (Figure 2.1b). Moreover, a second bright band may exist immediately below the top surface (Figure 2.1b). This feature was found in other troughs as well. Figure 2.2 shows troughs #4 and #3, where most features observed in #1 are encountered again: a bright band on the near wall (Figures 2.2a & c) and far wall (Figures 2.2b & d), soil clods, and crust fragments on the trough floor (especially trough #4). The numerous sharp indentations and "promontories" (marked by dotted arrows in Figures 2.2a & c) provide further evidence for induration immediately below the surface. Figure 2.3 presents images of trough #2, the trough that was studied extensively by ChemCam. The far (upslope) wall collapsed twice (compare Figures 2.3a to 2.3b to 2.4a). These collapses were not triggered by ChemCam laser shots, but might have been triggered by wind or by rover vibrations. Figures 2.3c & d show a high-resolution MAHLI view of what might be the indurated layer below the surface armor. Figure 2.3a shows again a double layering, - this time not on the near wall as in Figure 2.1b, but on the scoop entrance plane. The lower bright band has the same appearance as in troughs #1, #3, and #4. The upper bright band is only a millimeter thick and seems to be related to -or to form at least part of- the indurated layer near the top surface. Figure 2.4a shows the location of all ChemCam targets/shots in trough #2. Three out of six experiments in trough #2 (Kenyon, Kenyon High Albedo, Epworth_3) were linear transects of 10 or 15 laser spots designed to characterize subsurface stratigraphy (see Tables 1-2). All transects started at the bottom of the trough. Then the camera head (including the ChemCam laser) moved upwards by vertical increments as commanded (Tables 1 and 2). Figures 2.4b-d are laser-shot documentation images acquired by the RMI (Remote Microscopic Imager), ChemCam's own (grayscale) camera, that returns images with higher 6

7 resolution (> 40 μm/px) than any other camera onboard Curiosity except MAHLI. The filled circles in Figures 2.4b-d are best estimates of the location of each spot investigated by these experiments. Figure 2.5 shows scoop trough #5. In this trough (and in contrast to troughs #1-4) the bright band is quite pale (though still recognizable), appears higher on the exposed scoop entrance plane surface and inclined with respect to the top surface, possibly due to this trough's proximity to the crest. Samples from troughs #3 and #4 were analyzed by CheMin (Bish et al., 2013, Blake et al., 2013), and samples from trough #5 were analyzed by SAM (Leshin et al., 2013). We considered the possibility that the banding seen in the troughs might be an artifact of the scooping process rather than an intrinsic subsurface layer. Such scenarios would involve varying amounts of pressure-induced reorganization of mixed grain sizes by variable soil/scoop surface pressure exerted during a single scoop rotation. For example, in a situation where brighter, fine dust was intermixed with darker, somewhat coarser silt and very fine sand, greater transient compression of the scoop while sliding against the soil might force more of the finer, brighter (potentially dusty) materials up against the smooth outer surface of the scoop, creating a region brighter than average on the wall scar left behind. However, we conclude that the lower bright band (from here on referred to as "bright layer") is intrinsic to the deposit because: (a) it can also be followed on adjacent side walls of most of these troughs, where scoop pressure should have been minimal, and (b) it does not resemble obvious scooping artifacts observed during the Phoenix mission in 2008 (compare Figure S1). The bright layer appears to be roughly parallel to the top surface with the exception of trough #5 where it approaches the surface at a shallow angle. Based on Figure 2.3a (trough #2, the one interrogated by ChemCam) the bright layer represents a layer approximately 7 mm thick between ~11 and 18 mm below the local surface (~15 to 26 mm measured along the scoop entrance plane that is assumed to be inclined 45 with respect to the top surface). Some layered ripples were encountered by the Spirit rover in Gusev crater (Sullivan et al., 2008). These 7

8 stratifications were exposed naturally (rather than by wheel scuff) and have a range of layer thicknesses, but some examples are similar to those of Rocknest. Finally, Figure 2.6 shows the ChemCam and APXS target "Portage," located on the floor of a wheel scuff of the Rocknest deposit. The Portage target area displays coarse grains with diverse albedos, colors and morphologies. 2. ChemCam Data: Processing and comparison to other data sets Element or oxide abundances (as presented here) have been derived from ChemCam/LIBS raw spectra by two major steps (Wiens et al., 2012): (1) Level 1 processing of raw spectra that involves subtraction of the dark spectrum to correct for ambient light, de-noising, continuum removal, channel-to-wavelength calibration, and multiplication of the spectrum by the instrument response function; and (2) Level 2 processing that involves normalization of the Martian raw spectra to the total integrated intensity (for distance correction) and inferring compositional data using a Partial Least Square regression (PLS) model that is based on a training set of known geological samples. The PLS model has been developed and described in detail by Clegg et al. (2009), Anderson et al. (2011, 2012), and Wiens et al. (2013). As stated by Clegg et al. (2009), two types of PLS regression have been used to relate element concentration and ChemCam spectral data: element-by-element regression (referred to as PLS1), and simultaneous regression for all elements (referred to as PLS2). From the beginning it was recognized that PLS1 is more difficult to implement than PLS2, but is preferred because it is more flexible and allows for easy addition and removal of training samples depending on the element to be analyzed. All major oxide abundances (oxides of Si, Na, K, Al, Ca, Mg, Fe, and Ti) presented here have been obtained by PLS1. 8

9 Abundances of hydrogen (expressed in arbitrary units rather than wt. %) and Li (in ppm) are also referred to in subsequent sections, and these data are based on univariate peak fitting and integration (Schröder et al., 2013; Fabre et al., 2013; Ollila et al., 2013). Univariate peak analysis is generally considered the most accurate method for minor or trace elements within similar types of matrices. Therefore the relative hydrogen abundances (derived from ChemCam experiments at Portage and in trough #2) are believed to be consistent with each other since the overall soil matrix is the same. The same applies to Li abundances (in ppm). The hydrogen data used here are the same as adopted by Meslin et al. (2013), although the latter work was based on PLS2 instead of PLS1 for major-element abundances. Overall, multivariate analysis has been generally adopted for most major element quantification, although univariate analysis remains a useful complement; and is the only option to analyze rare outliers that might potentially contain interesting science and yet are outside the elemental ranges of the training dataset. The training set (originally 65 samples, but more than 200 as of the time of writing) is continuously being expanded in response to new types of rocks and soils encountered along the rover traverse. Despite ongoing progress in development of the training set as well as data processing, ChemCam-derived element abundances remain subject to considerable uncertainty, mainly in terms of accuracy. The uncertainty is higher for minor and trace elements (Ollila et al., 2013) as well as for rock matrices with poor laser coupling (such as carbonates), but major element abundances also have significant uncertainty. The topic is further addressed in a companion paper on Rocknest rocks (Blaney et al., 2013). Here we quote accuracies from Table 4 in Blaney et al. (2013), given as Root Mean Square Errors of Prediction (RMSEP), i.e. quadratic means obtained over the whole range of compositions: 7.1 wt.% (SiO 2 ), 0.6 wt.% (TiO 2 ), 3.7 wt.% (Al 2 O 3 ), 4.0 wt.% (FeO (total), from heron referred to as FeO T ), 3.0 wt.% (MgO), 3.0 wt.% (CaO), 0.7 wt.% (Na 2 O), and 0.9 wt.% (K 2 O). Precision (Table 3 in Blaney et al., 2013) is typically a few times smaller and within a given raster up to ten times smaller than those referring to accuracy. Obviously these uncertainties must be kept in mind when drawing conclusions 9

10 from ChemCam derived major-element abundances. Problematic results will be highlighted in the following sections. Several instruments onboard the Curiosity rover are capable of measuring (or at least constraining) chemical composition: ChemCam (Maurice et al., 2012 and Wiens et al., 2012), APXS (Alpha-Particle X- ray Spectrometer, Campbell et al., 2012), CheMin (Blake et al., 2012), and SAM (Mahaffy et al., 2012). Comparing ChemCam to APXS data is complex due to different spatial resolution (0.3 mm for ChemCam versus almost 17 mm for APXS) and different depth probing. By virtue of its small spot size ChemCam probes at the mineral grain level at each individual target spot rather than immediately providing whole rock compositions. Also, while APXS-derived oxide abundances are by definition normalized to 100 wt.% (dry mass), the sum of ChemCam PLS-derived oxides can remain notably below 100 wt.% because several elements present in Martian soils, particularly S, P, H, and C are not included in the major-element totals for ChemCam (SOM part of Meslin et al., 2013). When these elements can be accounted for, the total of major-element oxides serves as a sensitive check for accuracy (Meslin et al., 2013). Therefore substantial issues remain when comparing APXS to ChemCam data, even after averaging over a large number of ChemCam shots and spots. These caveats should be kept in mind when comparing these two data sets to each other. Comparison between ChemCam and CheMin data is hampered by the fact that ChemCam can return chemical composition of any surface material hit by the LIBS laser, whereas CheMin can only investigate particulate material that is in powder form with sizes below 150 μm. Moreover, CheMin utilizes X-ray diffraction and focuses therefore on mineralogy of the crystalline fraction of the sample. Rocknest fines are known to have a substantial abundance (30 to 40 wt.%) of X-ray amorphous material (Blake et al., 2013), which is difficult to fully characterize with CheMin. Finally, SAM can explore the chemistry of a sample in many ways (including pyrolysis and detection of evolved gases), but SAM data are not directly comparable to data from ChemCam. 10

11 3. Chemical Composition of Rocknest Soils This section describes the chemical composition of the Rocknest deposit. Figure 3.1 plots average PLS1 oxide abundances (open circle with central dot) together with their uncertainties in terms of precision and accuracy as stated in Section 2. These are averages over all ChemCam shots at all targets (except Schmutz_2) at the Rocknest deposit (Table 1). Note the large uncertainty for iron, titanium, and potassium oxides. In particular the abundance of potassium oxide has an uncertainty that is significantly larger than its average value. No such average abundances are plotted for H and Li because they are either strongly variable (H) or not well constrained (Li). However, Leshin et al. (2013) reported wt% H 2 O (based on SAM analyses of Rocknest fines) and the average abundance of Li was inferred by uni- and multivariate analysis of ChemCam data to be in the low ppm range (Ollila et al., 2013; Fabre et al., 2013). Also plotted in Figure 3.1 are averages over early shots into a given spot (vertical bars) versus later shots (post #10, plus signs). If early shots (at a given location) represent "dust", then offsets between vertical bars and plus signs suggest certain chemical trends as a function of grain size. Ideally, and assuming sufficient presence of dust at the shot location, the first one or two shots are expected to best represent dust (Meslin et al., 2013; Cousin et al., 2013), while later shots are increasingly affected by the underlying (coarser grained) substrate. In that sense, dust appears to be enriched in Mg, Fe, and Ti, and depleted in Na with respect to that substrate. The dust's enrichment in H was reported and discussed by Meslin et al. (2013). A quantitative determination of the dust's composition from the data discussed here is hampered by the unknown contribution of substrate even to the very first shot at each ChemCam target (Lasue et al., 2014). The above results refer to the type of material earliest encountered by a series of ChemCam shots at a 11

