mopdf.com

Journal of Geophysical Research: Planets10.1002/2017JE005474Reconnaissance Imaging Spectrometer for Mars (CRISM) instrument on board the Mars ReconnaissanceOrbiter indicated the presence of ferric smectite in this region (Arvidson et al., 2014). The investigationincluded a looping reconnaissance traverse from the eastern margin of Cape York, up to and along the ridgecrest, back down to the eastern margin, followed by intensive study of selected regions identified as being ofespecial geological interest.Subsequent to the exploration of Cape York, Opportunity was commanded to drive south to the next rim segment, Cape Tribulation. Along the way, cursory exploration of two small rim portions named Sutherland Pointand Nobbys Head was done (Figure 1b). Cape Tribulation was reached just east of its northern tip, a regionnamed Solander Point (Figure 1c). Opportunity rounded the northern tip, climbed along Murray Ridge, whichforms the spine of the northern portion of Cape Tribulation, investigated rocks and soils within Cook Haven(Arvidson et al., 2016), and then traversed southward along the western side of Murray Ridge. The latterincluded investigations of the rocks on the outboard bench and up on Murray Ridge. Opportunity also dida reconnaissance investigation of a short, 160 m long SW-NE trending ridge west of the Murray Ridge benchnamed Wdowiak Ridge (Figure 1c). On Sol 3847 (18 November 2014) Opportunity reached the northern endof a large, unnamed ridge and investigated bedrock in the Hueytown fracture zone on the outboard side ofthe ridge (Figure 1c).The rocks discussed here are all outcrop, ejecta-block and float-rock targets analyzed between Sols 2669 and3866 (28 July 2011 through 10 December 2014), from the last plains outcrop prior to reaching Spirit Point,through to the Hueytown fracture zone. Subsequent to our investigations at the Hueytown fracture zone,Opportunity began investigations in Marathon Valley. Rocks from this region are briefly mentioned fortextural comparisons, but they are not a focus of this paper. Soil analyses are not discussed.The instruments of the Athena payload (Squyres et al., 2003) were used to investigate materials along theEndeavour rim: the Alpha Particle X-ray Spectrometer (APXS; Rieder et al., 2003), the Microscopic Imager(MI; Herkenhoff et al., 2003), the Panoramic Camera (Pancam; Bell et al., 2003), and the Rock Abrasion Tool(RAT; Gorevan et al., 2003), all supported by imaging from the engineering cameras—Navigation Camerasand front and rear Hazard Avoidance Cameras (Maki et al., 2003). Prior to arrival at Cape York, theMiniature Thermal Emission Spectrometer (Christensen et al., 2003) had ceased operating. By the timeOpportunity had reached Cape York, the 57Co source of the MIMOS II Mössbauer Spectrometer(Klingelhöfer et al., 2003) had decayed to the point where useful measurements were no longer possible.The major focus of this paper is on the compositional information returned by the APXS and their use indefining alteration processes, but these data are not considered in isolation. We first put our study intogeological context using information derived from orbital and in situ mapping. Pancam and NavigationCamera images are used to interpret outcrop textures and structures, and Pancam spectra are used toconstrain mineralogy. The microtextures of the rocks are interpreted from MI images. The Mars observationsare then compared to a terrestrial analog site, the Ries Crater, and tied into information derived from cratering mechanics studies. Finally, the observations discussed here are developed into a geological and alterationhistory for the region around Endeavour Crater.2. The APXS Data SetThe APXS determines chemical compositions of rocks and soils using X-ray spectroscopy after irradiation withenergetic alpha particles and X-rays. It therefore resembles a combination of the standard laboratory methods of X-ray fluorescence spectrometry and particle-induced X-ray emission spectrometry (Rieder et al.,2003). The typical analysis field of view has a diameter of about 38 mm, with the instrument response beingstrongest in the central region. Concentrations are extracted from the X-ray spectra using the empiricalmethod described in Gellert et al. (2006). The areas of the characteristic peaks of each element are determined with a nonlinear least squares fit algorithm, and the peak areas are then quantified into elemental concentrations using the calibration sample set for MER, composed of about 50 geological reference materialsand additional simple chemical compounds (cf., Gellert et al., 2006; Rieder et al., 2003). For each major andminor element, the typical oxide—Na2O for quantified Na, MgO for Mg, etc.—is assumed. The major elementCl and trace elements Ni, Zn, and Br are treated as elemental in the data reduction. Iron is reported as FeObecause the Fe3 /Fe2 speciation could no longer be determined using the Mössbauer spectrometer. Thesum of all components is normalized to 100% to compensate for a variable standoff distance. In theMITTLEFEHLDT ET AL.1257

