¹Lingzhi Sun,¹Paul G. Lucey
Journal of Geophysical Research, Planets (in Press) Link to Article [https://doi.org/10.1029/2020JE006691]
¹Department of Earth Sciences, Hawai‘i Institute of Geophysics and Planetology, University of Hawai‘i at Mānoa, Honolulu, HI, USA
Published by arrangement with John Wiley & Sons

Mineral abundance and Mg# (100× molar Mg/(Mg + Fe)) are significant in understanding the crustal composition and thermal history of the Moon. In this study, we derive a new set of optical constants for olivine, orthopyroxene, and clinopyroxene using radiative transfer equations that include soil porosity and the opposition effect. Based on the new optical constants, we develop a mineral abundance and Mg# unmixing model, and build a spectral library composed of mineral mixtures of plagioclase, olivine, low‐Ca pyroxene (LCP) and high‐Ca pyroxene (HCP), and Mg# ranging within 40–90. The accuracy of this model in estimating mineral abundance and chemistry is better than 3 vol% for olivine, LCP and HCP, better than 6 vol% for plagioclase, and better than 10 for Mg#. This model is validated using forward and inverse modeling. For the forward modeling, we reproduce the spectra of powdered pure minerals and Lunar Sample Characterization Consortium (LSCC) lunar soils, and the modeled spectra are consistent with those measured in the laboratory. For the inverse modeling, we determined mineral abundances and Mg# of 19 LSCC soil spectra by searching the best match to the spectral library. The modeled mineral abundances of LSCC soils are consistent with those measured by X‐ray digital imaging. We derived a global Mg# map using our model and Moon Mineralogy Mapper images, and our Mg# map shows a peak concentration at 70, consistent with that measured by the Lunar Prospector gamma‐ray spectrometer.

¹Gabriel L. Eggers,¹James J. Wray,²Josef Dufek
Journal of Geophysical Research, Planets (in Press) Link to Article [https://doi.org/10.1029/2020JE006383]
¹School of Earth and Atmospheric Sciences, Georgia Institute of Technology, Atlanta, GA, USA
²Department of Earth Sciences, University of Oregon, Eugene, OR, USA
Published by arrangement with John Wiley & Sons

Decades of study of the igneous martian crust concluded that it was primarily basaltic, but a range of new investigations find evidence of evolved compositions. Foremost of these is a highly feldspathic unit within the Nili Patera caldera of Syrtis Major, the only detection with preserved volcanic context but which nonetheless remains ambiguous in exact composition and formation. We conduct compositional mapping of this feldspathic unit via near‐infrared spectroscopy from the Compact Reconnaissance Imaging Spectrometer for Mars instrument and find that the unit occupies at minimum 104 km2 at high confidence and an additional 41 km2 at moderately high confidence, meaning the unit is locally significant. We compare our mapping with that inferred from geomorphology and find that while texture and albedo are useful proxies, they are not perfectly reliable as substitutes for thorough compositional investigation. Study of the boundary between the feldspathic unit and surrounding mafic rock indicates the former formed early and may extend locally in the subsurface. We consider what compositional mixtures could explain the conflicting interpretations derived from visible/near‐infrared and thermal infrared spectroscopy, concluding it is likely due to thermophysical differences between the light‐toned feldspathic unit and the infilling dark mafic sand. We discuss proposed plutonic and volcanic formation scenarios for the feldspathic unit, considering Earth analogs and implications for the parent magmatic system, and offer observations in rock texture and composition that would clarify.

