¹Xiao-DuanZou et al. (>10)
Icarus (in Press) Link to Article [https://doi.org/10.1016/j.icarus.2020.114183]
¹Planetary Science Institute, Tucson, AZ, USA
Copyright Elsevier

NASA’s OSIRIS-REx spacecraft arrived at its sampling target, asteroid (101955) Bennu, in December 2018 and started a series of global observation campaigns. Here we investigate the global photometric properties of Bennu as observed by the OSIRIS-REx Visible and InfraRed Spectrometer (OVIRS) over the time period December 9, 2018, to September 26, 2019. In this study we used observations obtained over wavelengths ranging from 0.4 to 3.7 μm, with a solar phase angle range of 5.3° to 132.6°. Our aim is to characterize the global average disk-resolved photometric properties of Bennu with multiple models. The best-fit model is a McEwen model with an exponential phase function and an exponential polynomial partition function. We use this model to correct the OVIRS spectra of Bennu to a standard reference viewing and illumination geometry at visible to infrared wavelengths for the purposes of global spectral mapping. We derive a bolometric Bond albedo map in which Bennu’s surface values range from 0.021 to 0.027. We find a phase reddening effect, and our model is effective at removing this phase reddening. Our average model albedo shows a blueish spectrum with a > 10% absorption feature centered at 2.74 μm. Of all comparisons with previously visited asteroids and comets, only 28P/Neujmin, 2P/Encke, and (162173) Ryugu are darker than Bennu. We find that Bennu is a few percent brighter than Ryugu in the wavelengths respectively observed by the OSIRIS-REx and Hayabusa2 missions (from 0.48 to 0.86 μm). We also compare our spectroscopic photometry of Bennu with the OSIRIS-REx imaging photometry and with ground-based predictions.

^1,2,3Gu, L.,^2,4Wang, N.,^1,2,3Tang, X.,^2,3,5,6Changela, H.G.
Scanning 2020, 8406917 Link to Article [DOI: 10.1155/2020/8406917]
¹Electron Microscopy Laboratory, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing, China
²Key Laboratory of Earth and Planetary Physics, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing, China
³Innovation Academy for Earth Science, Chinese Academy of Sciences, Beijing, 10029, China
⁴University of Chinese Academy of Sciences, Beijing, China
⁵Qian Xuesen Laboratory of Space Technology, Chinese Academy of Space Technology, Beijing, China
⁶Department of Earth and Planetary Science, University of New Mexico, New Mexico, United States

We currently do not have a copyright agreement with this publisher and cannot display the abstract here

^1,2D.N.DellaGiustina et al. (>10)
Science 370, eabc3660 Link to Article [DOI: 10.1126/science.abc3660]
¹Lunar and Planetary Laboratory, University of Arizona, Tucson, AZ, USA.
²Department of Geosciences, University of Arizona, Tucson, AZ, USA.
Reprinted with permission from AAAS

Visible-wavelength color and reflectance provide information about the geologic history of planetary surfaces. Here we present multispectral images (0.44 to 0.89 micrometers) of near-Earth asteroid (101955) Bennu. The surface has variable colors overlain on a moderately blue global terrain. Two primary boulder types are distinguishable by their reflectance and texture. Space weathering of Bennu surface materials does not simply progress from red to blue (or vice versa). Instead, freshly exposed, redder surfaces initially brighten in the near-ultraviolet region (i.e., become bluer at shorter wavelengths), then brighten in the visible to near-infrared region, leading to Bennu’s moderately blue average color. Craters indicate that the time scale of these color changes is ~105 years. We attribute the reflectance and color variation to a combination of primordial heterogeneity and varying exposure ages.

^1,2H.H.Kaplan et al. (>10)
Science 370, eabc3557 Link to Article [DOI: 10.1126/science.abc3557]
¹NASA Goddard Space Flight Center, Greenbelt, MD, USA.
²Southwest Research Institute, Boulder, CO, USA.
Reprinted with permission from AAAS

The composition of asteroids and their connection to meteorites provide insight into geologic processes that occurred in the early Solar System. We present spectra of the Nightingale crater region on near-Earth asteroid Bennu with a distinct infrared absorption around 3.4 micrometers. Corresponding images of boulders show centimeters-thick, roughly meter-long bright veins. We interpret the veins as being composed of carbonates, similar to those found in aqueously altered carbonaceous chondrite meteorites. If the veins on Bennu are carbonates, fluid flow and hydrothermal deposition on Bennu’s parent body would have occurred on kilometer scales for thousands to millions of years. This suggests large-scale, open-system hydrothermal alteration of carbonaceous asteroids in the early Solar System.

¹Amy A. Simon et al. (>10)
Science 370, eabc3522 Link to Article [DOI: 10.1126/science.abc3522]
¹Solar System Exploration Division, NASA Goddard Space Flight Center, Greenbelt, MD, USA.
Reprinted with Permission from AAAS

Asteroid (101955) Bennu is a dark asteroid on an Earth-crossing orbit that is thought to have assembled from the fragments of an ancient collision. We use spatially resolved visible and near-infrared spectra of Bennu to investigate its surface properties and composition. In addition to a hydrated phyllosilicate band, we detect a ubiquitous 3.4-micrometer absorption feature, which we attribute to a mix of organic and carbonate materials. The shape and depth of this absorption feature vary across Bennu’s surface, spanning the range seen among similar main-belt asteroids. The distribution of the absorption feature does not correlate with temperature, reflectance, spectral slope, or hydrated minerals, although some of those characteristics correlate with each other. The deepest 3.4-micrometer absorptions occur on individual boulders. The variations may be due to differences in abundance, recent exposure, or space weathering.