Journal of Geophysical Research: Planets10.1002/2017JE005474analysis model, self-absorption is taken into account using the assumption of a homogeneous, glass-like sample. This assumption is probably never correct and is the underlying reason for a lower accuracy compared toanalyses of glass disks in standard X-ray fluorescence spectrometry. The absorption of the emitted X-rays,especially for lower Z elements that come from depths of only a few micrometers, depends on the composition of the host phase. Of necessity, absorption corrections for the APXS data use the averagesample composition.The results are reported with uncertainties for each element that represent 2σ precision errors of thepeak areas (e.g., Gellert et al., 2006; Ming et al., 2008). Precision uncertainties are well suited to judgethe similarity of samples rather than using the larger accuracy errors and can be used to group rocksby their similar compositions. The rocks likely share a similar mineralogy, and therefore, any inaccuratecorrections in the APXS analysis stemming from microscopic heterogeneity would be minimized for theserocks. The validity of using precision error bars for comparing and grouping rocks in classes is justified bythe nearly identical and consistent composition of fine-grained, homogeneous igneous rocks like theAdirondack basalts from Gusev Crater analyzed by sister rover Spirit (Gellert et al., 2006; McSweenet al., 2006).The relatively large accuracy error bars can be explained in part by the very different compositions of possibleminerals. For example, two possible Cl-rich minerals include NaCl and NaClO4, where the difference in oxygencauses differences in the absorption cross sections that are needed for accurate correction. Independentknowledge of the mineralogy and phase distributions within the targets would be required to improve theaccuracy of analyses. Table S1 of the supporting information gives the typical relative accuracy of the measurement, which is repeated from Table 1 by Gellert et al. (2006). These accuracy measures are comparedto the relative precision for the Shoemaker formation target Transvaal. This target has a composition closeto the mean Shoemaker formation breccia, and an integration time close to the median of all Shoemakerformation target integrations. Thus, the precision of this analysis is typical for the APXS measurementsreported here.3. Geological ContextThe oldest geologic structure in the region of Meridiani Planum is an ancient multiring basin that is at least800, and possibly 1,600 km, in diameter (Figure 2a); the lithologic units of Meridiani Planum were depositedon this structure (Newsom et al., 2003). Endeavour Crater was formed in materials of Noachian age. The basalunit in the immediate vicinity of the Meridiani plains is the Early to Middle Noachian highlands subduedcrater unit (Figure 2b) which is interpreted to be composed of a mixture of primary (volcanic and pyroclastic)and secondary (impact breccia, fluvial, and eolian sedimentary) rocks with a crater-density model age of 3.9 Ga (Hynek & Di Achille, 2017). This highlands unit is overlain by several hundreds of meters ofMeridiani etched plains units; the lower two are Middle to late Noachian in age; the topmost unit is LateNoachian/Early Hesperian in age (Figure 2b). The etched units are interpreted to be eolian and/or volcanicdeposits, with a combined crater-density model age also of roughly 3.9 Ga (Hynek & Di Achille, 2017;Hynek & Phillips, 2008). The Burns formation investigated by Opportunity is the uppermost part of the etchedunit stratigraphy. Based on mineralogy, composition, texture, and primary sedimentary features, the Burnsformation is interpreted to be a sulfate-rich eolian sandstone (e.g., Squyres et al., 2006). The region is cappedby the thin, surficial Hematite unit, mapped as Early Hesperian (Hynek & Di Achille, 2017). This is an unconsolidated lag deposit rich in hematitic concretions derived from erosion of the underlying the Burns formation,plus basaltic sands in eolian bedforms (Squyres et al., 2006).Endeavour Crater lies to the northeast