Hydrated minerals on Endeavour Crater's rim and interior, and surrounding plains: New insights from CRISM data

Authors


Corresponding author: E. Z. Noe Dobrea, Planetary Science Institute, 1700 E. Ft. Lowell, Ste. 106, Tucson, AZ 85719, USA. (eldar@psi.edu)

Abstract

[1] We have conducted a spectroscopic analysis of the rim and interior of Endeavour Crater using CRISM data in order to further constrain the mineralogical variability in the area and to identify targets of interest for in-situanalysis by Opportunity. Our analysis reveals that the spectral character of both the sulfates and the phyllosilicates in the area is more diverse than has been reported to date, with phyllosilicates present on the rim and interior of Endeavour crater as well as on the surrounding plains. Spectra of the sulfates adjacent to the rim and in the crater's interior mound exhibit features that are consistent with a component of Ca-sulfates. The spectral character of the phyllosilicates is consistent with that of Fe/Mg smectites, but there are clear spectral differences between the rim and interior phyllosilicates. Specifically, the phyllosilicates found inside the crater exhibit shallower, more rounded Fe/Mg-OH bands, a subtle 1.9μm hydration band, and a strong 1–2 μm spectral slope relative to the phyllosilicates on the rim, suggesting that they have experienced modification relative to the rim phyllosilicates. This modification may be attributed either to alteration via acidic leaching or to dehydration. Stratigraphically, we find that these altered phyllosilicates unconformably overlie the sulfate-bearing mound material, suggesting that they were emplaced by the reworking of rim phyllosilicates after the interior mound had reached its present day form.

1. Introduction

[2] The Mars Exploration Rover (MER) Opportunity reached the Cape York rim segment of Endeavour crater on August 9, 2011. Endeavour is a middle to late Noachian-aged, 20-km crater that has been largely infilled and buried by sulfate-bearing sedimentary deposits that unconformable mantle Meridiani Planum [Arvidson et al., 2006]. Its rim rises almost 100 meters above the surrounding plains, and contains clay-bearing materials that form part of the early Noachian highlands crust. Previously,Wray et al. [2009] identified Fe/Mg smectites in the rim material of Endeavour on the basis of absorption bands centered at approximately 1.9, 2.3, and 2.4 μm. In preparation for the arrival of Opportunity at these clay-bearing terrains, we re-analyzed the CRISM data with the intent of further constraining the mineralogy and hydration state of these minerals and to establish targets of interest to sample with Opportunity. Our analysis of the data reveals additional spectral variability and more complex history of clay-bearing material, and helps to further describe the geological history of materials measured within-situ observations by Opportunity.

[3] Past in-situ and remote sensing analyses of the Meridiani Planum plains unit measured by Opportunity have shown it to be sedimentary rock containing a combination of silicates, iron oxides, and sulfates. Of these, sulfates comprise ∼30–40% of the weight of the rock [e.g., Clark et al., 2005]. Mini-TES measurements suggest that the outcrops are composed primarily of a combination of one or more amorphous silica/glass/phyllosilicate phases, plus Mg-, Ca-, and Fe-bearing sulfates, primary silicates, and some Fe-oxides [Glotch et al., 2006]. Near-infrared orbital studies using OMEGA [Bibring et al., 2004] and CRISM [Murchie et al., 2007] have shown evidence for monohydrated sulfates in the freshest ten-meter scale areas of the walls of Victoria Crater and the rim of Santa Maria Crater [Arvidson et al., 2011], but not in older and flatter areas with more soil cover. Here we use CRISM data to show that the plains are also demonstrably hydrated and clay-bearing, and that multiple varieties and hydration states of sulfates and clays exist at Endeavour crater.