12 given location. That material can be termed "dust" as the finest fraction of surface material tends to agglomerate or stick to anything larger, but should not be rashly equated with modern near-surface airborne dust (Goetz et al., 2005, 2008, 2009; Madsen et al., 2009). In the following discussion the chemical properties of the first shots will be referred to "dust" as a generic term and for the sake of simple terminology, but the caveats mentioned here should be kept in mind. Finally three sets of literature data are plotted in Figure 3.1 (filled diamonds): Portage-APXS (together with the analytical uncertainty, Blake et al., 2013), "Martian dust" (Morris et al., 2006a), and average Martian soil (Taylor and McLennan, 2009). Error bars of the latter two data sets refer to accuracy. It should be noted that data for "Martian dust" actually refer to (presumably) dust-rich soils (Morris et al., 2006a). All three data sets (with associated uncertainties) serve as reference and will be plotted with the same symbols in most of the subsequent figures. While these literature data broadly agree with ChemCam data, the deviation in the case of iron is of concern. In particular Portage-APXS FeO T abundances are 70 % larger than the corresponding data from ChemCam. At the same time we note that the uncertainty for average Martian soil (red diamond, Taylor and McLennan, 2009) is largest in the case of iron. From that point of view, the iron abundance inferred from ChemCam analyses of the Rocknest deposit would appear to be near the low end of iron abundances that are found globally on Mars. Further chemical characterization of the Rocknest deposit is provided by focusing on a subset of chemical elements: (1) Molar ratio of Al/Si plotted versus molar ratio of (Fe+Mg)/Si (Figure 3.2), and (2) the total-alkali silica diagram (Figure 3.3). The former type of diagram was used previously to characterize rocks at Bradbury rise (Berger et al., 2013; Sautter et al., 2014) as well as soils encountered over the first 250 sols of the mission (Cousin et al., 2013). It has the advantage of efficiently separating important igneous and alteration minerals, as well as separating felsic bright minerals (e.g. feldspars, quartz which plot in the left part of the diagram) and mafic dark minerals (e.g. pyroxenes, olivine which 12

13 plot in the lower right part of the diagram). A full set of diagrams (oxide abundances, i.e. Na 2 O, K 2 O, Al 2 O 3, MgO, CaO, FeO (total), TiO 2, versus spot number as well as oxide abundances versus silica) are provided in the Supplementary Online Material (SOM). Figure 3.2a plots all 1860 data points (corresponding to 1860 laser shots and 7 targets as listed in Table 1). The data cloud plots in the central part of the diagram with only a few outliers. Given the precision of LIBS derived element abundances the central data cloud would preserve its shape if these data were reacquired on the same targets and processed in the same way as before. However, accuracy is poor (+/ and +/ along the x- and y-axis, respectively). Thus, if data processing/calibration changed the central data cloud could move around within a large fraction of the plot window and may even change somewhat in shape. The current outliers should, though, remain outliers despite potential later recalibration of the data. Also plotted are APXS data of Portage (Figure 2.6) and literature data for "Martian dust" and average Martian soil by respectively a blue, grey, and red diamond. The error bar for average Martian soil was computed as the standard deviation of the soil samples that were used for averaging. Taking into account the magnitude of that uncertainty and the accuracy of ChemCam PLS abundances in general, the agreement between Rocknest soils and those literature values is excellent, although adding the abundances of iron and magnesium on the abscissa may hide some potential divergence. Thus the diagram supports the idea that the fine-grained bulk of Rocknest deposit and material probed by the first few shots (the black ellipse in the center in Figure 3.2a) represent a fairly global unit (Bish et al., 2013; Blake et al., 2013; Leshin et al., 2013; Meslin et al., 2013). Figure 3.2b plots a subset of the data plotted in 3.2a, namely clusters of data points that are interpreted as coarse grains and pebbles buried in the deposit. Indeed, ChemCam data have reliable, well recognizable features (low fluctuations in total intensity, clustering and sharp discontinuities of Independent Component Analysis (ICA) data), whenever the laser penetrates into large solid grains (> about 500 μm, see SOM to Meslin et al., 2013; Cousin et al., 2013). This implies that the term "fines" 13

14 from a ChemCam perspective (as used in subsequent sections) includes sand grains up to a few hundred micrometer in size. Most of the clusters in Figure 3.2b are labeled by the abbreviated ChemCam target name and the spot number and are listed in Table 3 together with their likely chemical characteristics. In a heuristic way, the diversity of grains from the ChemCam perspective can be compared with the diversity of grains in terms of color, albedo and morphology as documented by high-resolution color images (Figure 2.6). However, it should be noted that ChemCam cannot readily detect sulfate rich particles that should be present in Rocknest deposit soil (Leshin et al., 2013; McAdam et al., 2013; Blake et al., 2013) and are tentatively identified in many high-resolution images (see e.g. light-colored spots pointed out in Figure 2.1a). Some of the clusters plotted in Figure 3.2b, in particular the outliers Kn,8, E2,1, and Kn,5 will be reconsidered in later sections as they can be further characterized by their position in other chemical data spaces. One of the most enigmatic grains and strong outliers in Figure 3.2 is E,5, a grain that is very low in alkali, Al, Si, Mg, and Ti, but contains 9-12 wt.% FeO T and is strongly enriched in Ca (>20 wt.% CaO) (see Forni et al., 2014, for further discussion). Figure 3.3 plots Rocknest deposit data in a total-alkali silica (TAS) diagram. Figure 3.3a plots all data separately with same plot symbols as in Figure 3.2. Selected outliers are labeled. In particular, the previously identified outliers Kn,8 and E2,1 turn out to be rich in alkali and silicon. Some of the outliers plotted may not be of igneous origin in which case they are unrelated to the classification scheme of the TAS diagram. Given the high spatial resolution of ChemCam data (~0.3 mm, Section 2), averaging of these data is key to interpreting Rocknest soils in terms of potential volcanic source rock types. Averages over all shots at given spots and over all targets are plotted, respectively, in Figures 3.3b and 3.3c. The latter averages plot fairly close to Gusev rocks and soils (red ellipse in Figures 3.3a-c) as well as global average soil (red diamond) and let us classify Rocknest deposit soils in average as picrobasalts and 14

15 tephrites in the frame of the TAS classification of volcanic rocks. Comparison with data returned by many other orbiting and landed missions (Figure 3.3d) suggests that Rocknest soils are more mafic than most other Martian soils and rocks. This agrees with APXS data for Portage (blue diamond). However, given the accuracy of the plotted ChemCam data (1.6 wt.% Na 2 O + K 2 O and 7.1 wt.% SiO 2 ), Rocknest deposit soils as a whole could either be similar to Gusev soils/rocks or they are more mafic than those, but a more felsic (more silica-rich) average composition is ruled out. Again it is emphasized that this conclusion refers to the broad average of ChemCam data of the entire Rocknest deposit (including felsic coarse grains). ChemCam data must scatter significantly in these types of diagrams (Figures 3.2 and 3.3) as a result of the diversity of coarse grains in the deposit, and as a result of ChemCam's high spatial resolution. Given earlier work the TAS diagram is well suited to set Rocknest deposit data in a broad context and to compare these data to data acquired during previous Mars missions. 4. Signs of Chemical Weathering? Figure 4.1 presents ternary diagrams for molar proportions of Al 2 O 3, (CaO + Na 2 O), and K 2 O (also referred to as A-CN-K, left part of figure) and Al 2 O 3, (CaO + Na 2 O + K 2 O), and (FeO T +MgO) (also referred to as A- CNK-FM, right part). In the upper part (Figure 4.1a) plot symbols distinguish between different ChemCam targets. A black ellipse highlights the domain where Martian Fe/Mg clays plot (Ehlmann et al., 2011). Filled black circles mark clay minerals: illite, montmorillonite (mont), smectite (with intermediate composition), nontronite (non), saponite (sap), and chlorites (chl) (Nesbitt and Wilson, 1992; Hurowitz and McLennan, 2007; Ehlmann et al., 2011). The lower part (Figure 4.1b) is similar to the upper one, but focuses on the difference between early (the first 10) and later shots. The distinction between those two groups of shots is set somewhat arbitrarily to 10 (as in Figure 3.1), although dust is probed by five (or 15

16 fewer) shots. Since dust is not the focus of the present work care was taken to eliminate in a conservative way all shots that might potentially be related to dust. Overall, the Rocknest deposit appears fairly unweathered as the large majority of ChemCam data are located below the plagioclase - K-feldspar join and the feldspar - (FeO T +MgO) join in the respective ternary diagram, and farther from the Al 2 O 3 apex in both. The same applies to the bulk chemical data (Portage-APXS, blue diamond) as well as to the calculated chemical compositions of the crystalline and the amorphous fraction (not shown in Figure 4.1, but see Morris et al., 2013; and Blake et al., 2013, for more on this). These three data points plot near the right edge of the main data cloud as expected (Section 3) and right above one another at the following ternary Al 2 O 3 coordinates: (crystalline), (Portage-APXS), (amorphous). Aqueous weathering accompanied by leaching of alkali and alkaline-earth cations should lead to accumulation of Al in the residual phase, especially at moderate ph where Al is least soluble (Stumm and Morgan, 1981; Richardson and McSween, 1989; Drever, 1997; Walther, 2005). On the ternary plots of Figure 4, materials weathered in this manner should plot farther from the (CaO + Na 2 O + K 2 O) apex and closer to the Al 2 O 3 -(FeO T +MgO) join than unweathered parent material (e.g., Hurowitz and McLennan, 2007). This is not observed in the Rocknest deposit data (Figure 4.1). The lower Al abundance in the amorphous fraction (as compared to the crystalline one) is thus more likely to be controlled by the original igneous material (type of volcanism) or by alteration under conditions in which Al is more soluble (e.g., low ph conditions), or a combination of these, rather than by weathering under moderate ph conditions. In summary, the ChemCam, APXS and CheMin data sets are inconsistent with extensive weathering at high water-to-rock ratios and moderate phs. As expected, ChemCam data for mudstone targets in the Yellowknife Bay area, some 50 m east of Rocknest, plot more strongly beyond these joins (Figure 2.C-D in McLennan et al., 2013). A few similar "excursions beyond these joins" are observed in Rocknest data. These are essentially two spots at the surface of the Rocknest deposit: Epworth2, spot #1 (E2, 1) and Epworth, spot #3 (E, 3). The former trends towards a 16

17 smectite of intermediate composition (Nesbitt and Wilson, 1992), while the latter barely exceeds the feldspar - (FeO T +MgO) join. Interestingly E, 3 coincides quite well with compositions of Martian Mg/Fe clays (Ehlmann et al., 2011). Thus ChemCam may well have hit some clay minerals, but the abundance of these clays may have been below the CheMin detection limit (typically at the percent level depending on the mineral phase; Bish et al., 2013), or these clays may have been bound to coarse grains (> 150 μm) that could not be delivered to CheMin. Indeed, ChemCam data indicate that the later shots at E, 3 (shots #18 through #27) hit a coarse grain. Data points attributed to dust (Figure 4.1b) plot strictly (without any exception) below the feldspar - (FeO T +MgO) and the plagioclase - K-feldspar joins (Figure 4.1b). The rare occurrence of weathering features (such as spots E2, 1 and E, 3) suggests that these features were inherited from (unknown) source region(s) that eventually supplied material to the Rocknest deposit. Disregarding these few outliers, the ternary diagrams (right part of Figure 4.1) reveal a subtle trend along a line sub-parallel to and below the feldspar - (FeO T +MgO) join and trending towards the upper left (and lower right). Overall the cloud of data points is elongated and oriented somewhat parallel to that join which is consistent with slow olivine weathering (Hurowitz and McLennan, 2007). CheMin found considerable amounts of olivine (>20 wt.%, Blake et al., 2013) in the fine-grained (< 150 μm) crystalline fraction of scoops #3 and #4 (Figures 2.2 and 2.5). Given that olivine is generally the most easily weathered mineral phase in basalt (Goldich, 1938; Delvigne et al., 1979; Morton and Hallsworth, 1999; Wilson, 2004; Velbel, 2009), the persistence of abundant olivine indicates that weathering rates must have been very slow, requiring in turn very low water-to-rock ratios (Hurowitz and McLennan, 2007). The requirement of low water-to-rock ratios would be particularly stringent in low-ph environments due to much-enhanced olivine dissolution rates at low ph (Stopar et al., 2006; Olsen and Rimstidt, 2007; Hausrath et al., 2008 and 2010). A trend towards the upper left (and below the feldspar - (FeO T +MgO) join) nearly identical with the one 17