¹Stephen R. Kane,²Giada N. Arney,³Paul K. Byrne,^1,4Paul A. Dalba,⁵Steven J. Desch,⁶Jonti Horner,⁷Noam R. Izenberg,⁷Kathleen E. Mandt,⁸Victoria S. Meadows,⁹Lynnae C. Quick
Journal of Geophysical Research, Planets (in Press) Link to Article [https://doi.org/10.1029/2020JE006643]
¹Department of Earth and Planetary Sciences, University of California, Riverside, CA, 92521 USA
²Planetary Systems Laboratory, NASA Goddard Space Flight Center, Greenbelt, MD, 20771 USA
³Planetary Research Group, Department of Marine, Earth, and Atmospheric Sciences, North Carolina State University, Raleigh, NC, 27695 USA
⁴NSF Astronomy and Astrophysics Postdoctoral Fellow
⁵School of Earth and Space Exploration, Arizona State University, Tempe, AZ, 85287 USA
⁶Centre for Astrophysics, University of Southern Queensland, Toowoomba, QLD, 4350 Australia
⁷Johns Hopkins University Applied Physics Laboratory, Laurel, MD, 20723 USA
⁸Department of Astronomy, University of Washington, Seattle, WA, 98195 USA
⁹Planetary Geology, Geophysics and Geochemistry Laboratory, NASA Goddard Space Flight Center, Greenbelt, MD, 20771 USA
Published by arrangement with John Wiley & Sons

Over the past several decades, thousands of planets have been discovered outside of our Solar System. These planets exhibit enormous diversity, and their large numbers provide a statistical opportunity to place our Solar System within the broader context of planetary structure, atmospheres, architectures, formation, and evolution. Meanwhile, the field of exoplanetary science is rapidly forging onward towards a goal of atmospheric characterization, inferring surface conditions and interiors, and assessing the potential for habitability. However, the interpretation of exoplanet data requires the development and validation of exoplanet models that depend on in‐situ data that, in the foreseeable future, are only obtainable from our Solar System. Thus, planetary and exoplanetary science would both greatly benefit from a symbiotic relationship with a two‐way flow of information. Here, we describe the critical lessons and outstanding questions from planetary science, the study of which are essential for addressing fundamental aspects for a variety of exoplanetary topics. We outline these lessons and questions for the major categories of Solar System bodies, including the terrestrial planets, the giant planets, moons, and minor bodies. We provide a discussion of how many of these planetary science issues may be translated into exoplanet observables that will yield critical insight into current and future exoplanet discoveries.

¹A.S.Yen et al. (>10)
Journal of Geophysical Research, Planets (in Press) Link to Article [https://doi.org/10.1029/2020JE006569]
¹Jet Propulsion Laboratory, California Institute of Technology, Pasadena, California, 91109Published by arrangement with John Wiley & Sons

In August 2015, the Curiosity Mars rover discovered tridymite, a high‐temperature silica polymorph, in Gale crater. The existing model for its occurrence suggests erosion and detrital sedimentation from silicic volcanic rocks in the crater rim or central peak. The chemistry and mineralogy of the tridymite‐bearing rocks, however, are not consistent with silicic volcanic material. Using data from Curiosity, including chemical composition from the Alpha Particle X‐ray Spectrometer, mineralogy from the CheMin instrument, and evolved gas and isotopic analyses from the Sample Analysis at Mars instrument, we show that the tridymite‐bearing rocks exhibit similar chemical patterns with silica‐rich alteration halos which crosscut the stratigraphy. We infer that the tridymite formed in‐place through hydrothermal processes and show additional chemical and mineralogical results from Gale crater consistent with hydrothermal activity occurring after sediment deposition and lithification.

¹Rachel Y. Sheppard,¹Ralph E. Milliken,²Mario Parente,²Yuki Itoh
Journal of Geophysical Research, Planets (in Press) Link to Article [https://doi.org/10.1029/2020JE006372]
¹Department of Earth, Environmental and Planetary Sciences, Brown University
²Department of Electrical and Computer Engineering, University of Massachusetts, Amherst
Published by Arrangement with John Wiley & Sons

