¹Thomas B.McCord,¹Jean-Philippe Combe,²Julie C.Castillo-Rogez,³Harry Y.McSween,⁴Thomas H.Prettyman
Geochemistry [Chemie der Erde] (in Press) Link to Article [https://doi.org/10.1016/j.chemer.2021.125745]
¹The Bear Fight Institute, 22 Fiddler’s Road, P.O. Box 667, Winthrop, WA, 98862 USA
²Jet Propulsion Laboratory, California Institute of Technology, 4800 Oak Grove Drive, Pasadena, CA, 91109 USA
³Department of Earth & Planetary Sciences, University of Tennessee, Knoxville, TN, 37996-1410, USA
⁴Planetary Science Institute, 1700 East Fort Lowell, Suite 106, Tucson, AZ, 85719-2395, USA
Copyright Elsevier

Ceres, a nearly 1000-km diameter body located in the Solar System’s main asteroid belt, has been classified under many categories: planet, comet, asteroid, minor planet and, presently, dwarf planet. No matter what the designation, Ceres has experienced major planetary processes. Its evolution has been controlled by water, making it a most unusual, interesting and accessible inner Solar System object that can inform the evolution of outer Solar System moons and other dwarf planets. Early telescopic observations suggested a hydroxylated mineralogy similar to carbonaceous chondrite meteorites and a size and mass indicating a bulk density that implied a water content of 17−27 wt%. Thermodynamic modeling of Ceres’ evolution indicated that thermal aqueous evolution likely occurred. The Dawn Mission produced a huge increase in our understanding of Ceres, confirming but vastly extending the early knowledge. Dawn, carrying multispectral cameras, a visible-infrared imaging spectrometer and a nuclear spectrometer, orbited Ceres between 2015–2018 (after orbiting Vesta) at a number of different altitudes, ultimately reaching 35 km from the surface at periapsis. Observations of almost the entire surface and gravity field mapping revealed multiple geological and internal features attributed to the effects of water. The surface displays cryovolcanic-like and flow structures, exposed phyllosilicates, carbonates, evaporites and water ice. The subsurface shows partial differentiation, decreasing viscosity with depth, and lateral density heterogeneity. Ceres appears to be geologically active today and possesses liquid water/brine pockets or even an extended liquid layer in the interior, confirming an “Ocean World” designation in today’s vernacular.

¹Keng-Hsien Chao,¹Rebecca deGraffenried,¹Mackenzie Lach,¹William Nelson,¹Kelly Truax,¹Eric Gaidos
Geochemistry [Chemie der Erde] (in Press) Link to Article [https://doi.org/10.1016/j.chemer.2020.125735]
¹Department of Earth Sciences, University of Hawai’i at Mānoa, Honolulu, HI 96822, USA
Copyright Elsevier

The magma ocean concept was first conceived to explain the geology of the Moon, but hemispherical or global oceans of silicate melt could be a widespread “lava world” phase of rocky planet accretion, and could persist on planets on short-period orbits around other stars. The formation and crystallization of magma oceans could be a defining stage in the assembly of a core, origin of a crust, initiation of tectonics, and formation of an atmosphere. The last decade has seen significant advances in our understanding of this phenomenon through analysis of terrestrial and extraterrestrial samples, planetary missions, and astronomical observations of exoplanets. This review describes the energetic basis of magma oceans and lava worlds and the lava lake analogs available for study on Earth and Io. It provides an overview of evidence for magma oceans throughout the Solar System and considers the factors that control the rocks these magma oceans leave behind. It describes research on theoretical and observed exoplanets that could host extant magma oceans and summarizes efforts to detect and characterize them. It reviews modeling of the evolution of magma oceans as a result of crystallization and evaporation, the interaction with the underlying solid mantle, and the effects of planetary rotation. The review also considers theoretical investigations on the formation of an atmosphere in concert with the magma ocean and in response to irradiation from the host star, and possible end-states. Finally, it describes needs and gaps in our knowledge and points to future opportunities with new planetary missions and space telescopes to identify and better characterize lava worlds around nearby stars.

¹R.Brasser,^2,3S.J.Mojzsis,⁴S.C.Werner,⁵O.Abramov
Icarus (in Press) Link to Article [https://doi.org/10.1016/j.icarus.2021.114389]
¹Earth Life Science Institute, Tokyo Institute of Technology, Ookayama, Meguro-ku, Tokyo 152-8550, Japan
²Department of Geological Sciences, University of Colorado, Boulder, CO 80309, USA
³Origins Research Institute, Research Centre for Astronomy and Earth Sciences, H-1112 Budapest, Hungary
⁴Centre for Earth Evolution and Dynamics, University of Oslo, N-0315 Oslo, Norway
⁵Planetary Science Institute, Tucson, AZ 85719, USA
Copyright Elsevier

Subsequent to the Moon’s formation, late accretion to the terrestrial planets strongly modified the physical and chemical nature of silicate crusts and mantles. This alteration came in the form of melting through impacts, as well as the belated addition of volatiles and the highly siderophile elements (HSEs). Even though late accretion is well established as having been an important process in the evolution of the young solar system, its intensity and temporal decline remain subject to debate. Much of this deliberation hinges on what can be inferred about late accretion to the Moon from its computed mantle HSE abundances. Current debate centres on whether the lunar HSE record is representative of its whole late accretion history or alternatively that these were only retained in the mantle and crust after a particular time, and if so, when. Here we employ improved Monte Carlo impact simulations of late accretion onto the Moon and Mars and present an updated chronology based on new dynamical simulations of leftover planetesimals and the E-belt – a now-empty hypothesised inner extension of the asteroid belt (Bottke et al., 2012). We take into account the inefficient retention of colliding material. The source of impactors on both planetary bodies is assumed to be the same, hence we use constraints from both objects simultaneously. We compute the crater and basin densities on the Moon and Mars, the largest objects to strike these planets and the amount of material they accreted. Outputs are used to infer the mass in leftover planetesimals at a particular time period, which is then compared to the lunar HSE abundance. From this estimate we calculate a preferred lunar HSE retention age of ca. 4450 Ma which means that the modelled lunar mantle HSE abundances trace almost all of lunar late accretion. Based on our results, the surface ages of the lunar highlands are at least 4370 Ma. We find that the mass of leftover planetesimals with diameters Di < 300 km at 4500 Ma that best fits the crater chronology is approximately 2 × 10−3 Earth mass (ME) while the mass of the E-belt was fixed at 4.5 × 10−4 ME. We also find that a leftover planetesimal mass in excess of 0.01 ME results in a lunar HSE retention age younger than major episodes of lunar differentiation and crust formation, which in turn violates geochemical constraints for the timing and intensity of late accretion to the Earth (Mojzsis et al., 2019).

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.