18 previously described could be achieved by exclusively physical sorting of mechanically exhumed olivine phenocrysts from basaltic rock fragments of the same size (McGlynn et al., 2012). In general, transport processes of different types (e.g. impact, fallback, fluvial and aeolian transport) can sort small grains according to (dominant) mineralogy and corresponding density. Because both chemical weathering and physical sorting of olivine from basaltic rock fragments can yield nearly identical trends when plotted on an A-CNK-FM diagram (right part of Figure 4.1), evidence independent of major-element composition will be required to distinguish between these two possible processes. However, in either case, compositional modification of the Rocknest sediment by chemical weathering ranges from minor to nonexistent. The right-hand diagram (A-CNK-FM, Figure 4.1a) might suggest coarse grains like Kn, 8 or Kn, 10 (the latter plotting on top of the former) to be formed by the result of more advanced weathering. However, Kn, 8 is far away from the (FeO T +MgO) apex, not because of low iron, but because it is virtually devoid of Mg (SOM Figures S3.5-S3.6). The left-hand plot (A-CN-K) shows that Kn, 8 has average abundance in CaO + Na 2 O and high abundance in K 2 O. In fact Kn, 8 is rich in Na, K, Si, Al and has near-average abundance in Ca, Fe, and Ti (Figures 3.2b and 3.3a, also SOM Figures S ). Given the presence of Fe- and Cabearing phases that generally have a high weathering potential, we conclude that Kn, 8's deficiency in Mg reflects its original igneous (felsic) mineralogy (likely including plagioclases and K-feldspars) rather than being caused by weathering of mafic mineral phases. This conclusion is consistent with the original classification of Kn, 8 as a felsic grain, as based on ICA (Independent Component Analysis) clustering (Meslin et al., 2013; Cousin et al., 2013). A close match to potential precursor rocks has not been found, but provenance studies (Section 7) suggest that Kn, 8 may be related to Rocknest rocks in the vicinity of the deposit. Hence Kn, 8, as well as Kn, 5, the mafic grain on the other side of the data point cloud (Figure 4.1a), reflects the diversity of igneous grains available in the source areas of the Rocknest deposit. Similarly, the outlier grain E, 5 does not document a weathering path off the central cloud of 18

19 data points in Figures 4.1, but was recognized as Ca-rich grain of likely magmatic origin (this remarkable grain is also rich in P and F; Forni et al., 2014). 5. Gradients of Chemical Composition in the Subsurface? The discovery of a bright subsurface layer in the Rocknest deposit motivated several dedicated MAHLI and ChemCam observations (Section 2). The goal of these campaigns was two-fold: (1) investigate in detail and if possible ensure that the apparent layering is not a scoop artifact, but an intrinsic feature of the deposit, and (2) search for vertical gradients in chemical composition. The first of these goals was achieved: The observation of layering on both near and far wall in several of the scoop troughs strongly suggests that this feature is indeed inherent to the deposit (Section 1). The second goal is much harder to achieve partly due to the low statistics which in turn is related to the high spatial resolution of ChemCam data: Less than 3 area% of the walls of trough #2 (scoop entrance wall and the far wall) that were visible from the ChemCam/Mastcam vantage point were hit by the ChemCam laser beam (Kenyon, Kenyon_HA, Epworth_3, spots #1-10, taking into account all 30 shots at each spot). Moreover the steep far wall was subject to frequent collapse, thus only the oblique scoop entrance wall could be investigated practically by ChemCam. Which chemical signature may be associated with the bright layer? None of the major oxides for which PLS1 abundances are available are consistently enriched or depleted within the bright layer as compared to fines above and below (see SOM Figures S2.1-S2.10 for all PLS1 oxides and H and Li). However, among those oxides only iron oxides (specifically ferric oxides with submicron grain size) can be efficient pigments such that very minor changes in the abundance of these pigments (likely smaller than 19

20 ChemCam's precision, ~1.8 wt.% for FeO T, see Section 2 and Figure 3.1) have a significant effect on albedo and color. Therefore we take a closer look at the iron oxide abundance in the shallow subsurface. Figure 5.1 serves both the present and the upcoming section. It plots all FeO T ChemCam data (in terms of PLS 1 abundances) that do address potential vertical gradients in chemical composition: These are the ChemCam experiments Kenyon (Kn) and Kenyon_High_Albedo (KnHA) as well as Epworth3, spots #1-10. The remaining 5 spots of the Epworth3 experiment are located at the very surface of the deposit, just like Epworth and Epworth2 (Figure 2.4, Tables 1-2). The vertical stippled lines in Figures 5.1a-c mark the range of spot numbers corresponding to the extent of the bright layer. Figures 5.1a and 5.1b suggest a slight enhancement in iron within the bright layer at the targets Kn and KnHA. However, spots Kn, 5 as well as KnHA, 5 and 7 (all located within the region of the bright layer) contain each an iron-rich coarse grain immediately below the surface of the scoop entrance plane. These grains are invisible in images, and can therefore not contribute to the observed albedo feature that must be associated with the fines. As a result, only one spot documents potential enhancement in iron within the bright layer: Kn, 6 that has iron-rich fines in the first ~20 shots and a deeply buried iron-poor grain in the last ~10 shots. Finally, no enhanced iron abundance is observed within the bright layer on the far wall (E3, Figure 5.1c). Given the absence of a clear ChemCam signature (even as far as Fe is concerned) the most likely explanation is a higher degree of oxidation within the bright layer, a chemical property that ChemCam cannot address. Assuming (for the moment) the iron abundance to be constant across the uppermost 40 mm of the subsurface, a different distribution of the available iron among ferrous and ferric phases within and outside the bright layer might explain the observation. Fine-grained ferric minerals are usually brighter than ferrous minerals. Ferric oxides (or oxy-hydroxides) with grains in the submicron size range would be the best candidate for the ferric phase in the region of the bright layer, although hydrogen data (SOM Figure S2.9) do not suggest hydration or hydroxylation. Even if the iron abundance is higher within 20

21 the region of the bright layer, we will also need the concept of a higher degree of oxidation to explain the higher albedo of the bright layer as ferrous minerals (olivine, pyroxenes, magnetite with mixed valence) tend to be dark(er). In summary, an enhanced degree of iron oxidation in the bright subsurface layer remains the preferred explanation whatever the (poorly constrained) chemical subsurface gradient may be. 6. Induration of the Top-Surface Section 1 described evidence for weak induration at depths of few millimeters, immediately below the top surface armoring (see especially Figures 2.2a, 2.2c, 2.3c, 2.3d). Such induration was observed frequently at Gusev crater and Meridiani Planum (Blake et al., 2013, and references therein). At the Rocknest deposit, narrow but distinct "cracks" are identified in high-resolution color images of the pristine (undisturbed) surface of the Rocknest deposit (see SOM, Figure S5.1) and are interpreted to be caused by the presence of a bonding agent between and below the top surface grains. What is the nature of this bonding agent? ChemCam experiment Epworth3 did not return useful information on the cm-scale subsurface layering of the Rocknest deposit as the laser hit a multiply collapsed wall (Figures ). However, spots #7-15 of Epworth3 did provide valuable data on the transition from fine-grained bulk material to coarse-grained armor. In particular, based on imagery (Figure 2.4d) spots #10-11 should be located exactly at this transition. While ChemCam data acquired at spot #10 are ambiguous with respect to grain size, data on spot #11 indicate that no coarse grain is present. From ChemCam's point of view, any bonding agent should be either part of the fines or deposited on the very surface of coarse grains. The interior of these 21

22 grains should certainly not be representative of the bonding agent. Hence ChemCam data acquired at spot E3, 11 fulfill the requirements to be representative of the bonding agent and indicate a weak trend towards high FeO T and slightly low TiO 2 (SOM Figure S5.2b). In particular the abundance in FeO T at spot #11 (~14 wt.% post shot #11, see SOM Figure S5.2b) is significantly higher than the average for the entire Rocknest deposit (~11 wt.%, see Figure 3.1). Moreover, abundances in FeO T at spots #10 and #11 are somewhat higher than those at earlier and later spots in the vertical E3 raster (Figure 5.1c). Further shots directly onto the surface (Epworth2 and Epworth, Figure 5.1d) show significant jumps in iron abundance, perhaps because iron-rich grains were hit, or because the laser reached that putative iron-rich indurated zone that embeds the top grains and locks them in position. To further test the hypothesis of such type of bonding agent, we focus on the PLS 1 plot of TiO 2 versus FeO T. Figure 6.1a plots all shots on all targets at the Rocknest deposit (Table 1) with prominent outlier shots labeled (Table 3). Figure 6.1b plots linear fits through all shots at a given spot, disregarding the first 5 shots for each fit with the intent of removing the signature of loose surface dust that should be representative of neither coarse grains nor bonding agent. Overall, two trends are identified: (1) a largely positive correlation between Fe and Ti, and (2) a negative correlation with data points trending towards low-ti and high-fe abundances (marked by a black ellipse in Figure 6.1a). Figures 6.1c and 6.1d plot the same data as Figure 6.1a, but focus on the difference between top surface and subsurface data, respectively. It can be seen that the black ellipse (located at same relative position in Figures 6.1a, c, and d) is mainly populated by top surface data (E, 2; E, 4; E2, 3; E2, 8; see SOM Figure S5.2 for chemical paths) and only by few subsurface data (Kn, 6). This may be interpreted as a further hint to the chemical signature of the bonding agent (high Fe, low Ti). However, as pointed out above, our search of the bonding agent must exclude the interior of coarse grains: Spot E, 2 may or may not contain a coarse grain. If only fines were hit at this spot, they must have a gradual change in composition. Spot E2, 8 is 22

23 dominated by a coarse grain with a remarkable gradual increase in FeO T throughout all 30 shots (SOM Figure S5.2d). The assignment of all (but the "dusty") shots to a coarse grain (as for E2, 8 data) does not exclude the possibility that some of these shots (e.g. the early ones) do represent the bonding agent on the grain's surface. E2, 3 contains a coarse grain, but shots #4 - #8 plot far from the dust (red dotted ellipse in Figures 6.1c-d) and may well represent fines related to the bonding agent (SOM Figure S5.2e). E2, 8 does not contain a coarse grain, but only shots #6 - #19 plot in the suggested area of the bonding agent (SOM Figure S5.2f). The few subsurface shots (Kn, 6, shots #4-9) plotting in the same area are fines and are clearly distinct from the dust signature. These shots were previously highlighted as high-iron fines in the bright layer (Section 5) and turn out to qualify equally well as bonding agent in terms of Fe and Ti abundances. In summary, these few shots are our best evidence for the bonding agent: E3, 11, shots #7 - #30; E2, 3, shots #4 - #8 and E2, 8, shots #6 - #19. The target E, 2 would add another 30 shots. E2 data are ambiguous in terms of grain size, although it is conceivable that particulate samples with various degrees of induration may give ChemCam data a range of characteristics between those of loose powders and those of competent rocks. We have observed trends towards high-iron and low titanium at or immediately below the top surface, where induration is observed according to high-resolution images. Thus the bonding agent is suggested to be enriched in iron and depleted in titanium. No shot is expected to sample exclusively the bonding agent, as this likely constitutes only a minor fraction of the bulk. Therefore data are also consistent with the bonding agent being devoid of (rather than depleted in) titanium. The potential enrichment of the indurated zone in other elements was also examined. In particular data do not suggest enrichment in Ca or Mg at the top surface as compared to the subsurface (SOM Figures 23