Previous studies have shown that Mt. Sharp has stratigraphic variation in mineralogy that may record a global transition from a climate more conducive to clay mineral formation to one marked by increased sulfate production. To better understand how small‐scale observations along the traverse path of NASA’s Curiosity rover might be linked to such large‐scale processes, it is necessary to understand the extent to which mineral signatures observed from orbit vary laterally and vertically. This study uses newly processed visible‐shortwave infrared CRISM data and corresponding visible images to re‐examine the mineralogy of lower Mt. Sharp, map mineral distribution, and evaluate stratigraphic relationships. We demonstrate the presence of darker‐toned strata that appears to be throughgoing with spectral signatures of monohydrated sulfate. Strata above and below this zone are lighter‐toned and contain polyhydrated sulfate and variable distribution of Fe/Mg clay minerals. Clay minerals are observed at multiple stratigraphic positions; unlike the kieserite zone these units cannot be traced laterally across Mt. Sharp. The kieserite zone appears to be stratigraphically confined, but in most locations the orbital data do not provide sufficient detail to determine whether mineral signatures conform to or cut across stratigraphic boundaries, leaving open the question as to whether the clay minerals and sulfates occur as detrital, primary chemical precipitates, and/or diagenetic phases. Future observations along Curiosity’s traverse will help distinguish between these possibilities. Rover observations of clay‐bearing strata in northwest Mt. Sharp may be more reflective of local conditions that could be distinct from those associated with other clay‐bearing strata.

^1,2C.D.Neish,³K.M.Cannon,^1,2L.L.Tornabene,^1,2R.L.Flemming,⁴M.Zanetti,^1,2E.Pilles
Icarus (in Press) Link to Article [https://doi.org/10.1016/j.icarus.2021.114392]
¹Department of Earth Sciences, The University of Western Ontario, London, ON N6A 5B7, Canada
²Institute for Earth and Space Exploration, The University of Western Ontario, London, ON N6A 5B7, Canada
³Department of Geology and Geological Engineering, Colorado School of Mines, Golden, CO 80401, United States of America
⁴NASA Marshall Space Flight Center, Huntsville, AL 35808, United States of America
Copyright Elsevier

Lunar impact melt deposits have unusual surface properties, unlike any measured terrestrial lava flow. Radar observations suggest that they are incredibly rough at decimeter scales, but they appear smooth in high-resolution, meter-scale optical images. The cause of their unusual surface roughness is unknown. In this work, we investigate the properties of impact melt deposits from seven lunar craters, ranging in size from 7.5 to 96 km in diameter, in an effort to understand the cause of their unique surface texture. We use data from the Lunar Reconnaissance Orbiter’s (LRO) Mini-RF instrument to characterize the small-scale roughness of the deposits, data from the LRO Camera (LROC) to characterize their meter-scale morphology, and data from Chandrayaan-1’s Moon Mineralogy Mapper (M3) to characterize their composition. This represents the most comprehensive study of the composition of lunar melt deposits completed to date. In particular, we applied a customized spectral unmixing model to the M3 data using laboratory spectra acquired from a range of possible lunar endmembers: pyroxene, olivine, fast-quenched lunar glass simulants, and impact melts and breccias (both synthetic and natural). We found that spectra derived from lunar melt deposits are typically modeled as a mix of the pyroxene and/or impact melts and breccias endmembers. Our modeled results suggest that lunar melt deposits are either crystalline deposits of pyroxene-rich rocks, or a mixture of glassy material and pyroxene minerals. The latter interpretation could explain the roughness observed in the Mini-RF data, if the melt deposits have a glassy surficial layer that shatters during impact gardening to produce decimeter scale blocks.

¹Toru Matsumoto,¹Takaaki Noguchi,¹Yu Tobimatsu,²Dennis Harries,^2,3Falko Langenhorst,⁴Akira Miyake,⁴Hiroshi Hidaka
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2021.02.013]
¹Faculty of Arts and Science, Kyushu University, 744 Motooka, Nisi-ku, Fukuoka 819-0395, Japan
²Institute of Geoscience, Friedrich Schiller University Jena, Carl-Zeiss-Promenade 10, 07745 Jena, Germany
³Hawai’i Institute of Geophysics and Planetology, School of Ocean and Earth Science and Technology, University of Hawai’i at Manoa, Honolulu, HI 96822, USA
⁴Division of Earth and Planetary Sciences, Kyoto University, Kitashirakawaoiwake-cho, Sakyo-ku, Kyoto-shi 606-8502, Japan
⁵Department of Earth and Planetary Sciences, Nagoya University, Furo-cho, Chikusa-ku, Nagoya Science building E, 464-8601, Japan
Copyright Elsevier

