¹Eva L. Scheller,^1,2Bethany L. Ehlmann
Journal of Geophysical Research (Planets) (In Press) Link to Article [https://doi.org/10.1029/2019JE006190]
¹Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, California, USA
²Jet Propulsion Laboratory, Pasadena, California Institute of Technology, Pasadena, California, USA
Published by arrangement with John Wiley & Sons

The western part of the Isidis basin structure hosts a well‐characterized Early Noachian to Amazonian stratigraphy. The Noachian Basement comprises its oldest exposed rocks (Early to Mid‐Noachian), and was previously considered a single LCP‐ and Fe/Mg‐smectite‐bearing unit. Here, we divide the Noachian Basement Group into 5 distinct geological units (Stratified Basement Unit, Blue Fractured Unit, Mixed Lithology Plains Unit, LCP‐bearing Plateaus Unit, Fe/Mg‐smectite‐bearing Mounds Unit), 2 geomorphological features (megabreccia and ridges), and a mineral deposit (kaolinite‐bearing bright materials), based on geomorphology, spectral characteristics, and stratigraphic relationships. Megabreccia contain four different pre‐Isidis lithologies, possibly including deeper crust or mantle materials, formed through mass‐wasting associated with transient crater collapse during Isidis basin formation. The Fe/Mg‐smectite‐bearing Stratified Basement Unit and LCP‐bearing Blue Fractured Unit likewise represent pre‐Isidis units within the Noachian Basement Group. Multiple Fe/Mg‐smectite‐bearing geological units with different stratigraphic positions and younger kaolinite‐bearing bright materials indicate several aqueous alteration episodes of different ages and styles. Units with slight changes in pyroxene spectral properties suggest a transition from low‐Ca pyroxene‐containing materials to those with higher proportions of pyroxenes higher in Ca and/or glass that could be related to different impact‐ and/or igneous processes, or provenance. This long history of Noachian and potentially Pre‐Noachian geological processes, including impact basin formation, aqueous alteration, and multiple igneous and sedimentary petrogeneses, records changing ancient Mars environmental conditions. All units defined by this study are available 20 km outside of Jezero crater for in‐situ analysis and sampling during a potential extended mission scenario for the Mars 2020 rover.

¹L.T.Elkins-Tanton et al. (>10)
Journal of Geophysical Research (Planets) (In Press) Link to Article [https://doi.org/10.1029/2019JE006296]
¹Arizona State University
Published by arrangement with John Wiley & Sons

Some years ago the consensus was that asteroid (16) Psyche was almost entirely metal. New data on density, radar properties, and spectral signatures indicate that the asteroid is something perhaps even more enigmatic: a mixed metal and silicate world. Here we combine observations of Psyche with data from meteorites and models for planetesimal formation to produce the best current hypotheses for Psyche’s properties and provenance. Psyche’s bulk density appears to be between 3,400 and 4,100 kg m‐3. Psyche is thus predicted to have between ~30 vol% and ~60 vol% metal, with the remainder likely low‐iron silicate rock and not more than ~20% porosity. Though their density is similar, mesosiderites are an unlikely analog to bulk Psyche because mesosiderites have far more iron‐rich silicates than Psyche appears to have. CB chondrites match both Psyche’s density and spectral properties, as can some pallasites, although typical pallasitic olivine contains too much iron to be consistent with the reflectance spectra. Final answers, as well as resolution of contradictions in the dataset of Psyche physical properties, for example, the thermal inertia measurements, may not be resolved until the NASA Psyche mission arrives in orbit at the asteroid. Despite the range of compositions and formation processes for Psyche allowed by the current data, the science payload of the Psyche mission (magnetometers, multi‐spectral imagers, neutron spectrometer, and a gamma‐ray spectrometer) will produce datasets that distinguish among the models.

¹BruceFegleyJr,¹Katharina Lodders,²Nathan S.Jacobson
Geochemistry [Chemie der Erde] (in Press) Link to Article [https://doi.org/10.1016/j.chemer.2019.125594]
¹Planetary Chemistry Laboratory, Dept. of Earth & Planetary Sciences and McDonnell Center for the Space Sciences, Washington University, St. Louis, MO 63130, USA
²Materials Division, NASA Glenn Research Center, MS106-1, 21000 Brookpark Road, Cleveland, OH 44135, USA
Copyright Elsevier

We review some issues relevant to volatile element chemistry during accretion of the Earth with an emphasis on historical development of ideas during the past century and on issues we think are important. These ideas and issues include the following: (1) whether or not the Earth accreted hot and the geochemical evidence for high temperatures during its formation, (2) some chemical consequences of the Earth’s formation before dissipation of solar nebular gas, (3) the building blocks of the Earth, (4) the composition of the Earth and its lithophile volatility trend, (5) chemistry of silicate vapor and steam atmospheres during Earth’s formation, (6) vapor – melt partitioning and possible loss of volatile elements, (7) insights from hot rocky extrasolar planets. We include tabulated chemical kinetic data for high-temperature elementary reactions in silicate vapor and steam atmospheres. We finish with a summary of the known and unknown issues along with suggestions for future work.