24 S3.4-S3.5). Thus Ca- or Mg-bearing phases (e.g. calcium carbonate, calcium sulfates, and iddingsite) are not favored as bonding agent. In some cases a millimeter-thick bright layer was observed very close to the top surface (see Figure 2.3a). It is tempting to relate it to the indurated layer described here. However, that second bright layer - not to be confused with the one discussed in Section 5 - was observed infrequently and might potentially be a scooping artifact although this interpretation is not favored (Section 1). 7. Provenance of Rocknest Soils? In this section Rocknest soils are described in terms of the oxide ratios K 2 O/Al 2 O 3 against TiO 2 /Al 2 O 3. This type of plot may be considered as an alternative way to characterize the geochemistry of the Rocknest deposit, in addition to other representations used in previous sections (Section 3: Al/Si versus (Fe+Mg)/Si, TAS; Section 4: A-MF-CNK, A-K-CN). It focuses on elements that are fairly immobile (K) to highly immobile (Ti, Al). Therefore this type of representation is suited for distinguishing between clasts of different igneous origins. It was previously used by McLennan et al. (2013) to characterize mudstone in the Yellowknife Bay area, about 50 m east of Rocknest. For consistency and easy comparison we plot these ratios with same axis ranges as done by McLennan et al. (2013). Obviously, clasts of different igneous origin may still plot in the same area, as only three chemical elements are considered. Therefore this diagram may be more appropriate to exclude the petrologic relationship between certain clasts than to claim their similarity and their provenance from the same source area. Both will be attempted in the following, but the caveats of the latter must be kept in mind. 24

25 Figure 7.1 presents such a diagram for all (1860) shots on Rocknest deposit soils. Clusters of data points in Figure 7.1a are labeled as in previous figures. Data for coarse grains (both in the subsurface and at the top surface) spread over a larger area than bulk fines. In particular, abscissa values (TiO 2 /Al 2 O 3 ) below ~0.04 appear to be reserved for top-surface grains (e.g. E2, #1; E2, #3; E3, #13 - #15). Other outlier grains plot at intermediate (Kn, 8) and large abscissa values (Kn, 5). The dust (Figure 7.1b) as probed by the first laser shots plots at abscissa values around ~0.10 and near the right edge of the main data cloud (consistent with an enrichment in titanium for the first few shots, Figure 3.1). Figure 7.2a presents the same type of diagram for five geologic units encountered during the first 100 sols of the mission: soil, conglomerate, floats, Bathurst and Rocknest rocks. Obviously, these units are not independent of each other, as the first one ("soil") incorporates detritus of all rock units. The group "float" refers here to all floats except Jake_M, a particular float rock of high importance (Stolper et al., 2013), which is plotted separately. In addition Figure 7.2a draws the outline of a domain (Region of Interest, ROI) that contains more than 99% of all Rocknest deposit soil data (compare ROI to Figure 7.1), while Figure 7.2b overplots the latter as large black circles. Figures 7.2c-h are similar to Figure 7.2a, but plot the different geologic units separately (next to the outline of Rocknest deposit data). Note that data for Jake_M (Figure 7.2f) are well replicated by a group of float rocks (Figure 7.2e). This suggests that Jake_M is a fairly common float rock on Bradbury rise. Figure 7.3 presents a statistical comparison based on the density of data points. We count the number of data points within a small square of the data space (here K 2 O/Al 2 O 3 versus TiO 2 /Al 2 O 3 ), then move that square by small steps all over the diagram. This procedure generates a two-dimensional map of data point densities. The counting procedure is applied to Rocknest deposit data as well as to data of each geologic unit. Finally the Rocknest deposit density map is multiplied by the one for each geologic unit 25

26 and the product is normalized such that its maximum equals 100. When plotted as color contour maps (Figure 7.3) chemical domains with high overlap (or correlation) appear as hot (red to yellow) spots. A few simple notes may be helpful to become familiar with the data plotted in Figures Overall, soils (Figure 7.2c) and float rocks (Figure 7.2e) are the geologic units that cover the largest areas in plot space. This has (at least) two causes: (1) Data on these two units were acquired over the largest number of sols when the rover was moving. Thus they also originate from a larger area along the rover's traverse. To the contrary, data on Rocknest rocks (Figure 7.2h) refer only to a very limited area located around the Rocknest deposit. Given the small area these rocks display a remarkable diversity (Blaney et al., 2013). Data on conglomerates were acquired on few sols only (especially on sols 14, 15, 19, 27), the same applies to Jake_M and Bathurst (Figures 7.2f-7.2g). (2) Soils and float rocks (more precisely: float rocks except conglomerates, Jake_M, and Bathurst) have been subjected to transport by wind and impacts, respectively. In addition a fraction of soils on Bradbury rise (explored during the first 100 sols) may have been formed by slow abrasion and alteration of float rocks and outcrops in the vicinity. Such transport processes can explain the diversity of these units. "Voids" in the diagram for float rocks (Figure 7.2e) may be in parts a sampling effect: Overplotting data for Jake_M, another float rock, would fill up some of these "voids". Figure 7.2 also provides insight on two very different outlier coarse grains of the Rocknest deposit that have been addressed in most previous sections: The mafic grain Kn, 5 and the felsic one Kn, 8 (see their plot location in Figure 7.1). As explained earlier, we cannot identify related rocks with certainty. However, the absence of data points in the relevant plot area allows exclude certain relationships in terms of igneous chemistry assuming that the sampling frequency was sufficiently large. Based on Figures 7.2c-h it appears that Kn, 8 is related to none of the highlighted geologic units except Rocknest rocks. Thus Kn, 8 may not be a very common grain, although this particular study allows for its genetic 26

27 relationship with the nearby Rocknest rocks (e.g. Rocknest 6, Peg, Pearson DP, see SOM Figures S6.1- S6.3). In particular, a Kn, 8-like chemical signature was found in neither soils nor conglomerates nor float rocks. This contrasts with the assertion by Cousin et al. (2013) who used the abundances in Cr to relate felsic coarse grains, including Kn, 8, to Bradbury float rocks, such as conglomerates. Kn, 8 has a felsic, high-silica composition (Section 3), appears not to be a result of weathering (Section 4), and was classified to belong to the felsic group of soils and rocks (Meslin et al., 2013; Cousin et al., 2013). The grain volume probed by ChemCam may not have been representative of the entire grain. Thus Kn, 8 was not necessarily derived from a felsic rock. Its provenance from another group of rocks, such as the Rocknest rocks (Meslin et al., 2013), remains a possibility. In contrast to Kn, 8, Figures 7.2c-h suggest that the mafic grain Kn, 5 is unrelated with all geological units except float rocks. Again, we can suggest (but not prove) a genetic relationship with particular float rocks (e.g. Thor_Lake, Pekanatui; Figure 7.2e). Overall, Figure 7.2 suggests Kn, 5 to be an uncommon grain (as Kn, 8). Finally we will use Figures to make some statements on the provenance of deposited material. With the explained caveats in mind, it may still be useful to explore the implications of Figures in terms of genetic relationships of the deposit. The relative fraction of data points within the Rocknest deposit ROI (Figure 7.2) suggests the following sequence of increasing contribution to the Rocknest deposit: Bathurst (18%) < conglomerates (19%) < floats (25%) < Rocknest rocks (27%) < Jake_M (37%) < soils (56%). In Figure 7.3 we plot the relative correlation (scaled to a maximum value of 100). The absolute correlation between both density maps can be quantified by the Pearson correlation coefficient as plotted in Figure 7.4. The sequence of different geologic units, sorted by increasing contribution to the material in the Rocknest deposit, becomes again (with approximate Pearson correlation coefficients 27

28 given in parentheses): Bathurst (~ 0.03) < conglomerates (~ 0.01) < floats (~ 0.03) < Rocknest rocks (~0.07) < Jake_M (~0.1) << soils (~0.6). Again, the correlation with the soil unit is strongest (as expected). Strong correlation with "Jake_M" confirms the above statement that Jake_M type rocks are common in the area of exploration. The remaining float rocks, i.e. all float rocks except Jake_M, have a similar or a lower correlation coefficient. 8. Discussion So far, we have been concerned with experimental data and the direct information contained in these data. In this section we will evaluate the chemical alteration of the Rocknest deposit and tie observations described here to other studies. Two zones were identified in the near subsurface: an indurated, thin (< 1 mm) iron-rich zone right below the top surface armor and a ~7 mm thick bright layer deeper in the subsurface (Fig. 8). The abundance of hydrogen (as inferred from ChemCam data) is highly variable. In particular, no consistent enrichment or depletion in hydrogen was found in either zone (SOM Figure S2.9), although bulk material of the Rocknest deposit clearly contains ~2 wt.% H 2 O as inferred from evolved gas analysis by the SAM instrument (Leshin et al., 2013). ChemCam data are consistent with a slightly higher level of iron within the region of the bright layer, although the iron enhancement occurs mainly in coarse grains (Kn, 5; KnHA, 5; KnHA, 7). However, the enhanced visible albedo must be associated with fines (as opposed to coarse grains) and is most easily explained by a higher degree of oxidation regardless of the overall iron gradient in the subsurface. The question remains open as to the geological significance and timing of the bright layer. It seems unlikely 28

29 that the bright layer could have existed while the deposit was moving (if it moved at all) as this would require the processes forming the bright layer to be faster than mobility-related mixing processes. If the deposit never moved and was built up on the lee side of an obstacle (say at the current location) then the bright layer may represent a discontinuity in conditions of deposition, such as a lower supply of wind-transported material or supply of different types of material. In particular, the bright layer may be a zone enriched in global dust. The latter is brighter and has a much higher degree of oxidation than average Martian soil (Fe 3+ /Fe tot ~39% for dust as opposed to ~23% for bright soil, Morris et al., 2006a). It is also possible that the bright layer was formed after the deposit already had acquired its terminal height by some kind of weak late-stage diagenesis, e.g. by short-range transport of soluble ions by thin films of liquid water (Arvidson et al., 2010). The typical distance of such transport processes would depend on the ionic strength of these transient aqueous solutions and on sublimation/freezing rates. In this respect it is interesting to note that the diurnal thermal skin depth (i.e. the penetration depth of the thermal wave) extends a few cm into the subsurface assuming typical thermal inertia and modern climate conditions (Cuzzi and Muhleman, 1972; Jakosky, 1979; Palluconi and Kieffer, 1981) and is thus similar to the depth of the Rocknest bright layer. The diurnal thermal skin depth is by one or two orders of magnitude smaller than the seasonal one and coincides with the depth probed by the REMS Ground Temperature Sensor (J. Gómez-Elvira et al., 2012). It is beyond the scope of the present paper to speculate if the diurnal thermal wave may have generated the Rocknest subsurface layer. However, subsurface layering was generally not observed by MER wheel scuffs into sand deposits. In particular Serpent dune (Gusev crater, some 2200 km SSE and ~10 deg. further south) as investigated by the Spirit rover has some similarity with Rocknest, but does not seem to be layered (Sullivan et al., 2008). Finally, it is unclear whether and how the albedo of the bright layer is related to enhanced iron abundance in that zone. As mentioned above, enhanced iron abundance was only encountered after a 29