Alteration of iron sulfides on the lunar surface by space weathering is poorly understood. We examined space weathering features of iron sulfides in lunar mature soil grains using scanning electron microscopy (SEM) and transmission electron microscopy (TEM). SEM observations reveal that iron sulfides have vesicular textures and iron whiskers on their surfaces. Iron sulfides observed using TEM are troilite and NC-pyrrhotite. The space-weathered rim on the iron sulfides is characterized by crystallographic misorientations and the disappearance of superstructure reflections of troilite in electron diffraction patterns. These crystallographic modifications are probably produced by solar wind irradiation. The rim contains opened vesicles that are aligned along the c-plane of the sulfides, as well as numerous tiny vesicles. The Fe/S ratio at the surface of the rim is higher than in non-altered regions, indicating selective sulfur loss from the surface. Iron whiskers protrude from the space weathered rim and consist of polycrystalline metallic iron. The sulfide rims and the iron whiskers are both coated with vapor-deposited materials rich in O and Si. The combined processes driven by the solar wind irradiation, heating during impact events, solar UV radiation, and the thermal cycling may cause vesicular textures, selective sulfur escape from the iron sulfides, and the formation of the iron whiskers. The rim textures support the notion that the enrichment of heavy sulfur isotopes in mature lunar soils is caused by space weathering of iron sulfides. The space weathered rims on lunar iron sulfides are similar to those observed in regolith samples from asteroid Itokawa. Therefore, alterations of sulfide surface might be common among airless bodies in the solar system.

¹Yoshinori Miyazaki,¹Jun Korenaga
Icarus (in Press) Link to Article [https://doi.org/10.1016/j.icarus.2021.114368]
¹Department of Earth and Planetary Sciences, Yale University, New Haven, CT 06511, USA
Copyright Elsevier

Chondrites are the likely building blocks of Earth, and identifying the group of chondrite that best represents Earth is a key to resolving the state of the early Earth. The origin of chondrites, however, remains controversial partly because of their puzzling major element compositions, some exhibiting depletion in Al, Ca, and Mg. Based on a new thermochemical evolution model of protoplanetary disks, we show that planetesimals with depletion patterns similar to ordinary and enstatite chondrites can originate at 1–2 AU outside where enstatite evaporates. Around the “evaporation front” of enstatite, the large inward flow of refractory minerals, including forsterite, takes place with a high pebble concentration, and the loss of those minerals results in depletion in Al, Ca, and Mg. The fractionation driven by the loss of forsterite would also create a complementary Mg-rich reservoir just inside the depleted region, creating two chemically distinct reservoirs adjacent to each other. The region around the evaporation front of enstatite has the highest dust concentration inside the snow line, and thus the streaming instability is most likely to be triggered therein. Planetesimals with two different major element compositions could naturally be created in the terrestrial region, which could evolve into parent bodies for Earth and chondrites. This can explain why Earth and enstatite chondrites share similar isotopic signatures but have different bulk compositions.