2. Methods

[4] Atmospheric correction of CRISM data was performed using the currently available version of the volcano-scan technique [e.g.,McGuire et al., 2009] as well as an alternate atmospheric-correction technique, described inNoe Dobrea et al. [2011]. This latter technique takes advantage of the sequence of high angle observations obtained by CRISM as the spacecraft approaches and then recedes from the target. The observations have emission angles between 20 and 70° and consequently observe the region of interest through multiple atmospheric path-lengths, which allows us to recover the shape of the atmospheric opacity spectrum for the region of interest at the time of the observation with no a-priori assumptions about the column abundances of water or ice relative to that of CO2. This technique is particularly useful in overcoming the systematic noise generated in the 2 μm region by the currently available volcano-scan technique. In particular, the volcano-scan technique impacts the spectral character from about 1.9–2.1μm, and generates systematic noise that interferes with the important 1.9 μm water combination band. The new technique only impacts the 1.98–2.08 μm region and hence does not mask out the 1.9 μm water combination band. The strength of this technique is that the presence or absence of the 1.9 μm hydration band can be identified in the spectrum without the need for spectral ratios, and hence it allows us to identify areas with no 1.9 μm absorption to use as denominators (spectral ratios are still useful in identifying dominant spectral features of isolated regions). Comparison of ratio spectra derived after VS-correction with ratio spectra derived after our correction results in the same ratio spectrum. However, if we had used the VS technique alone, we would not have been able to identify the denominator locations with no 1.9μm band with any fidelity. Figure 1 shows a comparison with the volcano scan atmospheric correction that was employed here over anhydrous terrains on the rim of Endeavour Crater, and demonstrates the efficacy of this new correction technique at identifying featureless spectra.

Figure 1.

Uncorrected CRISM FRT00008541 spectrum of anhydrous portion of Endeavour rim (red spectrum) compared to spectra processed using the volcano-scan method (green) and the Emission Phase Function (EPF) method (blue). The depth of the residual band at 1.9–2.1μm seen in the volcano-scan correction is sensitive to the opacity spectrum derived from the Olympus Mons observations.

3. Results

[5] Our analysis of CRISM data covering the plains around Endeavour crater as well as the rim and interior of the crater reveals additional insight into the processes that have occurred in the region.

3.1. Hydration

[6] Figure 2maps the strength of the 1.9-μm absorption due to molecular water in hydrated minerals (using parameter BD1900R in the CRISM Analysis Tool; BD1900H in Ehlmann et al. [2009]). We find that despite the presence of phyllosilicates known to exist in outcrops on the rim, the optical surface of the rim of Endeavour Crater contains, for the most part, the most anhydrous terrain in the scene (Figure 2b). Analysis of the EPF-corrected spectra from terrains exhibiting the lowest values of the BD1900R parameter show that these terrains do not exhibit a 1.9μm feature (Figure 1), or any narrow absorptions in the 1–2.6 μm range for that matter. The spectra do show a broad 2 μm band that is probably due to crystal field transitions in pyroxene, suggesting that the rim may contain unaltered mafic mineralogy. This is also true of the spectra of the dark barchanoid dunes present inside the crater. These are some of the few locations in CRISM data from Meridiani where that is the case, and we use spectra from these relatively anhydrous materials as denominators in much of the remainder of the study.

Figure 2.

(a) RGB composite of CRISM FRT00008541 and FRT0000CE1D showing approximate locations of spectra shown in Figure 3. R = 2.51, G = 1.51, and B = 1.08 μm. Image centered at approximately 5.30°W, 2.34°S. (b) BD1900R parameter map of the same area showing spatial variation in relative hydration. Location “D” refers to sample denominator spectrum shown in Figure 1. Dynamic range of image is approximately 0 < BD1900R < 0.01. North is up in all images. Close-up of unit 2 on these maps is shown in Figure S1.

3.2. Sulfates

[7] Wray et al. [2009] showed that the terrains around the western rim of Endeavour Crater contain iron and/or magnesium polyhydrated sulfates on the basis of a sharp absorption at 1.94 and a broader absorption at 2.4 μm observed in continuum-corrected spectra. Our analysis identifies the same sulfates when averaging large swaths of the plains material adjacent to the rim. However, more focused analyses using only 9 to 25 pixel averages find that some of the terrains adjacent to the rim also present a broad, shallow band centered around ∼2.21μm, which is consistent with either Ca-sulfates or hydrated silica (Figures 3b and Figure S2a in the auxiliary material). The 1.9 μm band has a minimum at 1.93 μm, which is consistent with that of gypsum (minimum at 1.94 μm), given the uncertainty in the data. Shortward of 2 μm, the relative reflectance of the CRISM spectrum drops rapidly, which results in a loss of spectral contrast. This sharp loss in reflectance is interpreted to be caused by Fe-bearing minerals in the rocks and/or coatings, which can dominate spectra shortward of ∼2μm, but become more transparent at longer wavelengths. Such a loss of reflectance may explain the absence or weakness of the ∼1.76 μm and 1.43 μm bands typically present in laboratory Ca-sulfate spectra.