^1,2M.Martinot,³J.Flahaut,⁴S.Besse,²C.Quantin-Nataf,³W.van Westrenen
Icarus (in Press) Link to Article [https://doi.org/10.1016/j.icarus.2020.113747]
¹Faculty of Science, Vrije Universiteit Amsterdam, De Boelelaan 1085, 1081 HV Amsterdam, The Netherlands
²Université Lyon 1, ENS-Lyon, CNRS, UMR 5276 LGL-TPE, F-69622, Villeurbanne, France
³Centre de Recherches Pétrographiques et Géochimiques, CNRS/Université de Lorraine F-54500, Vandoeuvre-lès-Nancy, France
⁴European Space Astronomy Centre, P.O. Box 78, 28691 Villanueva de la Canada, Madrid, Spain
Copyright Elsevier

Spectroscopic data from the Moon Mineralogy Mapper (M3) instrument are used to study the mineralogy of the central peak or peak ring of 75 craters located in the lunar anorthositic Feldspathic Highlands Terrane (FHT-a), as defined by Jolliff et al. (2000). The thickness of South-Pole Aitken (SPA) ejecta at the location of the selected craters is estimated. Crustal thickness models are used with empirical cratering equations to estimate the depth of origin of the material excavated in the studied central peaks, and its distance to the crust-mantle interface. The goal of this survey is to study the composition of the FHT-a crust, and the extent of its potential lateral and vertical heterogeneities. High-Calcium Pyroxene (HCP) and featureless spectra are mostly detected throughout the entire FHT-a, whereas the number of pure plagioclase detections is small. No relationship between the central peak composition and the distance to SPA or the depth within the SPA ejecta is observed. The SPA ejecta material cannot be spectrally distinguished from crustal material. We interpret the paucity of plagioclase spectra in the FHT-a, which contrasts with more frequent plagioclase detections in the central peaks of craters sampling the crust in younger lunar terranes using identical spectroscopic techniques Martinot et al. (2018b), as a possible effect of terrane maturation, or of mixing with mafic components that mask their signature in the visible near-infrared. Overall, the FHT-a appears homogeneous laterally. However, data hint at a pyroxene compositional change with increasing depth, from high-calcium content in the upper crust towards less calcic compositions with increasing depth, which is consistent with prior studies of the architecture of the lunar crust.

^1,3Matthew J.Genge,²Matthias Van Ginneken,³Martin D.Suttle
Planetary and Space Science (in Press) Link to Article [https://doi.org/10.1016/j.pss.2020.104900]
¹Department of Earth Science and Engineering, Imperial College London, Exhibition Road, London, SW7 2AZ, UK
²Department of Physics, University of Kent, Canterbury, Kent, UK
³Department of Earth Sciences, The Natural History Museum, Cromwell Road, London, SW7 2BD, UK

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¹A.C.Levasseur-Regourd,²C.Baruteau,^,2J.Lasue,^3,4J.Milli,⁵J.-B.Renard
Planetary and Space Science (in Press) Link to Article [https://doi.org/10.1016/j.pss.2020.104896]
¹LATMOS, Sorbonne Université, CNRS, CNES, Paris, France
²IRAP, Université de Toulouse, CNES, CNRS, UPS, Toulouse, France
³European Southern Observatory, Alonso de Córdova 3107, Casilla, 19001, Santiago, Chile
⁴Univ. Grenoble Alpes, IPAG, Grenoble, France
⁵LPC2E, Université d’Orléans, CNRS, Orléans, France

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¹Lydie Bonal,²Jérôme Gattacceca, ^1,3Alexandre Garenne,¹JolanthaEschrig, ²PierreRochette,² Lisa Krämer Ruggiu
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2020.03.009]
¹Institut de Planétologie et d’Astrophysique de Grenoble, Université Grenoble Alpes, CNRS CNES, 38000 Grenoble (France)
²CNRS, Aix Marseille Univ, IRD, Coll France, INRAE, CEREGE, Aix-en-Provence, France
³Nuclear and Chemical Sciences Division, Lawrence Livermore National Laboratory, Livermore, CA, USA
Copyright Elsevier

