¹Michael S.Bramble,¹Ralph E.Milliken,²William R.PattersonIII
Icarus (in Press) Link to Journal [https://doi.org/10.1016/j.icarus.2020.114251]
¹Department of Earth, Environmental, and Planetary Sciences, Brown University, Providence, RI, USA
²School of Engineering, Brown University, Providence, RI, USA
Copyright Elsevier

We investigate a suite of synthetic mineral mixtures designed to act as bulk mineralogical analogs to H, L, and LL ordinary chondrite meteorites in order to probe how the thermal emission characteristics of such materials change between ambient and simulated asteroid environmental conditions. Due to the parent body link with certain S-type asteroids, studying these analog mixtures in an environment that is relevant to actual asteroid surfaces advances our understanding of the thermal emission properties of one of the most common regolith types among the main-belt and near-Earth asteroid populations. The observed changes in spectral emissivity features due to cold, vacuum conditions are not as large as previously observed for single mineral (silicate) samples. We interpret this difference to be the result of metallic and opaque components weakening near-surface thermal gradients in the mixtures. As such, we predict that near-surface thermal gradients on ordinary chondrite parent bodies (e.g., S-type asteroids) are likely much weaker than would be inferred from cold, vacuum measurements of individual mineral components. We tested whether the increased spectral contrast observed in fine-grained (<25 μm) samples measured in a cold, vacuum environment increases the efficacy of least squares linear unmixing methods. It is found that the accuracy of such models does not improve relative to measurements made under ambient conditions, thus linear unmixing models are not expected to yield accurate estimates of the modal mineralogy of airless planetary surfaces if they are dominated by fine-grained regolith. Mixtures with coarse particle sizes (125–250 μm) that were modeled using the coarse particle size endmembers yielded results that were largely independent of the environmental conditions, but with larger errors in spectral fits for samples measured in a simulated asteroid environment. At simulated asteroid environmental conditions, the bulk silicate composition and metal content play a more important role in determining the thermal state and brightness temperature of the sample than at ambient conditions. Modest changes in metal content (10–25 wt%) lead to large differences in the brightness temperature of a sample. Under simulated asteroid conditions, an ~10 K increase in maximum brightness temperature that tracks with increased iron content is observed at fine particle sizes (<25 μm) between each of the analog ordinary chondrite groups. Based on these results, it may be difficult to distinguish H, L, and LL compositions of suspected ordinary chondrite parent bodies using only Earth- or space-based thermal emission spectra. This is inferred from the spectral similarity of the analog mixtures and the absence of significant variation in spectral emissivity associated with reported differences in bulk metal content for ordinary chondrites.

¹L.Riu et al. (>10)
Icarus (in Press) Link to Journal [https://doi.org/10.1016/j.icarus.2020.114253]
¹Institut of Space and Astronautical Science (ISAS), Japanese Aerospace eXploration Agency (JAXA), Sagamihara, Japan
Copyright Elsevier

C-type rubble pile asteroid (162173) Ryugu was observed and characterized up close for a year and a half by the instruments on-board the Japanese Aerospace eXploration Agency (JAXA) Hayabusa2 spacecraft. The asteroid exhibits relatively homogeneous spectral characteristics at near-infrared wavelengths (~1.8–3.2 μm), including a very low reflectance factor, a slight positive (“red”) slope towards longer wavelengths, and a narrow absorption feature centered at 2.72 μm that is attributed to the presence of OH− in phyllosilicate minerals. Numerous craters have been identified at the surface that provide good candidates for identifying and studying younger and/or more recently exposed near-surface material to further assess potential spectral/compositional heterogeneities. We present here the results of a spectral survey of all previously identified and referenced craters (Hirata et al. 2020) based on reflectance data acquired by the NIRS3 spectrometer, with an emphasis on the spectral characteristics between different craters as well as with their surrounding terrain. At a global scale, the spectral properties inside and outside of craters are found to be very similar, indicating that subsurface material is either compositionally similar to material at the surface that has a longer exposure age or that material at Ryugu’s optical surface is spectrally altered over relatively short timescales by external factors such as space weathering. Although, the imaging data from ONC camera suites show more morphological and color diversity in craters at a smaller scale than the resolution provided by the NIRS3 instrument, which could indicate a wider compositional diversity on Ryugu than that observed in the near-infrared and discussed in this paper. The 2.72 μm band depth exhibit a slight anti-correlation with the reflectance factor selected at 2 μm, which could indicate different surface properties (e.g., grain size and/or porosity) or different alteration processes (e.g., space weathering, shock metamorphism and/or solar heating). Four different spectral classes were identified based on their reflectance factor at 2 μm and 2.72 μm absorption strength. The most commonly spectral behavior associated with crater floors, is defined by a slightly lower reflectance at 2 μm and deeper band depth. These spectral characteristics are similar to those of subsurface material excavated by the Hayabusa2 small carry-on impactor (SCI) experiment, suggesting these spectral characteristics may represent materials with a younger surface exposure age. Alternatively, these materials may have experienced significant solar heating and desiccation to form finer grains that subsequently migrated towards and preferentially accumulated in areas of low geopotential, such as craters floors. It is believed that the Hayabusa2 mission successfully collected typical surface material as well as darker material excavated by the SCI experiment, and detailed analyses of those samples upon their return will allow for further testing of these formation and alteration hypotheses.

