LAYERED SULFATE-BEARING TERRAINS ON MARS: INSIGHTS FROM GALE CRATER AND MERIDIANI PLANUM. K.E. Powell1,2, R.E. Arvidson3, and C.S. Edwards1, 1Department of Physics & Astrono-my, Northern Arizona University, 2School of Earth & Space Exploration, Arizona State University, 3Department of Earth & Planetary Sciences, Washington University in St. Louis.

Introduction: Sulfate species have been detected

in late Noachian and Hesperian terrains on Mars lying stratigraphically above clay minerals, which has been interpreted as documenting a shift from wetter to more arid environments on the surface. Sulfate detections are associated with layered deposits in numerous loca-tions including Gale Crater, Meridiani Planum, Vallis Marineris, and Terra Sirenum, and Aram Chaos [1]. These sulfates and clays been identified using their diagnostic absorption features in visible and near-infrared reflectance (VNIR) data acquired from Mars orbit. Additionally, two rover missions have explored sites with massive sulfate deposits. The first, the MER Opportunity rover, landed in Meridiani in 2004 and traversed across the Burns formation sulfate-bearing sandstones, until it reached the rim of Endeavour Crater in 2011. The MSL Curiosity rover, after landing in Gale Crater in 2012, is gradually ascending the sed-imentary interior mound Mt. Sharp and approaching the “layered sulfate” section.

Methods: We identify sulfates using hyperspectral images from the CRISM instrument [2], from 0.4-2.7 µm at 12-36 m/pixel, which have been processed using the WUSTL pipeline [3]. After applying the volcano scan correction for atmospheric gases, we modeled surface scattering and atmospheric aerosols using a Hapke model and the DISORT radiative transfer code to retrieve single scattering albedo (SSA). These spec-tra were then further processed with a log-log maxi-mum likelihood method to retrieve the best estimate of SSA signal in the presence of Poisson noise.

Polyhydrated sulfates have diagnostic absorption features at 1.9 and 2.4 µm. In monohydrated sulfates the 1.9 µm absorption is shifted to 2.1 µm. Ca- and Fe- hydrated sulfates have additional diagnostic absorp-tions in the VNIR, while Mg- varieties are otherwise relatively featureless. What is observed on Mars with CRISM are real assemblages of minerals as opposed to pure phases and therefore absorption depths are greatly reduced relatively to laboratory spectra. Relative pro-portions of minerals, textures, grain sizes, and dust cover all affect the signal that reaches CRISM detec-tors.

Meridiani Planum: Meridiani is a relatively flat plain composed of a Noachian basaltic basement topped with hundreds of meters of sulfate-bearing sandstones. These sedimentary layers were formed by deposition and reworking in an episodically wet envi-

ronment, with episodes of diagenesis and weathering to form a crystalline hematite lag deposit [4, 5]. The lag deposit masks the CRISM spectral signature of sulfate in most locations. Sulfate minerals including kieserite and gypsum have been detected in impact crater walls and windswept regions [6]. The Oppor-tunity rover explored southern Meridiani Planum through a campaign of crater-hopping, using craters as a natural drill to expose strata [6]. The deepest expo-sures explored by Opportunity directly are ~10 meters thick at Victoria Crater. Opportunity results indicate that the top layers of Burns formation contain up to 40% sulfate and included Mg, Ca, and Fe species. This includes a significant jarosite component, which has not been detected by CRISM.

However, investigations of nearby Iazu Crater [7] indicate that the complete sulfate-bearing section is much thicker only ~20 km south of the rover’s final location (Figure 2b). CRISM detects polyhydrated Mg-sulfate in the walls of Iazu. These are an pre-impact equivalent ~115 m section of relatively light-toned deposits that show regular evidence of interspersed dark banding, visible at the HiRISE scale but too small for CRISM to resolve (Figure 2a). These sulfate-bearing layered deposits overlie dark, more resistant basalts with trace evidence of alteration to Fe/Mg smectite [7]. This site allows constraints on the thick-ness of the Burns formation in the vicinity of the Op-portunity traverse, and provides clues to our interpreta-tion of CRISM signatures of polyhydrated Mg-sulfate in the Meridiani region.