This paper focuses on the post-accretion history of CV3 chondrites, through a combination of petrographic and mineralogical characterization, magnetic measurements, spectral (Raman and Infrared) and thermo-gravimetric analysis of 31 meteorites (including 7 falls, 21 Antarctic and 3 non-Antarctic finds) spanning a wide metamorphic range.
We classify the 21 Antarctic chondrites and the Bukhara fall into the CVRed, CVOxA, and CVOxB subgroups. We establish quantitative parameters relevant for this sub-classification. In comparison to CVOx, CVRed chondrites are characterized by (i) a lower abundance of matrix, (ii) a higher abundance of metal, (iii) the presence of Ni-poor sulfides. In comparison to CVOxB, CVOxA are characterized by (i) similar matrix abundance, (ii) a higher abundance of metal, (iii) the presence of metal almost exclusively under the form of awaruite, (iv) lower Ni content of sulfides, (v) lower magnetic susceptibility and saturation remanence.
Both CVOx (CVOxA and CVOxB) and CVRed experienced aqueous alteration, and contain oxyhydroxides and phyllosilicates. We show that the abundance of these hydrated secondary minerals observed today in individual CV chondrites decreases with their peak metamorphic temperature. This is interpreted either as partial dehydration of these secondary minerals or limited hydration due to the rapid exhaustion of the water reservoir during parent body thermal metamorphism. Moreover, the lower abundance of oxyhydroxides (that have a lower thermal stability than phyllosilicates and may in large part postdate the peak of thermal metamorphism) in more metamorphosed CV chondrites is interpreted as lower availability of aqueous fluids during retrograde metamorphism in these meteorites.
Lastly, we show that in comparison to CVOxB, CVOxA are systematically (i) more metamorphosed, (ii) less hydrated, (iii) depleted in ferromagnetic minerals, (iv) but enriched in metal in the form of secondary awaruite. CVOxA may be thermally metamorphosed CVOxB. CVRed are significantly different from CVOx (matrix abundances, alteration products, opaque minerals), but span the same wide metamorphic range. This could be indicative of a laterally heterogeneous CV parent body, or suggest the existence of distinct parent bodies for CVOx and CVRed chondrites.

¹Olivier Poch et al. (>10)
Science 367, eaaw7462 Link to Article [DOI: 10.1126/science.aaw7462]
¹Université Grenoble Alpes, Centre National de la Recherche Scientifique (CNRS), Institut de Planétologie et d’Astrophysique de Grenoble (IPAG), 38000 Grenoble, France.

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¹Mario Fischer-Gödde,¹Bo-Magnus Elfers,¹Carsten Münker,²Kristoffer Szilas,³Wolfgang D. Maier,¹Nils Messling,^4,5,6Tomoaki Morishita,⁷Martin Van Kranendonk,⁸Hugh Smithies
Nature 579, 240-244 Link to Article [DOIhttps://doi.org/10.1038/s41586-020-2069-3]
¹Institut für Geologie und Mineralogie, University of Cologne, Cologne, Germany
²Department of Geosciences and Natural Resource Management, University of Copenhagen, Copenhagen, Denmark
³School of Earth and Ocean Sciences, Cardiff University, Cardiff, UK
⁴Faculty of Geosciences and Civil Engineering, Institute of Science and Engineering, Kanazawa University, Kanazawa, Japan
⁵Lamont-Doherty Earth Observatory, Columbia University, New York, NY, USA
⁶Volcanoes and Earth’s Interior Research Center, Research Institute for Marine Geodynamics, Japan Agency for Marine-Earth Science and Technology, Yokosuka, Japan
⁷Australian Centre for Astrobiology, University of New South Wales, Sydney, New South Wales, Australia
⁸Geological Survey of Western Australia, East Perth, Western Australia, Australia

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^1,2Tuan H.Vu,^1,2Mathieu Choukroun,^1,2Robert Hodyss,^1,2Paul V.Johnson
Icarus (in Press) Link to Article [https://doi.org/10.1016/j.icarus.2020.113746]
¹Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA 91109, USA
²NASA Astrobiology Institute, USA
Copyright Elsevier

The composition of Europa’s subsurface ocean is of great interest for understanding the internal geochemistry and potential habitability of this icy body. However, current constraints on the ocean composition need to rely largely on its expression on the surface. In this work, we combine chemical divide modeling with cryogenic Raman and X-ray diffraction experiments to examine freezing of a simple putative brine system containing Na⁺, Mg²⁺, Cl⁻, and SO₄²⁻ across a range of ionic concentrations and freezing rates to assess the feasibility of inferring the ocean’s chemical composition via in-situ techniques. The results suggest that multiple hydrated salts not predicted by chemical models are frequently encountered in the final solid phase, making accurate quantification of the subsurface liquid composition via surface observables rather challenging. In addition, flash freezing of diluted brines often produces water ice together with amorphous hydrated Mg salts, which may significantly hinder their detection. These findings can help inform both analytical protocols for a Raman spectrometer onboard a Europa surface lander as well as potential locations for exploration, in order to best provide meaningful constraints on emplacement mechanisms and the composition of frozen salt minerals on the surface.