30 significant number of laser shots and within coarse grains (as distinguished from fines by ChemCam data). However, monolayers or thin films of liquid water should have been available in the shallow subsurface under current environmental conditions (Haberle et al., 2001; Martínez and Renno, 2013) and more extensively under snow/ice covers during recent glacial periods (Head et al., 2003). This water would have penetrated the Rocknest deposit as a result of gravitation, capillary action or diffusion (Arvidson et al., 2010) and would have reacted with unaltered basaltic grains (e.g. olivine and pyroxene) in the bright layer to release the iron and re-deposit bright, ferric oxides or oxyhydroxides as a result of small-scale weathering. Such ferric oxides, when finely dispersed in the matrix, can have a significant effect on color and albedo long before they would affect the ChemCam-PLS1 estimate for FeO T. Both a discontinuity during build-up of a stationary deposit and a thermally driven circulation of transient water would explain the observation that the bright layer broadly follows the local topography. The one exception, the bright layer as it occurs in trough #5 near the crest of the landform, is inclined with respect to the present surface. This allows the possibility that the landform nearest the crest was mobilized at some time(s) after formation of the bright layer, thus truncating it. Contrary to the enigmatic bright layer in the cm-scale subsurface, the indurated layer near the very surface should be a more convincing candidate for weak late-stage diagenesis of the Rocknest deposit. Overall, this layer is weakly indurated by terrestrial standards. However its shear strength may be significant if the induration turns out to be only effective within a very narrow zone (as suggested by images). Given the clear presence of water in the deposit in general (Leshin et al., 2013; Meslin et al., 2013), we suggest that the bonding agent is hydrated to some degree and formed by leaching of ferrous iron, oxidation and subsequent precipitation. Hence the bonding agent is expected to contain poorly ordered ferric phases, and these phases should host some (or all) of the water present in agreement with 30

31 CheMin data that suggest all crystalline phases (above the detection limit) are essentially anhydrous (Blake et al., 2013). Ferric nanophase oxides (npox) have been found in significant abundances (up to 10 wt.% or so) in most soils at Gusev crater and Meridiani Planum (Morris et al., 2004, 2006, 2008; Ming et al., 2006, 2008). These phases are poorly crystalline to amorphous and are believed to be the most characteristic expression of slow basaltic alteration in the late-amazonian semi-arid Martian environment. Thus npox should also occur at similar abundances in the Rocknest deposit and are expected to constitute a significant part of its amorphous fraction (Blake et al., 2013). Based on in-situ Mössbauer spectroscopic data and studies of Mars analogue samples (such as Hawaiian palagonites) candidate npox phases are ferric oxyhydroxides and sulfur- and chlorine-bearing phases such as superparamagnetic goethite and hematite, lepidocrocite, akaganéite, schwertmannite, jarosite, ferrihydrite, iddingsite, or a ferric pigment similar to that found in palagonitic tephra (Morris et al., 2006b; note 33 in Blake et al., 2013). The bonding agent in the Rocknest deposit may be one (or several) of these npox phases. As mentioned earlier (Section 6) iddingsite is not a favored component as it contains significant amounts of Mg. With the exception of iddingsite, ChemCam data are consistent with all mentioned phases. In particular sulfurand chlorine-bearing phases are not ruled out due to ChemCam's low sensitivity to these elements. Akaganéite, β-feo(oh, Cl), was identified in crystalline form and at low abundance (1 to 2 wt.%) in the Yellowknife Bay mudstone, about 50 m east of Rocknest (Vaniman et al., 2013). However, pyrrhotite, akaganéite s putative precursor phase, is absent (or below detection limits) from the Rocknest material examined by CheMin (Vaniman et al., 2013). Consequently, if akaganéite is present in the Rocknest deposit, it occurs without its expected pyrrhotite precursor. Akaganéite is therefore not a favored candidate for the bonding agent in Rocknest. 31

32 Whatever may be the precise composition of the bonding agent, it likely represents a late-stage alteration product similar to npox. We suggest this bonding agent to be the latest addition of npox phases that formed after Rocknest had reached its terminal height and stabilized at the current location. Only that latter fraction of npox could act as a weak and slowly developing bonding agent by virtue of the protective pavement on the very surface of the deposit. This hypothesis is consistent with the existence of similar indurated zones near the Viking landers and in aeolian deposits in Gusev crater and at Meridiani Planum. It is, though, challenged by the question whether the product of alteration rate and time is large enough to generate such form of induration. Golombek et al. (2010) constrained the age of late wind ripple migration at Meridiani to be of the order of 100 ka. These ripples are possibly among the youngest macroscopic morphological features on Mars. Thus we suggest that the age of Rocknest deposit is at least 100 ka, but could well be several Ma. The latter age would imply Rocknest deposit to have undergone low-obliquity cycles with substantial ground ice at low latitudes and probable higher alteration rates during these cycles. Figure 9 shows images with similar spatial scale of soil profiles in Gale crater (a-c) and Gusev crater (d-e). Given its occurrence in three different areas explored by rovers this type of weak induration must be a wide-spread feature on the surface of Mars: Whenever fine-grained drift material becomes immobilized, the top-most part of that material becomes indurated. The depth of that induration should be limited to the penetration depth of transient liquid water that forms from atmospheric water vapor or snow. NpOx may play a significant role in this process. 32

33 9. Summary and Conclusions The Rocknest deposit is (like other ripples in Gusev crater and at Meridiani Planum) made up of finegrained bulk material covered by a coarse-grained millimeter-thick armor. The bulk volume also hosts a significant number of coarser grains, as glimpsed in some MAHLI images and indicated by ChemCam data. The most significant features of the shallow subsurface are: (a) a bright (~7 mm thick) layer in the subsurface, centered at a depth of ~15 mm, and (b) a sub-millimeter-thick zone of indurated materials immediately below the top-surface armor. Both zones are well documented by images. The hydration level of both zones remains unclear. As to the bright layer, no consistent chemical signature is found, except a weak iron enrichment associated with buried coarse grains. This suggests a discontinuity in the depositional history rather than post-depositional chemical transport mediated by small amounts of liquid water. In particular, the interpretation of the bright layer as a zone enriched in global dust would satisfy the observational constraints. As to the indurated zone, trends towards high iron and low titanium are interpreted to reveal an ironrich bonding agent. No enhancement or depletion in Ca or Mg is observed in this zone. The processes that lead to the formation of the bonding agent remain speculation, but are thought to involve thin films of transient liquid water penetrating the topmost subsurface. We suggest that the bonding agent is similar to one or several ferric npox phases that are known to be widespread on Mars based on MER- Mössbauer data. The build-up of weak induration would then be favored by the protective top-surface armor of coarse sand grains. Ignoring the indurated materials the Rocknest deposit appears to be relatively unweathered. A few topsurface grains plot closer to the Al 2 O 3 apex than most other data in ternary compositional diagrams (A- 33

34 CN-K, A-CNK-FM). However, these weak signs of alteration are likely inherited from whatever source region(s) supplied these materials to the Rocknest deposit, rather than being the result of in-place alteration. Given the severe limitation of alteration of the Rocknest deposit, its amorphous fraction (30 to 40 wt. %, Blake et al., 2013) should contain unaltered amorphous material (e.g. pristine volcanic glasses) along with npox phases. In average, the Rocknest deposit is chemically similar to soils and rocks encountered during previous missions to Mars and plots slightly on the mafic (low-silica) side of most Gusev soils and rocks. The finegrained bulk of the deposit (up to a few hundred micrometers in grain size, given ChemCam's spatial resolution) appears to plot in a similar area while the composition of the coarse grains (located both at the top and in the subsurface) ranges from mafic to felsic compositions (35 to 60 wt. % SiO 2, 43 wt. % SiO 2 in average). Thus the chemical diversity of the Rocknest deposit is associated with the coarse grains. Their diversity is fully consistent with high-resolution color images of soil patches and individual grains. Provenance studies (based on K 2 O/Al 2 O 3 versus TiO 2 /Al 2 O 3 diagrams) constrain potential source areas for coarse grains. These coarse grains are all in a pristine state, i.e. their chemistry reflects igneous origin rather than alteration. Based on oxide ratios Rocknest deposit soils correlate mostly with other soils (as expected), but also to a significant degree with selected float rocks, Jake_M in particular, and to a lesser degree to outcropping bedrock in the vicinity. Several lines of evidence suggest that Jake_M-like rocks are common on Bradbury rise and contributed significantly to the Rocknest deposit. Acknowledgements This work was partially funded by Deutsche Forschungsgemeinschaft (grant GO 2288/1-1). MBM 34

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44 Tables CCAM target seq ID (sol #) location Schmutz-2 (S2) CCAM01072 (12:20) 3x3 spots, 10 shots trough #2, bottom Δaz = mrad, Δel = -0.3 mrad Epworth (E) CCAM02072 (12:27) 1x5 spots, 30 shots top soil, far from trough #2 Δaz = mrad, Δel = +0.2 mrad Epworth-2 (E2) CCAM01079 (12:10) 1x10 spots, 30 shots Top soil, far from trough #2 Δaz = 0 mrad, Δel = +0.2 mrad Kenyon (Kn) CCAM01081 (sol 82, 09:20) 1x10 spots, 30 shots scoop entrance wall, trough #2 (Figure 1) Δaz = mrad, Δel = mrad Epworth-3 (E3) CCAM02084 (sol 85, 09:33) 1x15 spots, 30 shots far wall of trough #2 Δaz = 0 mrad, Δel = +1 mrad for 10 first shots Δaz = 0 mrad, Δel = +0.3 mrad for 5 last shots Portage (Por) CCAM02089 (sol 90, 09:28) 3x3 spots, 30 shots bottom of wheel scuff. Location also investigated by APXS. Δaz = -3.5 mrad, Δel = +3.5 mrad Kenyon-highalbedo (KnHA) CCAM02097 (12:34) 1x10 spots, 30 shots scoop entrance wall, trough #2, next to Kenyon (Figure 1), but narrower laser spot raster: Δaz = mrad, Δel = mrad 44

45 Table 1 Overview of CCAM experiments on Rocknest soils (7 targets, 1860 laser shots in total). Left column: Target names as well as short names (in parentheses) used for labeling in figures. Middle column: Sequence ID with the last 3 digits being the sol number. In parentheses Local True Solar Time (LTST) of acquisition of first RMI image. In some cases (Kn, E3, Por) the experiment was actually performed on the morning of the next sol, although it was part of the master plan of the previous sol). Right column: Location of shots and CCAM raster parameters. Vertical transects (Kn, E3, KnHA) started at the bottom of the trough and moved upwards in steps of Δel (rightmost column). Locality of all targets is shown in Figure s 1 and 2.4. Very small raster parameters (say <0.5 mrad) were not necessarily executed. In the case of e.g. Epworth the Remote Sensing Mast (RSM) and camera head may not have moved at all. 45

46 CCAM transects spot increment (mrad)* spot increment** total length** (mm) (Table 1) (mm) Kenyon (Kn) ( ) 1/2 = *2300 = 3.4 9*3.4 = 30.6 mm CCAM01081 Epworth-3 (E3), ( ) 1/2 = *2300 = 2.3 9*2.3 = 20.7 mm spots #1-10 CCAM02084 Kenyon High- ( ) 1/2 = *2300 = 2.6 9*2.6 = 23.4 mm Albedo (KnHA) CCAM02097 *Assumption: angular size of vertical Mastcam step = angular size of azimuthal Mastcam step **Length as measured on a target plane perpendicular to the camera line of sight. Table 2 Characteristics of CCAM transects (as commanded) on the wall of Rocknest trough #2. The distance from CCAM to any laser spot in/nearby the trough is taken as 2.3 m (accurate within 2 % for all spots). 46

