11:15 〜 11:30
[PPS06-08] Characterizing Hydration Carrier Phases in Jezero Crater Fan and Floor Rocks with SHERLOC on Mars 2020
キーワード：Mars, Perseverance rover, Raman spectroscopy, hydrated minerals, sulfate
The Mars 2020 Perseverance rover has explored igneous rocks within Jezero crater in the crater floor Séítah and Máaz formations (up to sol 380) and fluvio-lacustrine sedimentary rocks in the western fan front (sol 414-707) and the upper fan (sol 708-912). Prior work showed that igneous crater floor Séítah and Máaz formations have mafic mineralogy with alteration phases that indicate multiple episodes of aqueous alteration [e.g., 1]. Building upon previous work [2] that studied hydration carriers in the igneous crater floor, in this work [3], we extend the analyses of hydration to targets in the Jezero western fan delta, using the Scanning Habitable Environments with Raman and Luminescence for Organics and Chemicals (SHERLOC) instrument. We analyzed the deep-UV Raman spectra of stratigraphically in-place rock targets measured by SHERLOC to identify the alteration minerals and determined the hydration states by comparison to terrestrial laboratory data. We then considered variability within and between targets, relating to the geologic units, to understand whether/how the fluid environments changed across time and space in the Jezero crater system.
We find that sulfates are a principal hydration carrier phase in all units in the crater floor and western fan, though the cations and/or hydration states of the sulfates vary across the units. Carbonates are commonly found to co-occur with sulfates with more Fe-rich carbonates found in the crater floor and more Mg-rich carbonates in the fan. From comparison of laboratory data to Raman hydration peaks and sulfate peaks, we find no detections of highly hydrated sulfates e.g., epsomite (MgSO4•7H2O). The sulfate symmetric stretch at ~1000 cm-1 coupled with a hydration peak at ~3400 cm-1 indicate that MgSO4•nH2O (2 < n ≦ 5) is a likely hydration carrier phase in all units. Low-hydration MgSO4•nH2O (n = 1-2) occurs less commonly and is more prevalent in the fan than the crater floor. Hydrated Ca-sulfates (hydration peak at >3500 cm-1) matching bassanite (CaSO4•0.5H2O) are found only in the upper fan, while anhydrite (CaSO4) is found in all units. We have not been able to definitively attribute hydration peaks at ~3200 cm-1, but a possibility is low-hydration amorphous Mg-sulfates (n ≦ 1).
At the temperature and relative humidity conditions of SHERLOC measurements (~<3% RH, ~200-220 K), starkeyite (MgSO4•4H2O) and gypsum (CaSO4•2H2O) are expected to be the thermodynamically stable phases [4, 5]. The SHERLOC detections of MgSO4•nH2O (2 < n ≦ 5) imply these Mg-sulfates are in equilibrium with present-day surface environmental conditions. However, the detections of anhydrite and bassanite imply that processes that are not thermodynamically and kinetically favored at the surface now are required for their formations. While the direct precipitation of anhydrite commonly occurs at temperatures > 50°C, highly saline brines also allow anhydrite formation. SHERLOC detections of less soluble anhydrite at the stratigraphically higher fan front and more soluble hydrated Mg-sulfates at the stratigraphically lower crater floor likely indicate that at the time of formation, a saline brine flowing across from the fan to the floor precipitated first anhydrite and then Mg sulfates upon evapoconcentration without temperatures > 50°C. The persistence of anhydrite implies subsequent arid conditions that prohibit its hydration to gypsum. Collectively, the data imply aqueous deposition of sediments by fluids that could have been available intermittently or continuously, formation of salts, and subsequent aridity to preserve the observed hydration states.
Acknowledgements: We thank the SHERLOC and Mars 2020 science and engineering teams for the data that enabled this study.
References: [1] Farley, KA et al. (2022) Science, 377. [2] Siljeström, S et al. (2024) JGR Planets, 129. [3] Phua, Y et al. (in revision) JGR Planets. [4] Chipera, SJ et al. (2023) JGR Planets, 128. [5] Rapin, W et al. (2016) EPSL, 452.
We find that sulfates are a principal hydration carrier phase in all units in the crater floor and western fan, though the cations and/or hydration states of the sulfates vary across the units. Carbonates are commonly found to co-occur with sulfates with more Fe-rich carbonates found in the crater floor and more Mg-rich carbonates in the fan. From comparison of laboratory data to Raman hydration peaks and sulfate peaks, we find no detections of highly hydrated sulfates e.g., epsomite (MgSO4•7H2O). The sulfate symmetric stretch at ~1000 cm-1 coupled with a hydration peak at ~3400 cm-1 indicate that MgSO4•nH2O (2 < n ≦ 5) is a likely hydration carrier phase in all units. Low-hydration MgSO4•nH2O (n = 1-2) occurs less commonly and is more prevalent in the fan than the crater floor. Hydrated Ca-sulfates (hydration peak at >3500 cm-1) matching bassanite (CaSO4•0.5H2O) are found only in the upper fan, while anhydrite (CaSO4) is found in all units. We have not been able to definitively attribute hydration peaks at ~3200 cm-1, but a possibility is low-hydration amorphous Mg-sulfates (n ≦ 1).
At the temperature and relative humidity conditions of SHERLOC measurements (~<3% RH, ~200-220 K), starkeyite (MgSO4•4H2O) and gypsum (CaSO4•2H2O) are expected to be the thermodynamically stable phases [4, 5]. The SHERLOC detections of MgSO4•nH2O (2 < n ≦ 5) imply these Mg-sulfates are in equilibrium with present-day surface environmental conditions. However, the detections of anhydrite and bassanite imply that processes that are not thermodynamically and kinetically favored at the surface now are required for their formations. While the direct precipitation of anhydrite commonly occurs at temperatures > 50°C, highly saline brines also allow anhydrite formation. SHERLOC detections of less soluble anhydrite at the stratigraphically higher fan front and more soluble hydrated Mg-sulfates at the stratigraphically lower crater floor likely indicate that at the time of formation, a saline brine flowing across from the fan to the floor precipitated first anhydrite and then Mg sulfates upon evapoconcentration without temperatures > 50°C. The persistence of anhydrite implies subsequent arid conditions that prohibit its hydration to gypsum. Collectively, the data imply aqueous deposition of sediments by fluids that could have been available intermittently or continuously, formation of salts, and subsequent aridity to preserve the observed hydration states.
Acknowledgements: We thank the SHERLOC and Mars 2020 science and engineering teams for the data that enabled this study.
References: [1] Farley, KA et al. (2022) Science, 377. [2] Siljeström, S et al. (2024) JGR Planets, 129. [3] Phua, Y et al. (in revision) JGR Planets. [4] Chipera, SJ et al. (2023) JGR Planets, 128. [5] Rapin, W et al. (2016) EPSL, 452.