¹Vincent Milesi,^1,2Everett Shock,¹Tucker Ely,³Megan Lubetkin,⁴Sean P.Sylva,^5,6Zara Mirmalek,⁴Christopher R.German,⁷Darlene.S.S.Lim
Planetary and Space Science (in Press) Link to Article [https://doi.org/10.1016/j.pss.2020.105151]
¹GEOPIG, School of Earth and Space Exploration, Arizona State University, Tempe, AZ, 85287, USA
²School of Molecular Sciences, Arizona State University, Tempe, AZ, 85287, USA
³Ocean Exploration Trust, New London, CT, 06320, United States
⁴Woods Hole Oceanographic Institution, Woods Hole, MA, 02543, United States
⁵Harvard University, Cambridge, MA, United States
⁶Bay Area Environmental Research Institute, NASA Ames Research Park, Moffett Field, CA, 94035, United States
⁷Space Science and Astrobiology, NASA Ames Research Center, Moffett Field, CA, 94035, United States

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¹Yevgeniy P. Gurov,²Bevan M. French,¹Vitaliy V. Permiakov
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13601]
¹Institute of Geological Sciences, National Academy of Sciences of Ukraine, O. Gonchara Street 55b, Kiev, 01601 Ukraine
²Department of Paleobiology, Smithsonian Institution, NHB, E‐305B, P.O. Box 37012, Washington, District of Columbia, 20013‐7012 USA
Published by arrangement with John Wiley & Sons

Two forms of carbon‐rich microfossils were discovered in the breccias of the Onaping Formation, Sudbury impact structure. The first form is represented by single particles scattered in the matrix of the breccias and included in the vesicles in altered glass. These particles are leaf‐shaped, stem‐shaped, tubular, and spherical objects ranging from 5–10 μm to 200–300 µm in size. It is suggested that algal flora inhabiting the ocean basin before the Sudbury impact was the source of plant material in the Onaping Formation. The second form of carbon‐rich microparticles in the Onaping Formation is represented by plant detritus in carbon‐bearing fragments of mudstones included in the breccia matrix. The microparticles in the mudstones are mainly ribbon‐like shreds from 3–5 µm to 200–300 µm long, while rare particles have more complex shapes. The matrix of the mudstones is a fine‐grained clay‐like substance with a network of micron‐wide open‐joint fissures. Contents of carbon in the mudstone matrix are 12–15 wt%. Muddy bottom sediments of the pre‐impact sea are supposed as a source of mudstone fragments in the breccias. Fragments of mudstones and carbon‐rich microparticles are an important source of organic carbon in the breccias of the Onaping Formation. Discovery of microfossils in the breccias of the Onaping Formation suggests the presence of a previously unknown complex algal flora that inhabited the pre‐impact sea before the impact event 1.85 billion years ago at the very end of the Paleoproterozoic.

¹Chaves, L.,¹Thompson, M.,²Loeffler, M.
Microscopy and Microanalysis (in Press) Link to Article [DOI: 10.1017/S1431927620022114]
¹Purdue University, West Lafayette, IN, United States
²Northern Arizona University, Flagstaff, AZ, United States

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¹Deng, Z. et al. (>10)
Science Advances 6, abc4941 Link to Article [DOI: 10.1126/sciadv.abc4941]
¹Universite de Paris, Institut de physique du globe de Paris, CNRS, Paris, 75005, France

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¹Welsh, H.,¹Gueorguiev,¹G.-A.,²Kounaves, S.,¹Amundson, R.
Rapid Communications in Mass Spectrometry 34, e8905 Link to Article [DOI: 10.1002/rcm.8905]
¹Department of Environmental Science, Policy and Management, University of California, Berkeley, CA, United States
²Department of Chemistry, Tufts University, Medford, MA, United States