47 target & shot elements with non-average abundance Comments number (label) depleted enriched (chemistry, mineralogy, rock type) E,2 Ca, Mg, Ti, H Na, K, Si, Al, Fe coarse-grain signature ambiguous, perhaps fines with gradual change in composition, possibly related to Fe-rich bonding agent (Section 6) E,3 (cg) Ca K (with large scatter), Si, Al* mixed composition, signs of chemical weathering (Section 4) E,4 (cg) K, Ca, Mg, Ti, H Fe* coarse grain with compositions trending towards high iron and low titanium, some shots may be related to Fe-rich bonding agent (Section 6) E,5 (cg) Na*, K*, Si*, Al*, Mg*, Ti*, H* Ca*, F*, P*, Li outlier (likely of igneous origin, Forni et al., 2014): elements that occur in rock-forming minerals (with the exception of Ca and Fe) are absent or present at low abundances, PLS oxide sum very low E2,1 (cg) Ca, Mg K*, Si, Al, H composite grain, possibly containing K feldspar, large data scatter on all abundances, signs of chemical weathering (Section 4) E2,3 (cg) Ca, Mg Na, K*, Si*, Al, Fe early shots (#4 - #8): fines, possibly related to Ferich bonding agent (Section 6), later shots (#9 - #30): coarse grain E2,8 Na, K, Si, Al Ca fines with very broad range of chemical compositions, shots #6 - #19 possibly related to Ferich bonding agent (Section 6) E2,9 (cg) - Ti composite grain with fairly average composition E3,1 - Ti soil with fairly average composition, but high Ti, no coarse grain hit E3,2 (cg) K - composite grain with fairly average composition E3,3 (cg) Na*, K*, Si*, Al* Ca*, Fe, Li outlier, possibly similar to E,5 (with the exception of Mg, Ti and H) 47

48 E3,15 (cg) Ca, Mg, Ti, H Na*, K, Si, Al felsic, possibly containing K feldspar Kn,5 (cg) Ca, H Mg, Fe, Ti, Li mafic Kn,6 Mg, Fe, Ti Na, K, Ca, Al, Li early shots (#4 - #9): fines, possibly related to Ferich bonding agent (Section 6), shots #19 - #30: coarse grain of intermediate composition Kn,8 (cg) Mg* Na, K*, Si*, Al felsic, PLS oxide sum close to 100 wt.% KnHA,5 (cg) Ca, Mg, H K, Si, Al, Fe composite (felsic?) grain, enriched in Fe (mineral?) KnHA,7 (cg) Ca, Mg, H* Na, K, Si, Fe same as KnHA,5 KnHA,9 K - fairly mafic soil in terms of ChemCam data, close to Portage-APXS, no coarse grain hit Por,4 (cg?) - - fairly average compositions Por,8 (cg?) - - fairly average compositions Table 3. Chemical compositions encountered at the Rocknest deposit. ChemCam targets are listed alphabetically. Most analysis spots mentioned here are interpreted to contain coarse grains (cg) based on the scatter in chemical composition and total intensity. Most of these coarse grains are also plotted in Figure 3.2, and some of them have previously been discussed by Meslin et al. (2013). Focus is on enrichment or depletion of following elements: H, Li, Na, K, Mg, Ca, Ti, Fe, Al, Si. Elements not mentioned in the middle columns are present at roughly average abundance (given the typical scatter of the data). A raised circle (degree sign) and an asterisk indicate, respectively, slight and strong enrichment (or depletion). Estimates were "made by eye" taking into account both the average abundances for shots post #5 and the data scatter (See SOM Figures S2.1-10). Background colors highlight spots/shots of certain compositional trends: average (white), felsic (pink), mafic (grey, Kn,5 and KnHA,9), bonding (green), chemical weathering (blue), outliers (yellow). Comments (right-most column) and background colors are somewhat subjective highlighting various aspects of the present work. 48

49 Figures Figure 1. Troughs in the Rocknest deposit (labeled from 1 through 5). Instruments on the robotic-arm turret are specified, MAHLI is slightly above the brush and behind the turret and is therefore not visible in this image. A surface patch in the wheel scuff area called Portage (P) was studied extensively by MAHLI, ChemCam, and APXS. Image ID: Front Right Hazcam, sol 093 (FRA_ EDR_F FHAZ00205M_). 49

50 Figure 2.1. Trough #1 (Figure 1). The background image shows heterogeneous soil clods, numerous crust fragments, and subsurface banding (marked by white arrows). Scratches by the scoop are visible within and near the bright band. Inset (a) is a false-color stretch that highlights heterogeneity within a clod, likely displaying embedded ~0.5 mm grains. Inset (b) provides evidence for a double band (compare with Figure 2.3a). The sharp corners and reentrants of the crust fragments (one of them also highlighted in inset b) suggest induration. Modified from Goetz et al. (2013). Image ID: Background image and inset (b): 0066MH E2. Inset (a): 0066MH E1. 50

51 Figure 2.2. Trough #4 (a-b with inset) and trough #3 (c-d). See Figure 1 for trough labeling. The troughs have been imaged from two different vantage points: Top (a and c): MAHLI hovering over the troughs. The MAHLI images demonstrate continuation of the bright band on the near side of the trough (as marked by white solid lines [not to be confused with a misshaped arrow]). Bottom (b and d): Mastcam-100. The Mastcam-100 images demonstrate continuation of the bright band on the far side of the trough (white solid lines). Note again the scratches on the scoop entrance wall in all images. The floor of trough #4 is littered with soil clods. The sharp corners on the far side of trough #4 as well as the 6-7 mm large indentation on the far side of trough 51

52 #3 (dotted arrows in a and c) suggest significant induration. Image ID: 0084MH E1 (a and c). 0084mr e1 (b and d). 52

53 Figure 2.3. Trough #2 images in chronological order (with Local True Solar Time specified for each image). (a) Mastcam-100, sol 66 (right after digging). The image provides further evidence for a double subsurface band (compare Figure 2.1b). (b) Mastcam- 34, sol 67. Overnight most of the far wall has collapsed. This is the context image for high-resolution images (c-d). The MAHLI 53

54 images (c-d, sol 67) show the prominent difference between bright coarse-grained armor and dark fine-grained interior. Protrusions (solid white arrow in c) and sharp reentrants (black solid arrow in d) suggest induration near the top surface and immediately below the armor. Also the bulk (below the indurated zone) contains sand-sized particles: A dotted white arrow in (c) points to an embedded grain and the dotted arrows in (d) highlight voids of grains that have fallen out. Images (a-b) have been stretched to better display the subsurface banding. Images (c-d) are pseudo-true color images. Image ID: (a) 0066MR E1, (b) 0067ML E1, (c) 0067MH R0, (d) 0067MH R0. 54

55 Figure 2.4. Trough #2 as investigated by CCAM experiments Epworth (E), Epworth_2 (E2), Epworth_3 (E3), Schmutz_2 (S2), Kenyon (Kn), and Kenyon High Albedo (KnHA) (Table 1). (a) Context image (Mastcam-100, sol 84) for all CCAM runs. The deepshadow part of the far wall has been stretched and reveals bright particles leading up to CCAM target E. This may relate to the anomaly of one of the E spots (refer to main text). White solid lines mark the approximate position of the bright subsurface band and yellow dots mark the (most likely) position of CCAM shots for Kn, KnHA, and E3. Spot #5 is always represented by an ellipse so that the spot number can be read easily from the figure. It appears that the bright band is located within Kn spots #4 55

56 to 7, KnHA spots #1 to 7, and possibly E3 spots #3-9. Image (a) was acquired prior to CCAM experiments E3 and KnHA, therefore no laser shot holes (corresponding to the yellow dots) can be seen in this image. Note that only E3 spots #7-10 actually did hit the (undisturbed) wall, while E3 spots #3-6 hit a soil clod that fell down (prior to that CCAM run) when the far wall collapsed (Figure 2.3). Thus E3 spots #3-6 may have actually probed the same subsurface band as spots #5-8. (b) RMI image, sol 97, acquired after last KnHA shot at spot #10. (c-d) RMI images (sol 84 (actually morning of sol 85), acquired, respectively, before first and after last E3 shot. Red, blue, and green arrows mark tie points for correlation of a with, respectively, b, c, and d. One of the green arrows in (a) is dotted and highlights as a special tie point a prominent protrusion of the top surface crust that casts a long shadow on the trough wall. Note that the E3 transect is oblique in the Mastcam-100 image (a), but vertical in (c-d) due to the significantly different vantage point of Mastcam-100 as compared to RMI (and Mastcam-34). Image ID: (a) 0084MR E1_DRWL, (b) CR0_ PRC_F CCAM02097L1, (c) CR0_ PRC_F CCAM02084L1, (d) CR0_ PRC_F CCAM02084L1 (b acquired ~13 sols after a, c & d acquired ~20 h after a). 56

57 Figure 2.5 All troughs as imaged by Mastcam-34, and trough #5 as imaged by Mastcam-100 (insets). The insets are identical and emphasize the bright band that appears to be higher up in trough #5 and inclined in comparison to troughs #3 and #4. All images are color stretched. Image ID: Background image: 0093ML E1. Insets: 0093MR E1. 57

58 Figure 2.6. The ChemCam-APXS target "Portage". (a) Wheel scuff for context (compare Figure 1). Portion of left NavCam, sol 89. The image shown is about 40 cm wide (equal to the width of Curiosity's wheel). The cleat marks are spaced ~59 mm apart. (b) MAHLI, sol 58. The insets highlight very different grains (with shapes varying from angular to spherical) and are placed near the particles that are magnified. The insets are rotated counter-clockwise 90 (with respect to the background image) in order to 58

59 facilitate recognition of the particles' shape to the human eye (after-rotation lighting is from upper left). Coarse grains in the wheel scuff area do not have a dusty coating. These coatings might have been lost either when the grains fell down from the scuff wall, or from interactions with the rover wheel. The absence of dusty coatings reveals more of the grains' morphologies and colors. The spherule (impact melt or possibly cosmic spherule) in the lower inset is 0.53 mm in diameter (Minitti et al., 2013). (c) MAHLI, sol 89. The image was acquired after APXS integration on the same sol. Imprint of APXS contact plate in the lower left part of the image. The plus signs in the right half specify the most likely position of ChemCam laser shots (inferred by comparison to the RMI image CR0_ PRC_F CCAM02089L1, see Figure S4). Solid arrows and the solid ellipse emphasize the diversity of coarse grains at this locality. Dotted arrows and the dotted ellipse emphasize ubiquitous whitish particles and flakes. APXS data do not suggest enhanced salt content (as compared to earlier missions, Blake et al., 2013), but close inspection of soil patches (black rectangles) show the entire soil is speckled by whitish dots similar to Phoenix soils (Goetz et al., 2010). The big bright spot is ~1.3 mm across and is magnified in the inset in the upper right quadrant of (c). Morphology suggests that this is a competent solid grain rather than an agglomerate of fine-grained material. This grain has an unusually high albedo and some textural features at the limit of resolution. Figures (b) and (c) have same scale. Figure (c) is ~63 mm wide. Image ID: (a) Detail from left NavCam, sol 89, NLA_ EDR_F NCAM00307M_, (b) 0058MH R0 (focus merge, 48.3 μm/px). Insets are from 0058MH R0 (focus merge, ~ 30 μm/px). (c) Detail from 0089MH E1 (also lossless as 0089MH C0 with same field of view). Inset (magnifying the big bright grain, ~1.3 mm, in the upper left quadrant) is a detail from 0089MH E1. 59