^1,2Noël Chaumard,^1,3Céline Defouilloy,^1,4Andreas T.Hertwig,¹Noriko T.Kita
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2021.02.012]
¹WiscSIMS, Department of Geoscience, University of Wisconsin-Madison, 1215 W. Dayton Street, Madison, WI 53706-1692, USA
²Fi Group, Direction scientifique, 14 terrasse Bellini, 92800 Puteaux, France
³CAMECA, 29 quai des Grésillons, 92622 Gennevilliers Cedex, France
⁴Institut für Geowissenschaften, Universität Heidelberg, Im Neuenheimer Feld 234-236, 69120 Heidelberg, Germany
Copyright Elsevier

In-situ oxygen three-isotope analyses of chondrules and isolated olivine grains in the Paris (CM) chondrite were conducted by secondary ion mass spectrometry (SIMS). Multiple analyses of olivine and/or pyroxene in each chondrule show indistinguishable Δ17O values, except for minor occurrences of relict olivine grains (and one low-Ca pyroxene). A mean Δ17O value of these homogeneous multiple analyses was obtained for each chondrule, which represent oxygen isotope ratios of the chondrule melt. The Δ17O values of individual chondrules range from –7‰ to –2‰ and generally increase with decreasing Mg# of olivine and pyroxene in individual chondrules. Most type I (FeO-poor) chondrules have high Mg# (∼99) and variable Δ17O values from –7.0‰ to –3.3‰. Other type I chondrules (Mg# ≤97), type II (FeO-rich) chondrules, and two isolated FeO-rich olivine grains have host Δ17O values from –3‰ to –2‰. Eight chondrules contain relict grains that are either 16O-rich or 16O-poor relative to their host chondrule and show a wide range of Δ17O values from –13‰ to 0‰.

The results from chondrules in the Paris meteorite are similar to those in Murchison (CM). Collectively, the Δ17O values of chondrules in CM chondrites continuously increase from –7‰ to –2‰ with decreasing Mg# from 99 to 37. The majority of type I chondrules (Mg# >98) show Δ17O values from –6‰ to –4‰, while the majority of and type II chondrules (Mg# 60-70) show Δ17O values of –2.5‰. The covariation of Δ17O versus Mg# observed among chondrules in CM chondrites may suggest that most chondrules in carbonaceous chondrites formed in a single large region across the snow line where the contribution of 16O-poor ice to chondrule precursors and dust enrichment factors varied significantly.

^1,2A.J.King,¹P.F.Schofield,¹S.S.Russell
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2021.02.011]
¹Planetary Materials Group, Department of Earth Sciences, Natural History Museum, Cromwell Road, London SW7 5BD, UK
²School of Physical Sciences, The Open University, Walton Hall, Milton Keynes MK7 6AA, UK
Copyright Elsevier

The CM carbonaceous chondrite meteorites provide a record of low temperature (<150 °C) aqueous reactions in the early solar system. A number of CM chondrites also experienced short-lived, post-hydration thermal metamorphism at temperatures of ∼200 °C to >750 °C. The exact conditions of thermal metamorphism and the relationship between the unheated and heated CM chondrites are not well constrained but are crucial to understanding the formation and evolution of hydrous asteroids. Here we have used position-sensitive-detector X-ray diffraction (PSD-XRD), thermogravimetric analysis (TGA) and transmission infrared (IR) spectroscopy to characterise the mineralogy and water contents of 14 heated CM and ungrouped carbonaceous chondrites. We show that heated CM chondrites underwent the same degree of aqueous alteration as the unheated CMs, however upon thermal metamorphism their mineralogy initially (300–500 °C) changed from hydrated phyllosilicates to a dehydrated amorphous phyllosilicate phase. At higher temperatures (>500 °C) we observe recrystallisation of olivine and Fe-sulphides and the formation of metal. Thermal metamorphism also caused the water contents of heated CM chondrites to decrease from ∼13 wt% to ∼3 wt% and a subsequent reduction in the intensity of the 3 μm feature in IR spectra. We estimate that the heated CM chondrites have lost ∼15 – >65% of the water they contained at the end of aqueous alteration. If impacts were the main cause of metamorphism, this is consistent with shock pressures of ∼20–50 GPa. However, not all heated CM chondrites retain shock features suggesting that some were instead heated by solar radiation. Evidence from the Hayabusa2 and ORSIRS-REx missions suggest that dehydrated materials may be common on the surfaces of primitive asteroids and our results will support upcoming analysis of samples returned from asteroids Ryugu and Bennu.