Figure 3.

(a) CRISM ratio spectrum (black) exhibiting shallow absorptions at 1.93, 2.30, and 2.40 μm compared to laboratory spectra of Fe and Mg smectites. CRISM spectrum extracted from location 1 in Figure 2; FRT00008541 1000 pixel average. Error bars indicate the 3σchannel-to-channel variability. Laboratory spectra are from CRISM spectral library (nontronite: NCJB26; saponite; LASA58; illite: LAIL01). Continuum-corrected plot is shown inFigure S2a. (b) Comparison of CRISM ratio spectrum (green) bearing a deep absorption at 1.94 μm and a shallow absorption around 2.2 μm to the spectra of various sulfates and hydrated silica. CRISM spectrum extracted from location 3 in Figure 2; FRT0008541 541 pixel average. Continuum-corrected plot is shown inFigure S2b. (c) CRISM spectra of surface units exhibiting absorptions at 1.93, 2.3, and 2.4 μm. The spectral character of these absorptions varies with location, from the most well-defined on the rim to the shallowest and most rounded on the plains. Numbers in bracket at right correspond to locations shown inFigure 2.

3.3. Phyllosilicates

[8] Our analysis of CRISM data from the rim, interior, and plains around Endeavour crater identifies terrains exhibiting absorptions at 1.93, 2.30, and approximately 2.4 μm (Figure 3c). We find that the spectral character of the terrains associated with these absorptions varies with location. Particularly surprising is the identification of these absorptions in the spectra of the plains outside Endeavour Crater. An average of spectra from 1000 pixels of the plains material (1 in Figure 2a) ratioed to spectra (taken from the same image columns) of anhydrous rim material exhibits very weak absorption features at 1.93, 2.30, and 2.40, suggesting the presence of Fe/Mg smectites in the plains material (Figure 3a). Unfortunately, the large number of pixels needed in order to identify these absorptions at 5σabove the channel-to-channel variation precludes spatial mapping of the strength of these bands on a local scale. Since most of the plains units do not contain any anhydrous units such as those found in the rim and dunes at Endeavour, it is not possible to repeat this experiment elsewhere in the region to determine the spatial variability of the phyllosilicates. However, data from MER show no significant compositional variability between points measured on the plains throughout its traverse from Eagle crater, suggesting that their distribution is also homogeneous throughout the plains units.

[9] In addition to these absorptions, we also note variations in the spectral slope between 1 and 2 μm, in the band-depth of the 1.9μm feature, and in the depth, sharpness, and band centers of the 2.3 and 2.4 μm bands. In particular, the smectite spectra from the rim of Endeavour crater have a relatively shallow spectral slope in the 1–2 μm region, an easily discernable band at 1.93 μm, a sharp absorption at 2.30 and a shoulder at about 2.39–2.40 μm. In contrast, the smectite spectra from the crater's interior and the surrounding plains units exhibit a steeper slope in the 1–2 μm region, a shallower 1.9 μm band, a shallower and more rounded 2.3 μm band, and a displacement of the 2.4 μm shoulder to 2.43 μm. The 1–2 μm slope is often seen in the spectra of smectites on Mars and has been attributed to Fe2+ in phyllosilicates or an associated phase [e.g., Bishop et al., 2008]. The 1.9 μm band in the spectra of the crater interior smectites only appears as a shoulder superposed on the steep 1–2 μm spectral slope, and its relatively small band-depth may be caused by either relatively lower quantities of H2O in the rock, or by loss of spectral contrast as a consequence of overlap with the strong band that generates the 1–2 μm slope. The loss of spectral contrast and sharpness of the 2.3 μm band may be attributable to loss of crystallinity or lower abundance. However, lower abundances cannot explain the shift of the 2.4 μm feature towards 2.43 μm. It also is possible that the 2.43 μm shoulder is not caused by a shift, but instead it is caused by mixing with some other mineral phase that exhibits an absorption at 2.43 μm. Minerals exhibiting absorption at 2.43 μm include some sulfates, zeolites, and borates.

