¹Lowery, C.M. et al. (>10)
Oceanography 32, 120-134 Link to Article [DOI: 10.5670/oceanog.2019.133]
¹University of Texas Institute for Geophysics, Jackson School of Geosciences, Austin, TX, United States

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¹Stennikov, A.V.,¹Voropaev, S.A.,¹Fedulov, V.S.,¹Dushenko, N.V.,¹Naimushin, S.G.
Solar System Research 53, 199-207 Link to Article [DOI: 10.1134/S0038094619030055]
¹Vernadsky Institute of Geochemistry and Analytical Chemistry, Russian Academy of Sciences, Moscow, 119991, Russian Federation

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¹K.M.Primm,¹D.E.Stillman,²T.I.Michaels
Icarus (in Press) Link to Article [https://doi.org/10.1016/j.icarus.2019.06.003]
¹Dept. of Space Studies, Southwest Research Institute, 1050 Walnut St. #300, Boulder, CO 80302, USA
²SETI Institute, 189 Bernardo Ave Suite 200, Mountain View, CA 94043, USA
Copyright Elsevier

Liquid solution stability has been a highly studied topic in the Martian community since the detection of perchlorate (ClO4−) at the Phoenix landing site and the global detection of chloride (Cl−) by THEMIS (Thermal Emission Imagining System, onboard Mars Odyssey). Understanding how brines form and react to changing environmental conditions helps identify potentially habitable environments on Mars, both at present and in the past. Here we measure the extent of metastability of magnesium chloride (MgCl2) and sodium chloride (NaCl) brines when freezing. We find that the metastable eutectic temperature of MgCl2 depends on the maximum temperature (Tmax) reached before freezing. If Tmax < −15 °C, the metastable eutectic temperature (mTeu) is only 3 °C below the stable eutectic temperature, and if Tmax > −15 °C, mTeu is 15 °C below the stable eutectic temperature (Teu). We speculate that this metastable behavior follows the phase diagram for the transition into the 8 hydrate for MgCl2, thus, yielding a different freezing temperature, the peritectic for MgCl2·8H2O. However, mTeu for NaCl is independent of Tmax and was constantly at 3 °C below Teu with no peritectic (consistent with the phase diagram). We also found that MgCl2 brine can exist for at least 60 h at 5 °C below its Teu. Applying our findings, we determined the potential time evolution of brines at Palikir crater, using a time-series of modeled temperature profiles. Surficial layers melt more frequently, but layers at 2–3 cm depth are able to warm above Tmax > −15 °C and maintain brine for longer than surficial layers. The evaporation rate of brine buried by 2–3 cm of regolith is greatly reduced due to the generally cold temperatures, solute concentration, and by the regolith overburden. We also found that at Palikir crater, only the deep subsurface (~9.5 cm depth) has water activities (~0.75) high enough to support life. Overall, the metastable properties of brines can drastically affect their formation and longevity on Mars, and should be considered in future models.

¹I.Varatharajan,¹A.Maturilli,¹J.Helbert,²G.Alemanno,³H.Hiesinger
Earth and Planetary Science Letters (in Press) Link to Article [https://doi.org/10.1016/j.epsl.2019.05.020]
¹Deutsches Zentrum für Luft- Und Raumfahrt
²Universita del Salento
³Westfälische Wilhelms-Universität Münster
Copyright Elsevier

To detect the mineral diversity of a planet’s surface, it is essential to study the spectral variations over a broad wavelength range at relevant simulated laboratory conditions. The MESSENGER (Mercury Surface, Space Environment, Geochemistry, and Ranging) mission to Mercury discovered that irrespective of its formation closest to the Sun, Mercury is richer in volatiles than previously expected. This is especially true for sulfur (S), with an average abundance of 4 wt%. It has been proposed that sulfur in the interior of Mercury can be brought to the surface through volcanic activity in the form of sulfides as slag deposits in Mercury hollows and pyroclastic deposits. However, comprehensive spectral library of sulfide minerals measured under vacuum conditions in a wide spectral range (0.2–100 μm) was lacking. This affects the detectability and understanding of the distribution, abundance, and type of sulfides on Mercury using remote-sensing spectral observations. In the case of Mercury, the effect of thermal weathering affecting the spectral behavior of these sulfides must be studied carefully for their effective detection. In this study, we present a spectral library of synthetic sulfides including MgS, FeS, CaS, CrS, TiS, NaS, and MnS. For each sample, we performed emissivity measurements in the thermal infrared range (TIR: ∼7–14 μm) for sample temperatures from 100 °C–500 °C, covering the daytime temperature cycle on Mercury’s surface. In addition, for each sample we measured the spectral reflectance of fresh and thermally processed sulfides over a wide spectral range (0.2–100 μm) and at four different phase angles, 26°, 40°, 60°, 80°. This spectral library facilitates the detection of sulfides by past and future missions to Mercury by any optical spectrometer of any spectral range. Specifically, the emissivity measurements in this study will support the Mercury Radiometer and Thermal Imaging Spectrometer (MERTIS) instrument on the ESA/JAXA BepiColombo mission, which will study the surface mineralogy over a wavelength range of 7–14 μm at a spatial resolution of 500 m/pixel. The measured reflectance of these sulfides in 0.2–100 μm at various phase angles will support the interpretation of measurements from past (MDIS, MASCS on MESSENGER) and future missions (SIMBIO-SYS on BepiColombo).