¹D.P. Quinn,²B.L. Ehlmann
Journal of Geophysical Research, Planets (in Press) Link to Article [https://doi.org/10.1029/2018JE005706]
¹Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, California, USA
²Jet Propulsion Laboratory, California Institute of Technology, Pasadena, California, USA
Published by arrangement with John Wiley & Sons

Ancient stratigraphy on Isidis Basin’s western margin records the history of water on early Mars. Noachian units are overlain by layered, basaltic‐composition sedimentary rocks that are enriched in polyhydrated sulfates and capped by more resistant units. The layered sulfates –uniquely exposed at northeast Syrtis Major – comprise a sedimentary sequence up to 600‐m thick that has undergone a multi‐stage history of deposition, alteration, and erosion. Siliciclastic sediments enriched in polyhydrated sulfates are bedded at m‐scale and were deposited on slopes up to 10°, embaying and thinning against pre‐existing Noachian highlands around the Isidis basin rim. The layered sulfates were modified by volume‐loss fracturing during diagenesis. Resultant fractures hosted channelized flow and jarosite mineral precipitation to form resistant ridges upon erosion. The structural form of the layered sulfates is consistent with packages of sediment fallen from either atmospheric or aqueous suspension; coupling with substantial diagenetic volume‐loss may favor deepwater basin sedimentation. After formation, the layered sulfates were capped by a “smooth capping unit” and then eroded to form paleovalleys. Hesperian Syrtis Major lavas were channelized by this paleotopography, capping it in some places and filling it in others. Later fluvial features and phyllosilicate‐bearing lacustrine deposits, sharing a regional base level of ~‐2300m, were superimposed on the sulfate‐lava stratigraphy. The layered sulfates suggest surface bodies of water and active groundwater upwelling during the Noachian–Hesperian transition. The northeast Syrtis Major stratigraphy records at least four distinct phases of surface and subsurface aqueous activity spanning from late Noachian to early Amazonian time.

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).

¹Sedykh, E.M.,¹Gromyak, I.N.,¹Lorents, K.A.,¹Skripnik, A.Y.,¹Kolotov, V.P.
Journal of Analytical Chemistry 74, 393-400 Link to Article [DOI: 10.1134/S1061934819040129]
¹Vernadsky Institute of Geochemistry and Analytical Chemistry, Russian Academy of Sciences, Moscow, 119991, Russian Federation

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¹Maksimova, A.A.,²Unsalan, O.,¹Chukin, A.V.,¹Oshtrakh, M.I.
Hyperfine Interactions 240, 47 Link to Article [DOI: 10.1007/s10751-019-1593-8]
¹Institute of Physics and Technology, Ural Federal University, Ekaterinburg, 620002, Russian Federation
²Department of Physics, Faculty of Science, Ege University, Izmir, Bornova 35100, Turkey

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^1,2,3Yun Jiang,³Heng Chen,³Bruce Fegley Jr.,³Katharina Lodders,⁴Weibiao Hsu,⁵Stein B.Jacobsen,^3,5KunWang(王昆)
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2019.06.003]
¹CAS Key Laboratory of Planetary Sciences, Purple Mountain Observatory, Chinese Academy of Sciences, Nanjing 210008, China
²CAS Center for Excellence in Comparative Planetology, China
³Department of Earth and Planetary Sciences and McDonnell Center for the Space Sciences, Washington University in St. Louis, One Brookings Drive, St. Louis, MO 63130, USA
⁴Space Science Institute, Macau University of Science and Technology, Macau
⁵Department of Earth and Planetary Sciences, Harvard University, 20 Oxford Street, Cambridge, MA 02138, USA
Copyright Elsevier

Tektites are mm to cm sized glassy objects generated through high-energy meteoroid impacts on the surface of the Earth under high temperature and pressure, and reducing conditions. They are the products of large-scale catastrophic events in Earth’s history and can be used to understand the behavior of moderately volatile elements (e.g., K and Zn) during impact vaporization events. Here, we report bulk K isotopic compositions of tektites from three different strewn fields and “in-situ” profile analysis of both K and Zn isotopes in one complete tektite. All tektites span a narrow range in their K isotopic compositions (δ⁴¹K_BSE: −0.10 ± 0.03‰ to 0.16 ± 0.04‰), revealing no discernible K isotopic fractionation from the Bulk Silicate Earth (BSE) and upper continental crust materials, which is consistent with previous results. In contrast, Zn isotopes show a large variation (δ⁶⁶Zn: −0.39 ± 0.02‰ to 2.38 ± 0.03‰) even within one specimen. In order to provide a coherent explanation for the different behavior of moderately volatile elements (K, Zn and Cu), we have conducted thermochemical calculations to compute the partial vapor pressures of Cu₂O, K₂O, and ZnO dissolved in silicate melts as a function of temperature, pressure, oxygen and chlorine fugacities. In a large range of the parameter space, the calculations show that Cu and Zn can be vaporized much easier than K and thus produce large isotopic fractionation. In contrast, the lithophile element K is more prone to remain in the silicate melt because of its very low activity coefficient in the melt, and thus the K isotopes remain unfractionated. This study provides new constraints on the formation of tektites and is consistent with a “bubble-stripping” model to explain the extreme water and volatiles depletion in tektites.

^1,2,3M.E.Newcombe, ¹J.R.Beckett,¹M.B.Baker, ⁴S.Newman,¹Y.Guan,¹J.M.Eiler¹E.M.Stolper
Geochmica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2019.05.033]
¹Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, CA 91125, USA
²Lamont Doherty Earth Observatory, 61 Route 9W, Palisades, NY 10964, USA
³Department of Terrestrial Magnetism, Carnegie Institution of Washington, 5241 Broad Branch Road NW, Washington, DC 20015
⁴Bay Area Air Quality District, 375 Beale St., Suite 600, San Francisco, CA 94105
Copyright Elsevier

^1,2Jaramillo, E.A.,³Royle, S.H.,^4,5Claire, M.W.,^1,3Kounaves, S.P.,³Sephton, M.A.
Geophysical Research Letters 46, 3090-3098 Link to Article [DOI: 10.1029/2018GL081335]
¹Department of Chemistry, Tufts University, Medford, MA, United States
²Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA, United States
³Department of Earth Science and Engineering, Imperial College London, London, United Kingdom
⁴School of Earth and Environmental Sciences and Centre for Exoplanet Science, University of St. Andrews, Saint Andrews, United Kingdom
⁵Blue Marble Space Institute of Science, Seattle, WA, United States

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