3.4. Stratigraphy

[10] We have mapped the distribution of clays and sulfates in FRT00008541 using the D2300 parameter for clays [Pelkey et al., 2007] and the BD1900R parameter as a proxy for sulfates. These maps have been verified by direct analysis of spectra extracted from each unit. We find that units exhibiting high values of both D2300 and BD1900R are found to contain predominantly phyllosilicate signatures, whereas units exhibiting only high BD1900R values contain spectra consistent with hydrated sulfates.

[11] Overlying the CRISM spectral parameter maps on HiRISE images allows us to determine the geological unit associated with each spectral type. We find that the sulfates are associated with two types of units in the area: 1) light-toned outcrops occurring around the rim and within Endeavour Crater, and 2) a darker, “etched” unit that appears laterally continuous to the light-toned unit. The light toned-unit is often found as a bench of material occurring not only around the rim of Endeavour, but anywhere that overlying dark mantling material has been removed. Removal of the bench unit typically, but not always, results in the exposure of the darker etched unit. The 2.2μm feature occurs in both of these units.

[12] Clays are found in three different types of geologic units: 1) the plains unit, 2) the rim of Endeavour Crater, and 3) the interior of Endeavour Crater. The more crystalline smectites (spectra 6 and 7 in Figure 3c), described in Wray et al. [2009], are found only on the crater's rim, and are typically associated with polygonally-fractured surfaces and also with some talus material. On the other hand, the less crystalline clays (spectra 4 and 5 inFigure 3c) are found inside the crater and in the Meridiani Plains. These clays occur in association with dark mantling material that overlies lighter-toned sulfate-bearing material in the Endeavour crater interior (Figure 4b) and which is in turn overlain by darker barchanoid dunes. Since the material occurs in areas of positive relief with superposed small craters, it is consistent with an in-place layer and not a wind-blown transient deposit, although its thermal inertia is lower than the underlying sulfate-bearing deposits [Chojnacki et al., 2010].

Figure 4.

(a) Spectral parameter map of the interior of Endeavour Crater overlaid on HiRISE image PSP_007849_1775 showing Fe/Mg phyllosilicates (as parameter D2300 of Pelkey et al. [2007]) in green and sulfates (parameter BD1900R of Ehlmann et al. [2009]) in blue. The green unit at the image center is the provenance of spectrum 5 in (Figure 3d). (b) Same as Figure 4a, with the overlay removed for clarity. The phyllosilicates occur in association with a dark mantling unit that overlies the sulfate-bearing units and is in turn overlain by darker barchanoid dunes. It can have either a smooth or ridged texture.

4. Discussion

[13] Perhaps one of the more provocative results of this investigation is the evidence for Fe/Mg clays in the plains unit, either as part of the bedrock or of the dark mantling sands. This result supports the suggestion by Glotch et al. [2006] that nontronite is required to best fit the MiniTES data from the Meridiani plains outcrops, and is also consistent with Mössbauer Spectrometer data indicating an unknown ferric phase (“Fe3D3”) in those same outcrops. This unknown ferric phase is most commonly assumed to be a ferric oxide/hydroxide/oxysulfate, but is alternatively consistent with Fe3+in Fe-smectites [Klingelhöfer et al., 2004; Morris et al., 2006]. Alternatively, these phyllosilicates could explain the high-silica phase proposed byRogers and Aharonson [2008] to occur in the Meridiani sands.