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¹John H.D.Harrison,^1,2Oliver Shorttle,¹Amy Bonsor
Earth and Planetary Science Letters 554, 116694 Link to Article [https://doi.org/10.1016/j.epsl.2020.116694]
¹Institute of Astronomy, University of Cambridge, Madingley Road, Cambridge, CB3 0HA, UK
²Department of Earth Sciences, University of Cambridge, Downing Street, Cambridge, CB2 3EQ, UK
Copyright Elsevier

The loss and gain of volatile elements during planet formation is key for setting their subsequent climate, geodynamics, and habitability. Two broad regimes of volatile element transport in and out of planetary building blocks have been identified: that occurring when the nebula is still present, and that occurring after it has dissipated. Evidence for volatile element loss in planetary bodies after the dissipation of the solar nebula is found in the high Mn to Na abundance ratio of Mars, the Moon, and many of the solar system’s minor bodies. This volatile loss is expected to occur when the bodies are heated by planetary collisions and short-lived radionuclides, and enter a global magma ocean stage early in their history. The bulk composition of exo-planetary bodies can be determined by observing white dwarfs which have accreted planetary material. The abundances of Na, Mn, and Mg have been measured for the accreting material in four polluted white dwarf systems. Whilst the Mn/Na abundances of three white dwarf systems are consistent with the fractionations expected during nebula condensation, the high Mn/Na abundance ratio of GD362 means that it is not (). We find that heating of the planetary system orbiting GD362 during the star’s giant branch evolution is insufficient to produce such a high Mn/Na. We, therefore, propose that volatile loss occurred in a manner analogous to that of the solar system bodies, either due to impacts shortly after their formation or from heating by short-lived radionuclides. We present potential evidence for a magma ocean stage on the exo-planetary body which currently pollutes the atmosphere of GD362.

¹K. Righter,²R. Rowland II,²L. R. Danielson,³M. Humayun,³S. Yang,
⁴N. Mayer (Waeselmann),⁵K. Pando
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13598]
¹NASA Johnson Space Center, Mailcode XI2, 2101 NASA Parkway, Houston, Texas, 77058 USA
²Los Alamos National Laboratory, Mail Stop P952, Los Alamos, New Mexico, 87545 USA
³National High Magnetic Field Laboratory, and Department of Earth, Ocean & Atmospheric Science, Florida State University, Tallahassee, Florida, 32310 USA
⁴Mineralogisch‐Petrographisches Institut (MPI), University of Hamburg, Grindelallee, 48, 20146 Hamburg, Germany
⁵Jacobs JETS, 2101 NASA Parkway, NASA Johnson Space Center, Houston, Texas, 77058 USA
Published by arrangement with John Wiley & Sons

Trace elements and extant and extinct isotopic attributes in Martian meteorites have been used to argue that Mars accreted quickly, differentiated into core and mantle, and established several mantle reservoirs, possibly within 10 Ma of T0. The partitioning of trace elements in the deep mantle has been relatively unstudied, despite the need for such knowledge in understanding magma ocean crystallization and the origin of depleted and enriched mantle reservoirs. The siderophile element composition of the Martian mantle and lithophile isotopic systems such as Sr, Hf, and Nd are thought to record evidence for early metal–silicate equilibrium and deep magma ocean at an intermediate depth and pressure of 800 km or 14 GPa. We have carried out experiments across this pressure range to better understand the mineral/melt partitioning of a wide range of elements. These new data are used to evaluate differentiation models for Mars and to help interpret the available isotopic data. The relatively incompatible nature of Re compared to mildly compatible Os means that the crystallization of a deep magma ocean will lead to residual liquids with superchondritic Re/Os, and solids with subchondritic Re/Os. Such material available in the mantle could be the source of enriched isotopic reservoir that produced shergottites with +γOs values. Slightly subchondritic Re/Os ratios in the crystallizing solids would provide a reservoir that could produce −γOs values. Melting of mixtures of these two enriched and depleted endmembers could explain the Nd‐Os isotopic correlations and systematics of shergottites.

¹Seamus Anderson et al. (>10)
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13593]
Space Science and Technology Center, Curtin University, GPO Box U1987, Perth, Western Australia, 6845 Australia
Published by arrangement with John Wiley & Sons

We present a novel methodology for recovering meteorite falls observed and constrained by fireball networks, using drones and machine learning algorithms. This approach uses images of the local terrain for a given fall site to train an artificial neural network, designed to detect meteorite candidates. We have field tested our methodology to show a meteorite detection rate between 75% and 97%, while also providing an efficient mechanism to eliminate false positives. Our tests at a number of locations within Western Australia also showcase the ability for this training scheme to generalize a model to learn localized terrain features. Our model training approach was also able to correctly identify three meteorites in their native fall sites that were found using traditional searching techniques. Our methodology will be used to recover meteorite falls in a wide range of locations within globe‐spanning fireball networks.