of Miyamoto Crater (Figure 2a) (Grant et al., 2016; Newsom et al., 2003),an 160 km diameter impact structure containing Fe-Mg-rich smectite phases on its floor (Wiseman et al.,2008). Formation of the smectites is thought to have been engendered by the hydrological environmentof western Arabia Terra in which groundwaters from the highlands to the south emerged from local topographic lows and promoted in situ alteration of primary or impact-generated rocks (Andrews-Hanna &Lewis, 2011; Andrews-Hanna et al., 2007). The Endeavour impact occurred well within the region wherethe continuous ejecta blanket of Miyamoto Crater would have been, and the preimpact target stratigraphywould have included polymict breccias from that earlier impact. These could have been altered as werethe Miyamoto Crater floor rocks.MITTLEFEHLDT ET AL.1258

Journal of Geophysical Research: Planets10.1002/2017JE005474Figure 2. Portion of the geologic map (a) and cross section (b) of the Meridiani Planum region surrounding the Mars Exploration Rover Opportunity area ofinvestigation (Hynek & Di Achille, 2017), and the schematic stratigraphy of the region explored by Opportunity (c) (modified after Crumpler, Arvidson, Bell, et al.,2015). Unit key only covers those discussed in the text. Cross section vertical exaggeration is 78 . White dotted circle—approximate location of Miyamoto Craterrim; yellow dotted arcs—approximate inner rim crest and first ring of multiring basin that underlies Meridiani Planum (after Newsom et al., 2003).Most of Endeavour Crater and portions of its rim are unconformably buried by the sulfate-rich sandstones ofthe Burns formation (Figure 2c) (Arvidson et al., 2011; Grant et al., 2016; Squyres et al., 2006). Portions of thecrater rim rise above the Burns formation strata, forming a discontinuous ring of rim segments. There is noevidence, such as fragments of Burns rocks or hematitic concretion clusters high on the rim, that the Burnsformation covered these rim segments. Golombek et al. (2006) estimated that 80 m of rock has been erodedin Meridiani Planum since the Hesperian, and Grant et al. (2016) estimated that Burns formation rocks mighthave been 80–100 m higher than at present in the region of Cape Tribulation. These estimates are less thanthe current Cape Tribulation height above the plains. Erosion has variably degraded the crater rims with onthe order of 100–200 m having been removed, mostly before deposition of the Burns formation sands(Crumpler, Arvidson, Bell, et al., 2015; Grant et al., 2016). Some of the rim segments show the infrared spectralsignature of Fe-Mg-smectite clays in data returned by the CRISM instrument on board Mars ReconnaissanceMITTLEFEHLDT ET AL.1259

Journal of Geophysical Research: Planets10.1002/2017JE005474Orbiter (Fox et al., 2016; Noe Dobrea et al., 2012; Wray et al., 2009), suggesting that they have undergone aqueous alteration under conditions of circumneutral pH. A localized area in the region explored by Opportunityduring the sols covered here has yielded detections of phyllosilicates by CRISM (Figure 1c). On the inboardside of Cape York is a small area on a feature dubbed by the team as Matijevic Hill that is thought to containa few weight percent ferric smectites (Arvidson et al., 2014).Burns formation sandstones are dominated by Mg-, Ca-, and Fe-sulfates, a silicic component and ferric oxides(e.g., Clark et al., 2005; Klingelhöfer et al., 2004; McLennan et al., 2005; Morris et al., 2006a). The sandstones aremostly eolian in origin, with some aqueous facies that indicate local fluvial reworking, and a minor component of mudstones indicating localized deposition in quiet water, possibly a lacustrine setting (Edgar et al.,2012, 2014; Grotzinger et al., 2005, 2006; Hayes et al., 2011). The sediments have undergone groundwaterinfluenced cementation and diagenesis and are noteworthy for containing abundant hematitic concretions.They document a period of aqueous activity postdating the formation of Endeavour Crater in which groundwaters interacted with and altered mafic composition rocks (e.g., Hurowitz et al., 2010). The solutions evaporated to form sulfate-rich evaporitic muds, which were subsequently redistributed by wind and water underincreasingly arid conditions to form sandstones. Rocks of the Burns formation are not a focus of this paper,but we do discuss those Burns formation targets from near the margins of the Endeavour rim for comparisonwith rocks on the rim proper (Table S2). These targets are referred to here as “Burns margin.” We include inTable S2 the last Burns formation target analyzed