[14] The identification of phyllosilicates in the interior of Endeavour is also important because these phyllosilicates form part of the dark mantling deposit that overlies the sulfates, and do not form part of the original bedrock into which the impact occurred. This is one of only a few examples identified on Mars to date [e.g., Wray et al., 2010] where smectite clays clearly overlie sulfates. The unit in which these phyllosilicates occur appears to unconformably mantle the crater's interior mound, suggesting that it was deposited after the mound reached its present-day state. Therefore, we consider it to be reworked material, possibly derived from the crater rim. Smectite clays alone occur in small, clay/dust-sized particles that can be entrained in suspension. Since the Endeavour interior clays are localized to the dark mantling deposit, we infer that the phyllosilicates form part of more resistant silt- or sand-sized grains, perhaps as part of an alteration rind of these grains. A reasonable speculation would therefore be that these phyllosilicates represent a light alteration on the grain surfaces of otherwise unaltered materials, such as lightly altered basaltic sand.

[15] Spectroscopically, the shape of the 2.3 and 2.4 μm absorptions, the apparent shift of the 2.38 μm band to 2.43 μm, and the strong Fe2+ slope in the 1–2 μm region of the Endeavour interior spectra are intriguing, because they suggest that either these phyllosilicates may have experienced some type of alteration that affected their structure or that they are mixed with some other phase or phases.

[16] The structure of phyllosilicates can be affected by both by exposure to acids and by dehydration. Altheide et al. [2010]showed that Fe smectites are particularly susceptible to acidic conditions. Exposure at pH 3 and below results in the leaching of Fe and Mg from the clay, the formation of amorphous silica, and the loss of contrast and sharpness of absorption features associated to metal-OH combination bands. The spectroscopic identification of Fe/Mg clays, which are easily altered by acidic conditions, suggests that the smectite-bearing rocks on Endeavour's rim were formed in alkaline conditions and that they were not subsequently exposed to the strong acids inferred to have been prevalent during the formation of the Meridiani plains units [e.g.,Squyres et al., 2004]. On the other hand, the clays identified inside the crater do exhibit a loss of sharpness and spectral contrast in the metal-OH bands, as well as the development of a strong spectral slope in the 1–2μm region. Interaction with acids could potentially cause the loss of crystallinity, which would result in the loss of spectral contrast and sharpness of the metal-OH combination bands. However, the development of the red slope in the 1–2μm region is not explained by this process. Alternately, desiccation experiments by Morris et al. [2011] show a loss of spectral contrast in the 1.4, 1.9, 2.3, and 2.4 μm bands as well as the formation of a 1–2 μm red slope in the spectra of Fe-smectites as absorbed water is lost to dehydration and the interlayer is collapsed. These changes are consistent with some of the spectral character observed in the Endeavour interior clays although they also do not explain the apparent shift in wavelength of the 2.4μm shoulder, which suggests that the 2.43 μm shoulder may in fact be due to another mineral phase mixed with the smectite. Unfortunately, multiple sulfates, zeolites, and borates exhibit a similar 2.43 μm absorption, so this alone cannot be used to uniquely constrain the putative additional phase.

[17] Finally, we find that the boundary between the sulfate-bearing plains and the smectite-bearing rim is of particular interest because of its apparent enhancement in Ca-sulfates relative to the plains materials. Modeling byAndrews-Hanna and Lewis [2011] suggests that Meridiani may have experienced significant groundwater upwelling. Vaniman et al. [2011] and Wilson and Bish [2011]found that the interaction of Ca-bearing nontronite with Mg-sulfate brines will produce a more Ca-enriched brine and a more Mg-rich nontronite. The enhancement of Ca-sulfates near the rim could therefore indicate precipitation of Ca-sulfates resulting from the interaction of Mg-brines with the Noachian nontronites, consistent with Opportunity's recent observation of gypsum-rich veins bordering Endeavour's rim [Squyres et al., 2012]. The observations presented here are intriguing because they identify potential sources (the rim) and sinks (crater interior) of phyllosilicates in the area, as well as the mineralogical products derived from the potential modification and transport of these phyllosilicates into the crater's interior. However, the formative process of the phyllosilicates, which could be diagenetic, sedimentary, or hydrothermal, is not know. This question will hopefully be clarified via in-situ studies by Opportunity.

Acknowledgments

[18] The Editor thanks John Mustard and William H. Farrand for their assistance in evaluating this paper.

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