Gale Crater: Mt. Sharp, the ~5 km high interior mound in Gale Crater (Figure 2d), has been document-ed to contain clay minerals stratigraphically below sulfate minerals [8, 9]. The areas containing sulfate are bright-toned and sculpted into tall rounded mounds and buttes by aeolian processes. The primary mineral iden-tified in the vicinity of the Curiosity traverse is poly-hydrated Mg-sulfate, although monohydrated sulfates have been identified in other parts of the mound [8,10]. The sulfate-bearing section spans >600 meters in ele-vation with dips typically <5 deg. The exposures in the walls of these features have the strongest spectral sig-natures of sulfates and also contain periodic dark bands at the HiRISE scale (Figure 2c). Curiosity has not yet reached the layered sulfate unit at the time of writing.

Synthesis: The layered sulfate sections in Meridi-ani and Gale have similar spectral signatures in the VNIR, although the Gale spectra have consistently

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higher albedo. In both cases the signature is consistent with polyhydrated Mg sulfate, basaltic minerals and dust. These detections do not preclude the presence of other hydrated phases, including other species of sul-fate. Both locations show layering at the meter to sub-meter scale and alternating dark and bright banding, too fine to be resolved by CRISM.

The Meridiani sulfates formed in a playa environ-ment through groundwater upwelling [5]. The Gale deposits were formed in a closed basin, and it is yet to be determined if the sulfate section was deposited and/or altered by groundwater upwelling [11]. Under-lying the sulfates in Gale are mudstones and sand-stones, which have significant evidence for alteration and diagenesis even in areas without clear hydrated mineral detections from CRISM. In contrast, the Noa-chian basalts at Meridiani have only limited evidence for aqueous alteration.

Curiosity will have the opportunity to investigate the full mineral assemblage in the Mt. Sharp layered sulfate section and compare it to measurements previ-ous made of the Burns formation. The mission has the advantage of a more extensive instrument payload than Opportunity’s, and should be able to access a larger stratigraphic section. It remains to be determined how the mineralogical makeup and geologic expression of the two sites will be similar or different, given the above orbital similarities between them. Careful con-sideration of ground-based measurements at each site can inform the interpretation of sulfate-bearing terrains in other areas on Mars that have not been visited by landed spacecraft. We can then make predictions on what sulfate assemblages we can expect when we de-tect polyhydrated sulfate elsewhere with CRISM. No-tably lacking in both locations are repeated sequences of monohydrated and polyhydrated sulfate, reported in other location on Mars [1].

The exploration of the Mt. Sharp layered sulfate section with Curiosity will shed light on the local vs. global nature of formation, alteration, and diagenesis of layered sulfate units on Mars and the implications for environmental trends. Thus the late stages of the Curiosity mission will provide important perspective for our understanding of Martian geologic history as a whole.

References: [1] Ehlmann, B.L. & Edwards, C.S. (2014), Ann. Rev. E&PS, 42, 291-315. [2] Murchie, S.L., et al. (2007) JGR, 112, E05S03. [3] Politte, D.V. et al. (2019) LPSC L, Abstract #2690. [4] McLennan, S.M. et al. (2005), EPSL, 240, 95-121. [5] Grotzinger, J.P. et al. (2005), EPSL, 240, 11-72. [6] Arvidson, R.E.. et al. (2015) JGR, 120, 429-451. [7] Powell, K.E. et al. (2017) JGR, 122, 1138–1156. [8] Milliken, R.E., et al. (2010), GRL, 37, L04201. [9] Fraeman, A.A., et

al. (2016) JGR, 121, 1713-1736. [10] Hughes, M.N. et al. (2019), LPSC L, Abstract #3196 [11] Andrews-Hanna, J.C. et al. (2012), LPSC XLIV, Abstract #2706.

Figure 1: Comparison of CRISM SSA spectra of the wall of Iazu Crater, Meridiani Planum (red) and Mt. Sharp, Gale Crater (blue). Absorptions at 1.9 and 2.4 µm (gray lines), combined with a lack of other diag-nostic features, identify polyhydrated Mg sulfate. Spectra are 3x3 pixel averages.

Figure 2: Examples of layered sulfate-bearing terrains on Mars. a) Layering in the north wall of Iazu Crater, overlying darker basalts (HiRISE). b) Endeavour and Iazu Craters in Meridiani Planum (THEMIS VIS). Op-portunity traverse shown in white. c) Gediz Vallis wall in Mt. Sharp (HiRISE). d) Northwest Mount Sharp, Gale Crater (HiRISE). Curiosity traverse through sol 2316 in shown white. Locations of a) and c) given by arrows. Locations of spectra from Figure 1 indicated by an x.

6316.pdfNinth International Conference on Mars 2019 (LPI Contrib. No. 2089)