before reaching Cape York, Gibraltar, and two Burns formation targets from the saddle between Cape York and Cape Tribulation, Tawny, and Black Shoulder. These targets are approximately 320, 340, and 190 m from the nearest rim margins and are not included under thesobriquet “Burns margin” in the discussion.The rocks of the Endeavour Crater rim have been divided into three units which are, oldest to youngest; theMatijevic, Shoemaker, and Grasberg formations (Figure 2c) (Crumpler, Arvidson, Bell, et al., 2015). A continuous bench of bright rock surrounding Cape York, Sutherland Point, and Nobbys Head, and partially along themargin of Cape Tribulation, is discernable in High Resolution Imaging Science Experiment images of the western rim of Endeavour Crater (Figures 1b and 1c). This bench is part of the Grasberg formation (Crumpler,Arvidson, Bell, et al., 2015). Benches of bright rock are visible in High Resolution Imaging ScienceExperiment images around other rim segments of Endeavour Crater, and these are interpreted to beGrasberg formation outcrops (Grant et al., 2016). The spine of Cape York is formed by Shoemaker Ridgeand is the type locality for the Shoemaker formation. This name is given to the polymict impact breccias ofbasaltic composition that comprise the major lithology of the Endeavour Crater rim (Squyres et al., 2012).The Matijevic formation, consisting of bright clastic rock of basaltic composition (Arvidson et al., 2014), hasbeen encountered only on the inboard side of Cape York at the base of Matijevic Hill (Figure 1b). MurrayRidge is notable for having localized concentrations of dark-rock float (Figures 3a and 3b), and WdowiakRidge is capped by fine-grained dark rocks (Figure 3c). The former are allochthonous, while the latter cannotbe placed within the local stratigraphic framework. Both are of uncertain provenance.4. Rock Outcrop and Microscopic TexturesTo set the stage for the discussion of unit compositions to follow, we present observations on outcrop morphology, and macroscopic and microscopic textures of the various lithologies on Endeavour Crater rim in thissection. We also discuss constraints on mineralogy derived from Pancam spectra. The order in which the rockunits are discussed mirrors the discussion of the compositions of lithologies in section 5 and is not in stratigraphic sequence. Section 5 is ordered by the specific science issues we wish to explore. Observations forsome of the rock types have been described previously (Arvidson et al., 2014, 2016; Clark et al., 2016;Crumpler, Arvidson, Bell, et al., 2015; Farrand et al., 2013, 2014; Squyres et al., 2012). The outcrop morphologyand textures for the units discussed here are summarized in Table 1. Details for the Pancam images used inthis paper are given in Table S3 of the supporting information.4.1. Grasberg FormationThe Grasberg formation is the oldest of the postimpact formations in the area and occurs as a shallowly tiltedbench on the margins of both rim segments investigated by Opportunity. The description of the formationgiven here is largely derived from Crumpler, Arvidson, Bell, et al. (2015) plus new observations; Crumpler,Arvidson, Bell, et al. (2015) will be cited for specific interpretations but not for basic descriptive information.MITTLEFEHLDT ET AL.1260

Journal of Geophysical Research: Planets10.1002/2017JE005474Figure 3. (a) Portion of the Sols 3387–3389 site 179/position 0 Navigation Cameras mosaic showing dark-rock float on Solander Point; Murray Ridge in the background. Tick Bush is 20 cm across. (b) Portion of the Sol 3609 Panoramic Camera (Pancam) L257 false-color mosaic showing dark-rock float on the McClureBeverlin Escarpment of Murray Ridge. Labeled boulders A and B are 16 and 18 cm across at their bases. (c) Portion of the Sol 3750 L257 Pancam false-color mosaicshowing the dark cap-rock on the northeast tip of Wdowiak Ridge. (The left Pancam filters numbers 2, 5, and 7 are centered on 753, 535, and 432 nm. Unlessotherwise noted, all Pancam false-color images used are based on these filters.)The Grasberg formation consists of an upper bright unit and a lower dark unit with a total formation thicknessestimated as 1–2 m. The rocks are homogeneous and fine grained and are planar in outcrop (Figure 4). TheGrasberg formation presents hackly outcrop surfaces that are fractured into polygonal blocks or slabs(Figures 4a and 4e). Sedimentary structures are lacking in most outcrops, but an exception is the lower unittarget Poverty Bush from Solander Point which shows fine-scale, wavy laminations (Figure 4e, arrows).Outcrops