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

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 ronment, with episodes of diagenesis and weathering in late Noachian and Hesperian terrains on Mars lying to form a crystalline hematite lag deposit [4, 5]. The stratigraphically above clay minerals, which has been lag deposit masks the CRISM spectral signature of interpreted as documenting a shift from wetter to more sulfate in most locations. Sulfate minerals including arid environments on the surface. Sulfate detections kieserite and gypsum have been detected in impact are associated with layered deposits in numerous loca- crater walls and windswept regions [6]. The Oppor- tions including Gale Crater, Meridiani Planum, Vallis tunity rover explored southern Meridiani Planum Marineris, and Terra Sirenum, and Aram Chaos [1]. through a campaign of crater-hopping, using craters as These sulfates and clays been identified using their a natural drill to expose strata [6]. The deepest expo- diagnostic absorption features in visible and near- sures explored by Opportunity directly are ~10 meters infrared reflectance (VNIR) data acquired from Mars thick at Victoria Crater. Opportunity results indicate orbit. Additionally, two rover missions have explored that the top layers of Burns formation contain up to sites with massive sulfate deposits. The first, the MER 40% sulfate and included Mg, Ca, and Fe species. This Opportunity rover, landed in Meridiani in 2004 and includes a significant jarosite component, which has traversed across the Burns formation sulfate-bearing not been detected by CRISM. sandstones, until it reached the rim of Endeavour However, investigations of nearby Iazu Crater [7] Crater in 2011. The MSL Curiosity rover, after landing indicate that the complete sulfate-bearing section is in Gale Crater in 2012, is gradually ascending the sed- much thicker only ~20 km south of the rover’s final imentary interior mound Mt. Sharp and approaching location (Figure 2b). CRISM detects polyhydrated Mg- the “layered sulfate” section. sulfate in the walls of Iazu. These are an pre-impact Methods: We identify sulfates using hyperspectral equivalent ~115 m section of relatively light-toned images from the CRISM instrument [2], from 0.4-2.7 deposits that show regular evidence of interspersed µm at 12-36 m/pixel, which have been processed using dark banding, visible at the HiRISE scale but too small the WUSTL pipeline [3]. After applying the volcano for CRISM to resolve (Figure 2a). These sulfate- scan correction for atmospheric gases, we modeled bearing layered deposits overlie dark, more resistant surface scattering and atmospheric aerosols using a basalts with trace evidence of alteration to Fe/Mg Hapke model and the DISORT radiative transfer code smectite [7]. This site allows constraints on the thick- to retrieve single scattering albedo (SSA). These spec- ness of the Burns formation in the vicinity of the Op- tra were then further processed with a log-log maxi- portunity traverse, and provides clues to our interpreta- mum likelihood method to retrieve the best estimate of tion of CRISM signatures of polyhydrated Mg-sulfate SSA signal in the presence of Poisson noise. in the Meridiani region. Polyhydrated sulfates have diagnostic absorption Gale Crater: Mt. Sharp, the ~5 km high interior features at 1.9 and 2.4 µm. In monohydrated sulfates mound in Gale Crater (Figure 2d), has been document- the 1.9 µm absorption is shifted to 2.1 µm. Ca- and Fe- ed to contain clay minerals stratigraphically below hydrated sulfates have additional diagnostic absorp- sulfate minerals [8, 9]. The areas containing sulfate are tions in the VNIR, while Mg- varieties are otherwise bright-toned and sculpted into tall rounded mounds and relatively featureless. What is observed on Mars with buttes by aeolian processes. The primary mineral iden- CRISM are real assemblages of minerals as opposed to tified in the vicinity of the Curiosity traverse is poly- pure phases and therefore absorption depths are greatly hydrated Mg-sulfate, although monohydrated sulfates reduced relatively to laboratory spectra. Relative pro- have been identified in other parts of the mound [8,10]. portions of minerals, textures, grain sizes, and dust The sulfate-bearing section spans >600 meters in ele- cover all affect the signal that reaches CRISM detec- vation with dips typically <5 deg. The exposures in the tors. walls of these features have the strongest spectral sig- Meridiani Planum: Meridiani is a relatively flat natures of sulfates and also contain periodic dark bands plain composed of a Noachian basaltic basement at the HiRISE scale (Figure 2c). Curiosity has not yet topped with hundreds of meters of sulfate-bearing reached the layered sulfate unit at the time of writing. sandstones. These sedimentary layers were formed by Synthesis: The layered sulfate sections in Meridi- deposition and reworking in an episodically wet envi- ani and Gale have similar spectral signatures in the VNIR, although the Gale spectra have consistently Ninth International Conference on Mars 2019 (LPI Contrib. No. 2089) 6316.pdf

higher albedo. In both cases the signature is consistent al. (2016) JGR, 121, 1713-1736. [10] Hughes, M.N. et with polyhydrated Mg sulfate, basaltic minerals and al. (2019), LPSC L, Abstract #3196 [11] Andrews- dust. These detections do not preclude the presence of Hanna, J.C. et al. (2012), LPSC XLIV, Abstract #2706. 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. Figure 1: Comparison of CRISM SSA spectra of the Curiosity will have the opportunity to investigate wall of Iazu Crater, Meridiani Planum (red) and Mt. the full mineral assemblage in the Mt. Sharp layered Sharp, Gale Crater (blue). Absorptions at 1.9 and 2.4 sulfate section and compare it to measurements previ- µm (gray lines), combined with a lack of other diag- ous made of the Burns formation. The mission has the nostic features, identify polyhydrated Mg sulfate. advantage of a more extensive instrument payload than Spectra are 3x3 pixel averages. 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 Figure 2: Examples of layered sulfate-bearing terrains whole. on Mars. a) Layering in the north wall of Iazu Crater, References: [1] Ehlmann, B.L. & Edwards, C.S. overlying darker basalts (HiRISE). b) Endeavour and (2014), Ann. Rev. E&PS, 42, 291-315. [2] Murchie, Iazu Craters in Meridiani Planum (THEMIS VIS). Op- S.L., et al. (2007) JGR, 112, E05S03. [3] Politte, D.V. portunity traverse shown in white. c) Gediz Vallis wall et al. (2019) LPSC L, Abstract #2690. [4] McLennan, in Mt. Sharp (HiRISE). d) Northwest Mount Sharp, S.M. et al. (2005), EPSL, 240, 95-121. [5] Grotzinger, Gale Crater (HiRISE). Curiosity traverse through sol J.P. et al. (2005), EPSL, 240, 11-72. [6] Arvidson, 2316 in shown white. Locations of a) and c) given by R.E.. et al. (2015) JGR, 120, 429-451. [7] Powell, K.E. arrows. Locations of spectra from Figure 1 indicated et al. (2017) JGR, 122, 1138–1156. [8] Milliken, R.E., by an x. et al. (2010), GRL, 37, L04201. [9] Fraeman, A.A., et