60 Figure 3.1. Abundances of oxides in soils of the Rocknest deposit (all soil targets except Schmutz_2). Plotted are PLS1 average abundances (open circle with central dot) and both types of uncertainties (precision and accuracy). FeO T, TiO 2, and especially K 2 O abundances have poor accuracy. Also plotted are chemical compositions associated with subsequent shots (vertical bars and plus signs). For each oxide (as well as for H and Li) 11 data points are plotted: The first vertical bar is the abundance inferred from the 1 st shot. The other 9 vertical bars are average abundances calculated, respectively, from the first 2, 3, 4,..., and 10 shots. The plus sign marks the average over abundances inferred from all shots from #11 and up to shot #30. Trends from early shots suggest dust to be enriched in Mg, Fe, Ti, H, and to be depleted in alkalis. Literature data for the Portage target (Figure 2.6) as investigated by APXS (Blake et al., 2013), for "Martian dust" (Morris et al., 2006a, abbreviated as M2006) and for average Martian soil (Taylor and McLennan, 2009, abbreviated as TM2009) are overplotted as colorized filled diamonds. Error bars refer to accuracy in the case of Martian dust and average Martian soil and represent the analytical uncertainty in the case of Portage- APXS. Overall literature data agree well with ChemCam derived data except for FeO T. 60

61 Figure 3.2. Al/Si versus (Fe+Mg)/Si. (a) All shots onto Rocknest deposit soils. Targets are distinguished by different plot symbols and colors. Large blue, gray, and red diamonds represent, respectively, Portage APXS (Blake et al., 2013), average dust as inferred from MER (Morris et al., 2006a), and average Martian soil (Taylor and McLennan, 2009). Most data points plot on the "felsic side" of the plot (left of average soil [red diamond] and left of Portage-APXS [blue diamond]). Data for dust (shots #1-3 at any spot) mostly fit within the black ellipse. Selected minerals (plagioclase, albite, montmorillonite, nontronite, silica, augite, pigeonite, olivine) are mentioned together with their (x, y) coordinates in this particular plot. An asterisk indicates that these coordinates are from Blake et al. (2013). (b) Selection of data points with either strongly clustered (potentially multi-clustered) or continuously varying chemical composition. Only consecutive shots are shown. None of the strongly clustered data points (likely coarse grains, see Table 3) seem to have a ChemCam signature corresponding to a pure mineral. Kn,8 and E,5 (and many others) display the typical ChemCam characteristics of coarse grains. KnHA,9 displays a continuously varying chemical composition (likely not a coarse grain). Conclusions to be drawn from these diagrams must account for large uncertainty (see main text for further discussion). 61

62 Figure 3.3. Total alkali versus silica diagram for soils of the Rocknest deposit (Table 1, same legend as in Figure 3.2). Classes of volcanic rocks are specified following the usual definitions (PB = pricrobasalt, B = basalt, BA = basaltic andesite, A = andesite, TB = trachy-basalt, BTA = basaltic trachy-andesite, TA = trachy-andesite). (a) All shots. Outlier coarse grains (Epworth targets and Kenyon, see Table 1) are labeled. (b) Like (a), but averaged over all shots at a given spot. (c) Like (a), but averaged over all shots and spots (thus one point per ChemCam target). All data are subjected to large uncertainty (see main text). (d) Data for the Martian crust (in-situ data on soils and rocks, orbital data, SNC meteorites) (from McSween et al., 2009., with permission). The red solid ellipse in (a-c) specifies the main area of Gusev soils and rocks as shown in (d). 62

63 Figure 4.1. Al 2 O 3 -(CaO+Na 2 O)-K 2 O (A-CN-K, left part) and Al 2 O 3 -(CaO+Na 2 O+K 2 O)-(FeO T +MgO) (A-CNK-FM, right part) diagrams for soils of the Rocknest deposit (Table 1). These diagrams can be compared to similar ones for the Yellowknife Bay mudstone some 50 m further east (McLennan et al., 2013). (a) Distinction between different ChemCam targets. Plot symbols are as in Figure 3.2. Relevant clay minerals are overplotted as black filled circles (Hurowitz and McLennan, 2007; Ehlmann et al., 2011): illite, montmorillonite (mont), smectite of intermediate composition (Nesbitt and Wilson, 1992), nontronite (non), saponite (sap) and chlorite (chl). Rocknest deposit soils -except a few coarse grains (E2, 1; E, 3)- cross neither the plagioclase (plag) - potassium feldspar (K-fs) join (left-hand diagram) nor the (FeO T +MgO) - feldspar (fs) join (right-hand diagram). The black ellipse (coinciding with E, 3) marks the composition of Martian Mg/Fe clays (mainly in Mawrth Vallis and Nili Fossae). (b) Distinction between early shots (light brown symbols) and later shots (open blue squares) showing that windblown dust strictly remains below the weathering line in either diagram. The data "cloud" for dust specifies where the global component of loose material in Gale crater is located in these diagrams. Strong deviation off this "cloud" suggests either a local contribution or some 63

64 chemical weathering. The "dust cloud" is slightly displaced towards the (FeO T +MgO) apex (as compared to the remaining data points) in agreement with Figure

65 Figure 5.1. Abundance of FeO T versus spot number from ChemCam transects. At each spot the ChemCam laser was fired 30 times (Table 1) resulting in 30 different PLS1 estimates for FeO(total). Vertical lines mark the upper and lower limit of the bright layer (Figure 2.4) for each of the 3 transects: (a) Kenyon, (b) Kenyon_HA, and (c) Enworth_3, spots 1 through 10. The remaining spots of ChemCam run Epworth_3 (#11-15) are at the top surface of the deposit, as are all spots of ChemCam runs Epworth_2 and Epworth (d). In all plots (a-d) averages of the first 5 and the last 25 shots (at each spot) are plotted by, respectively, a blue "x" and an open red square. The iron abundance may be slightly higher within than outside the bright layer. This feature is weakly observed in (a-b), but absent in (c). However, the location of Epworth_3 spots relative to the subsurface stratigraphy is unclear as the laser was shot into a previously collapsed far wall. Enhanced ferrous iron (as e.g. in olivine and pyroxenes) would lower the albedo. Hence iron within the bright layer may be more oxidized. Spots E3,10 and E3,11 (marked by arrows) are not affected by the collapse and are located at the transition from the fine-grained darker subsurface to the coarse-grained top surface. Images suggest significant chemical induration in this domain (Section 6). Figure 5.1 is also shown in the SOM (Figure S2.7) next to several other PLS1 oxide abundances. 65

66 Figure 6.1. (a) TiO 2 versus FeO(total) for soils of the Rocknest deposit (Table 1). This figure is also included in the SOM (Figure S3.8). Plot symbols are as in previous figures and major outliers are specified (Table 3). A group of outliers (E2,3; E2,8; E,2; E,4) characterized by high Fe and low Ti is marked by a solid black ellipse. These outliers are interpreted to deviate from average composition due to admixture of a small amount of an iron-rich bonding agent. (b) Like (a), but ChemCam data for each target (except Schmutz_2 and Portage) are fitted by and presented as straight lines (E: solid green, E2: dotted blue, Kn: dashed purple, E3: dash-dot-dotted cyan, KnHA: long-dashed purple; same colors as in (a)). The first 5 shots were excluded from each fit in order to focus on bulk material rather than loose surface dust. Compositions at most spots trend from the lower left to the upper right in this diagram (positive correlation between iron and titanium), while some (E2,8; E,2; E,4) trend to the lower right. (c) Like (a), but restricted to ChemCam targets at the top surface. (d) Like (a), but restricted to subsurface ChemCam targets. The first 5 shots plot dominantly within the red dotted ellipse (c and d) near the top right of the data cloud (enriched in Fe and Ti, consistent with Figure 3.1). The black solid ellipse (c and d) hosts 66

67 predominantly top surface compositions (> 70), not subsurface compositions (< 10). This is consistent with images that suggest induration near the top surface (immediately below the armoring grains). 67

68 Figure 7.1. K 2 O/Al 2 O 3 versus TiO 2 /Al 2 O 3 for (a) different targets at the Rocknest deposit, and (b) for all targets (shot # 11) versus dust (shots #1-10). These diagrams can be compared to a similar diagram on Yellowknife Bay mudstone (~50 m further east of Rocknest, Figure 4A in McLennan et al., 2013). Spread of data in this diagram should reflect diversity of source areas, e.g. the grains Kn,5, Kn,8, KnHA, 4, and KnHA,7 are inferred to originate each from a different type of magmatic source, while KnHA,4 & 5 possibly originate from the same source. The left part of the diagram (say at abscissa values < 0.04) appears to be dominated by top surface data (E; E2; E3, spots 12-15). In particular, lots of E3 data plot at abscissa values < 0.04 and more than half of those data points belong to the top surface part of E3 (spot #12-15, see Figure 2.4). See Table 3 for further characterization of grains. The geochemical diversity of top surface material (especially armoring grains) appears to be larger than that of bulk material. Transport of millimeter-sized grains by saltation and creep is restricted to short distances, and likely to within Gale crater. The diversity of the top surface material suggests that other (more efficient) transport mechanisms (such 68

69 as impact) were active at the same time. The dust (light brown filled circles in (b)) has high ratios TiO 2 /Al 2 O 3 (consistent with Figure 3.1) and thus plots to the right of most data points in (b). None of these points plots at small abscissa values. ChemCam dust data as well as average data from the literature (grey and red diamonds) serve as reference in this diagram. The former has the advantage to provide a ChemCam internal reference that is subjected to the same statistical and systematic errors as all other ChemCam data. 69

70 70

71 Figure 7.2. K 2 O/Al 2 O 3 versus TiO 2 /Al 2 O 3 for Rocknest deposit soils and surrounding units (soils, float rocks, outcropping bedrock) investigated by ChemCam. Compare to similar diagram for Yellowknife Bay mudstone (Figure 4A in McLennan et al., 2013). (a) Float rocks, soils and bedrock encountered during the first 100 sols of the mission. Jake_M, an important float rock (Stolper et al., 2013), is plotted separately (cyan open square), next to other float rocks (blue plus sign). (b) Same as (a), but with all Rocknest deposit soils (Figure 7.1a) overplotted as filled black circles. The Region of Interest (ROI, delimited by black line) encompasses >99% of all Rocknest deposit data. (c-h) Each of the 6 geological units (in (a)) plotted separately together with the ROI. The number of data points within the ROI ratioed to the total number of points plotted is given on top of each graph. Some areas and outlying clusters are labeled (for further details refer to SOM Figures S6.1-S6.3). 71

72 Figure 7.3. K 2 O/Al 2 O 3 versus TiO 2 /Al 2 O 3 : Product of data point densities for Rocknest deposit soils and other surface materials or stratigraphic units as selected in Figure 7.2. The bright line shows the approximate boundary of Rocknest deposit soil data (as in previous figures). For all plots the number of data points was counted within small squares of size = (along x) such that 30 such squares fit on the entire x-axis. Then these squares were moved all over the diagram in steps of (i.e. one thirtieth of the square size). All products are normalized to 100 (see common color scalebar in the upper right). Thus only the 72

73 (relative) topography of these correlations can be compared to each other (absolute Pearson correlation coefficients are written on top of each plot window and are also plotted in Figure 7.4). 73

74 Figure 7.4. Correlation between Rocknest deposit soils and other surface materials (or stratigraphic units) in terms of Pearson correlation coefficients. These coefficients refer to the chemical data space of K 2 O/Al 2 O 3 versus TiO 2 /Al 2 O 3 (see Figures ) and depend somewhat on the mesh parameters (a parameter set of (30, 30) was used in Figure 7.3). As expected Rocknest deposit soils correlate best with other soils nearby. The contributions by Bathurst and conglomerates is smallest (correlation coefficients around zero), although statistics are poor in the case of Bathurst due to the low number of data points (see Figure 7.2). The contribution by Rocknest rocks is very small despite their close vicinity to the Rocknest deposit. The correlation coefficient for Jake_M is somewhat uncertain, but appears to be at least as large as the one for Rocknest rocks. This suggests that Jake_M is a common rock type in the area explored by the rover over the first 100 sols. Overall, the relative magnitude of these correlations likely reflects the relative abundance and resistance to erosion of surrounding surface materials. 74