can exhibit fine-scale jointing (Figure 4c). Outcrops of the lower Grasberg unit are commonly transected by bright veins tens of cm in length and of roughly cm-scale width (Figure 4d). Short, bright streaks inthe upper Grasberg unit could represent smaller versions of the coarse veins that are common in the lowerunit (Figure 4a, arrows). The contact between the lower and upper units is defined only by a color transition,and no obvious textural or morphological difference is evident; the upper unit might simply reflect an indurated cap rock formed by weathering (Crumpler, Arvidson, Bell, et al., 2015). Rocks of both units are composed of grains with diameters smaller than the 100 μm (3 pixels) resolution of the MI (Figure 5); clastictextures are generally not observed. If the texture is primary, then the homogeneous, fine-grained naturesuggests deposition occurred in a relatively low-energy environment. Wind-polished surfaces show small pitsthat could belie initial porosity (Figures 5c and 5d), but these are not evident in the interior of the onlyGrasberg formation target that was abraded (Figure 5a). If that upper unit target is representative of the formation, then the Grasberg formation consists of homogeneous fine-grained rock later cut by veins (cf.Crumpler, Arvidson, Bell, et al., 2015).MITTLEFEHLDT ET AL.1261

Journal of Geophysical Research: Planets10.1002/2017JE005474Table 1Summary of Rock Units at Endeavour Crater RimFormationUnitBurnsn/aGrasbergUpperLowerGreeley HavenShoemakerChester LakeCopper CliffTisdaleMurray RidgeHueytownMatijevicDark rocksMatrixSpherule-richVeneerFloatWdowiak RidgeabVolatile/mobile elementccharacteristicMorphology and texture“Silicate” characteristicLaminated to cross-laminated mediumto coarse, well-sorted sand, 1–2 mmPlanar, fractured, homogeneous, 100 μmDevoid of structure, homogeneous, 100 μmBreccia, cm-sized angular/subroundedclasts in fine-grained matrixAs for Greeley Haven; with prominentlineation of clastsAs for Greeley Haven; with1–2 mm spherules,fine, anastomosing bright veinsAs for Chester LakeAs for Greeley HavenAs for Greeley Haven; poorer inclasts, generally smaller sizeTabular, clastic, poorly laminated, 100 μmLinear, fin-like, 2–4 mmmatrix-supported spherulesTabular surface lamination, homogeneousAllochthonous blocks, homogeneous, 100 μmAs for floatLow to very low Al; very high K; abraded only:high P, Fe; very high MgLow Al, Mn; very low Mg; very high K, FeLow Mn; very low Mg; very high K, Fe(average)Very high S; high tovery high ZnVery high Cl, Zn, BrVery high Zn, Br(average)Low Si(average)High Ni(average)Low Ca; very low Mg; high Fe; very high P, Ni(average)(average)Very high Zn, Br(average)Very high SLow K, Ti; very high Si, P, NiVery low P, Ca, Ti; high Ni; very high SiLow SLow SHigh Ni; very high PLow Fe; very low Mg, Cr; very high Al, MnLow Cr; very low Mg; high Na; very high AlVery high Cl, BrLow S, ClLow S, ClabArvidson et al. (2014); Crumpler, Arvidson, Bell, et al. (2015); Edgar et al. (2012); Grotzinger et al. (2005, 2006); Squyres et al. (2012); and this work. Elementsnormalized to be free of volatile/mobile elements (S, Cl, Zn, and Br); compared to an average of Shoemaker formation breccias, excluding Tisdale and anomalousctargets (see text). Volatile/mobile elements compared to an average of Shoemaker formation breccias, excluding Tisdale and anomalous targets.The Grasberg formation is distinct from the Burns formation sandstones in mineralogy and texture. The visible to near infrared (VNIR) reflectance spectra of the upper Grasberg resembles purple-colored Burns formation outcrops that have higher 482 to 535 nm slopes as described by Farrand et al. (2007). However, the upperGrasberg has deeper 535 and 904 nm band depths indicative of higher fractions of crystalline red hematite inthat unit and thus is mineralogically distinct from the Burns formation (Farrand et al., 2014). The very finegrained nature of the Grasberg formation is also distinct from coarser, sand-sized Burns formation sandstones(e.g., Grotzinger et al., 2005, and see Crumpler, Arvidson, Bell, et al., 2015).On Cape York, the Grasberg formation dips 10 away from the rim segment in all directions and is interpreted to lie on an erosional pediment forming the lower slopes of Cape York (Crumpler, Arvidson, Bell,et al., 2015). The geome

Division, NASA/Johnson Space Center, Houston, TX, USA, 4Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA, USA, 5Space Science Institute, Boulder, CO, USA, 6Biological and Environmental Sciences, Faculty of . forms the spine of the northern portion of Cape Tribul