75 Figure 8. Diagram of the internal structure of the Rocknest deposit as inferred from ChemCam and imaging data. The diagram is not to scale. The bonding agent near the top surface is inferred to be rich in iron. The bright layer further down is likely more oxidized (i.e. a zone with higher ratio Fe(III)/Fe(total)) than material above and below and may record a depositional event. 75

76 Figure 9. Induration of uppermost soil layers at the Rocknest deposit, Gale crater, and at Serpent ripple and the El Dorado dune field, Gusev crater. All images are within 10% at the same scale (see scale bar). (a)-(c) show evidence for induration immediately below the top surface of troughs #1, #3, and #4 (compare Figures ). In particular, compare the Mastcam image in (a) to the MAHLI image in Figure 2.1 that was acquired 5 sols later. Dotted arrows highlight sharp promontories and reentrants attesting to crust formation. (d) and (e) Similar observations on soil profiles, exposed by MER wheel scuffs at, respectively, Serpent ripple and Gallant_Knight, El Dorado in Gusev crater. The thick solid arrows (highlighted in yellow) point to locations where high-resolution MI images were acquired (see SOM Figure S7). These images show that Serpent ripple is armored by coarse sand (millimeter-sized grains like Rocknest) while the armor of El Dorado ripples is made up of medium to fine sand ( μm). Despite different resolution (Curiosity-Mastcam/MAHLI, a-c, and MER-Pancam, d-e) these images show similar features of crust formation under different types of sand armors and suggest that top surface soil is weakly indurated on a global scale, provided that this soil has been immobilized by a protective top surface armor of sand grains. Image ID: (a) 0061MR E1, (b) and (c) 0084MH E1, (d) A072, P2352, (e) A711, P2536. Pancam approximate true-color images were obtained from and stretched for the purpose of this figure. 76

77 Internal Structure and Evidence for Diagenesis of the Rocknest Aeolian Deposit, Gale Crater, Mars W. Goetz et al. (SOM, Supplemental Online Material) Figure S.1 shows scoop troughs and walls made during the Phoenix Mars mission (May-Oct. 2008, ~69 N). No subsurface layering was seen in the dry top surface (less than 20 cm thick) overlying the ice table. Some subsurface features have different appearance in different scoop troughs and are interpreted as scooping artifacts. None of these features are similar to those at the Rocknest deposit, Gale crater. Figure S1 Trench walls made by the Phoenix robotic arm and scoop (for comparison to Curiosity) (a) Sol 11, Dodo Goldilocks trench. White solid circle highlights two identical spots in (a) and (b). The white arrow in the left part of the image marks the transition to a "brighter" subsurface area. Such a "bright band", however, was only found occasionally, and is here the result of soil compaction by the scoop. This interpretation is suggested by the 1

78 sharp rectilinear transition from the upper to the lower (brighter) area. The scoop wall in the right part of the image does also have a lower (brighter) area whose albedo is caused by soil compaction and generation of corresponding void space (white arrow). This feature expresses inefficient induration of very fine-grained particulate material. (b) Sol 18, same trench as in (a), but extended as a result of further scooping. The very same bright area (as in a) can be seen in the bottom part of the trench. The top part shows the high-fidelity cast of the scoop's backside. Dotted arrows highlight both the brightness lineation as well as the latter casts. (c) Sol 29, Wonderland trench. The voids in the trench wall suggest slightly coarser-grained material (than in a) and/or some degree of soil induration. (d) Sol 118, Upper Cupboard trench. Brighter areas (marked by solid arrows) may well correspond to true albedo variations. The dotted arrow marks again the sharp lineation caused by the scoop. (e) Sol 128, La Mancha trench. Bright areas (white arrows) express the local scoop force on the soil material and are therefore dipping with respect to the top surface. All images were acquired in the afternoon (note the shadows), except (c) that was acquired late in the morning. Scale: A single (one-scoop wide) trench is 8 cm wide. Images (a-e) are approximately, but not exactly at the same scale. Image ID: All images have been stretched to emphasize various scooping features and are thus in false color. (a) SS011IOF _11C20R2CBA18TB, (b) SS018IOF _125C0R2CBA18TB, (c) SS029IOF _13540R2CBA18TB, (d) SS118IOF _1DB10R2CBA18TB, (e) SS128IOF _1EBD0R2CBA18TB. Images: NASA, U of A, Texas A&M Univ.. 2

79 Figures S plot PLS1 oxide abundances of the Rocknest deposit for major elements (Na, K, Si, Al, Ca, Mg, Fe, Ti, all in wt%) and for hydrogen (in arbitrary units) and Li (in ppm) versus spot number. Some of these figures are also shown in the main text. Arrows highlight particularly low or high abundances. Some specific coarse grains (such as Kn,8; Kn,5; E3,3; E,5) that are outliers in terms of chemical composition are also highlighted. Finally, special attention is given to E3,10 and E3,11 that are located at the transition from the fine-grained darker subsurface to the coarse-grained top surface. Images suggest significant chemical bonding (induration) in this domain. Figure S2.1 Na 2O versus spot number. E3,3 and E5 are low in Na as well as the transition domain (E3,10 and E3,11) where chemical bonding is expected. 3

80 Figure S2.2 K 2O versus spot number. Figure S2.3 SiO 2 versus spot number. 4

81 Figure S2.4 Al 2O 3 versus spot number. Figure S2.5 CaO versus spot number. 5

82 Figure S2.6 MgO versus spot number. Figure S2.7 FeOT versus spot number. 6

83 Figure S2.8 TiO 2 versus spot number. 7

84 Figure S2.9 H 2O versus spot number. Data plotted here (in arbitrary units) are the same as those presented by Meslin et al. (2013). Leshin et al. (2013) reported wt% H 2O (based on SAM data). 8

85 Figure S2.10 Li abundance (ppm) versus spot number. The abundances (inferred by univariate analysis, Fabre et al., 2013) scatter considerably: (15.9 +/- 10.5) ppm (average +/- std. dev.) for all 1500 ChemCam data plotted here. Multivariate analyses yield Li abundances of only a few ppm, but they have very large uncertainty (Ollila et al., 2013). However, the identification of Li in Rocknest deposit soils is solid as the Li emission line (λ~671 nm) is well identified in most ChemCam spectra. More significant levels of Li (up to 80 ppm) were found in surrounding float rocks and outcropping bedrock (Ollila et al., 2013). 9

86 Figures S show PLS 1 oxide abundances inferred from all (1860) ChemCam shots, color-coded according to the ChemCam target (see Table 1 in the main text). The oxides (K 2O, Na 2O, Na 2O+K 2O, CaO, Al 2O 3, MgO, FeO T, TiO 2) are plotted against SiO 2. Figure S3.8 plots TiO 2 against FeO T. In all plots the APXS data for Portage (wheel scuff area of Rocknest, Blake et al., 2013; Meslin et al., 2013) as well as data for global dust (Morris et al., 2006, referred to as M2006 in the legend) and average Martian soil (Taylor and McLennan, 2009) are overplotted by large plot symbols (diamonds). Figure S3.1 Na 2O plotted against SiO 2. The correlation between Na 2O and SiO 2 is very weak (the very large majority of data points do not lie on a straight line). Specific outliers are emphasized (see Table 1 for target labels). 10

87 Figure S3.2 K 2O plotted against SiO 2. Particles with more than 40 wt% SiO 2 do contain some potassium. The linear behavior suggests presence of alkali feldspars, although the fine-grained fraction does only contain very minor amounts of alkali feldspar (1.4 wt% sanidine [referring to the crystalline part of the sample], Blake et al. (2013)). Thus most potassium must be either in large solid grains or in the amorphous phase (Morris et al., 2013). 11

88 Figure S3.3 Al 2O 3 plotted against SiO 2. The overall linearity suggests abundant tectosilicates. These are dominantly plagioclases (41 wt% plagioclase of intermediate composition [referring to the crystalline part of the sample], Blake et al. (2013)). 12

89 Figure S3.4 CaO plotted against SiO 2. Visual inspection suggests a major anticorrelation between CaO and SiO 2. However, removing the outliers E,5, E3,3, Kn,8, and E2,1 (only 120 out of 1860 data points) degrades the anticorrelation significantly. The very large majority of points are not on a straight line. 13

90 Figure S3.5 MgO plotted against SiO2. 14

91 Figure S3.6 FeO T (total iron oxide) plotted against SiO 2. Note that most ChemCam data points plot at significantly lower FeO T abundance than either APXS (Portage) or global average soil (Taylor & McLennan, 2009). 15

92 Figure S3.7 TiO 2 plotted against SiO 2. 16

93 Figure S3.8 TiO 2 plotted against FeO T. 17

94 Figure S4 RMI image after ChemCam Portage experiment (CR0_ PRC_F CCAM02089L1). The numbers specify the sequence of ChemCam spots that are very close to the area investigated by APXS (note part of the circular imprint of the APXS contact plate in the lower left quadrant of the image). 18

95 19

96 Figure S5.1 Undisturbed top soil of the Rocknest deposit. The inset (top) marks the approximate position (black rectangle), where the shown high-resolution image was taken. The top-most (few millimeters thick) domain of the deposit appears strongly indurated. Top and bottom images are identical. Cracks in the indurated zone are highlighted by yellow lines (bottom). The bottom and top images are also provided as full-resolution images (in PNG format). For eye inspection of the cracks, toggle back and forth between both images in full-screen mode. A modified version of this figure was published in Meslin et al. (2013, SOM). Image ID: 058MH C0 (sol 58, cropped). Inset on top: NLA_ EDR_F NCAM00441M1 (sol 55). 20

97 Figure S5.2 Chemical paths in FeO T-TiO 2 space (zooms of Figure 6.1 in the main text) at six spots with (potential) bonding agent. E3,10 (a) and E3,11 (b) are locations where a bonding agent is expected based on imagery (see main text, Figures ). E,2 (c) and E2,4 (d) are the best examples for a (somewhat scattered) trend to lower titanium and higher iron. The path of E2,3 (e) is slightly different, but entirely contained in the area of interest (high Fe & low Ti). E2,8 (f) has a very long path in the diagram (both iron and titanium vary by a factor of 2) with mid-range shots (#6-19) located in the area of interest. Compositions at all 6 spots trend -at least partly- to high Fe and low Ti. Shots #1-5 (probing dust) are plotted (and labeled), but are not taken into account for linear regression (straight lines also plotted, line styles as in Figure 6.1b in the main text). Cemented/bonded soils can possibly mimic "coarse grains" 21

98 from a ChemCam point of view. In any case, it seems reasonable that ChemCam shots on bonded coarse grains inherit their chemical signature partly from the grain's bulk and partly from the bonding agent (depending on shot #). 22

99 Figure S6.1 Targets in the vicinity of Rocknest deposit (overview). 23

100 Figure S6.2 Targets in the vicinity of Rocknest deposit (soils and conglomerates). Plot symbols are the same as in Figure S

101 Figure S6.3 Targets in the vicinity of Rocknest deposit (floats, Jake_M, Bathurst_Inlet, and Rocknest rocks). Plot symbols are the same as in Figure S

102 Figure S7 Pancam (a, c) and corresponding MI images (b, d, see yellow highlighted arrows) of soil profiles exposed by MER wheel scuffs in Gusev crater. Serpent ripple and Gallant_Knight/El Dorado is armored, respectively, by a monolayer of millimeter-sized grains and a few layers of μm large grains (concerning the difference between coarse-grained and ordinary ripples, see Sullivan et al., 2008; see also Figure 13 in Goetz et al., 2010, for plots of size distributions). Pancam images (a) and (c) are about 50 cm wide. MI images (b) and (d) are about 31 mm wide (see scale bars). This figure supports the discussion of indurated soils outside Gale crater (compare to Figure 9 in the main text). 26