¹Tamilarasan Subramani,¹Kristina Lilova,¹Mykola Abramchuk,¹Kurt D. Leinenweber,¹Alexandra Navrotsky
Proceedings of the National Academy of Sciences of the United States of America Link to Article [DOI:
https://doi.org/10.1073/pnas.2017312117]
¹School of Molecular Sciences and Center for Materials of the Universe, Arizona State University, Tempe, AZ 85281

Iron sulfide minerals are widespread on Earth and likely in planetary bodies in and beyond our solar system. Using measured enthalpies of formation for three magnetic iron sulfide phases: bulk and nanophase Fe3S4 spinel (greigite), and its high-pressure monoclinic phase, we show that greigite is a stable phase in the Fe–S phase diagram at ambient temperature. The thermodynamic stability and low surface energy of greigite supports the common occurrence of fine-grained Fe3S4 in many anoxic terrestrial settings. The high-pressure monoclinic phase, thermodynamically metastable below about 3 GPa, shows a calculated negative P-T slope for its formation from the spinel. The stability of these three phases suggests their potential existence on Mercury and their magnetism may contribute to its present magnetic field.

¹L. M. Barge,¹E. Flores,²D. VanderVelde,¹J. M. Weber,³M. M. Baum,³A. Castonguay
Journal of Geophysical Research (Planets) (in Press) Link to Article [https://doi.org/10.1029/2020JE006423]
¹NASA Jet Propulsion Laboratory, California Institute of Technology, 4800 Oak Grove Drive, Pasadena, CA, 91109 USA
²Department of Chemistry, California Institute of Technology, 1200 E. California Blvd., Pasadena, CA, 91125 USA
³Department of Chemistry, Oak Crest Institute of Science, 128‐132 W. Chestnut Ave., Monrovia, CA, 91016 USA
Published by arrangement with John Wiley & Sons

Geological conditions play a significant role in prebiotic / abiotic organic chemistry, especially when reactive minerals are present. Previous studies of the prebiotic synthesis of amino acids and other products in mineral‐containing systems have shown that a diverse array of compounds can be produced, depending on the experimental conditions. However, these previous experiments have not simulated the effects of varying geochemical conditions, in which factors such as pH, iron redox state, or chemical concentrations may vary over time and space in a natural environment. In geochemical systems that contain overlapping gradients, many permutations of individual conditions could exist and affect the outcome of an organic reaction network. We investigated reactions of pyruvate and glyoxylate, two compounds that are central to the emergence of metabolism, in simulated geological gradients of redox, pH, and ammonia concentration. Our results show that the positioning of pyruvate/glyoxylate reactions in this environmental parameter space determines the organic product distribution that results. Therefore, the distribution pattern of amino acids and alpha‐hydroxy acids produced prebiotically in a system reflects the specific reaction conditions, and would be distinct at various locations in an environment depending on local geochemistry. This is significant for origin of life chemistry in which the composition and function of oligomers could be affected by the environmentally‐driven distribution of monomers available. Also, for astrobiology and planetary science where organic distribution patterns are sometimes considered as a possible biosignature, it is important to consider environmentally‐driven abiotic organic reactions that might produce similar effects.

¹Shaofan Che,¹Adrian J.Brearley
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2020.10.031]
¹Department of Earth and Planetary Sciences, MSC03-2040, 1 University of New Mexico, Albuquerque, NM 87131-0001, USA
Copyright Elsevier

Type C CAIs are a rare group of refractory inclusions in carbonaceous chondrites that are compositionally and isotopically distinct from the more commonly observed igneous CAIs (i.e., Type As and Type Bs). We have investigated a forsterite-bearing Type C CAI ALNH-04 from the Allende CV3 chondrite. This CAI has both textural and compositional similarities to some of the Type C CAIs previously reported; however, there are notable differences that imply that ALNH-04 may have formed from a different precursor from other Type C inclusions. Based on the bulk composition of ALNH-04 and the minor element contents of forsterite, we suggest that the forsterite grains were inherited from a Forsterite-bearing Type B CAI (FoB) precursor. The presence of augite on the periphery of ALNH-04 implies a re-melting event that probably occurred in a chondrule-forming region.

Another interesting feature of ALNH-04 is the secondary iron-alkali-halogen zoning sequence as manifested by varying proportions of nepheline, sodalite, fayalitic olivine, and sulfides in different regions of the CAI. Nepheline ± sodalite have replaced anorthite in the outer part of the inclusion, giving way to the presence of ubiquitous sodalite with minor nepheline, partially replacing anorthite at grain boundaries and fractures in the interior of the inclusion. Sulfides and Fe-bearing olivine form an iron-rich alteration zone. The textural relationships between nepheline and sodalite show no evidence of a direct replacement relationship between the two phases. Combined with the SEM observations, the microstructures are most consistent with a two-stage fluid alteration process: (1) nepheline replaced anorthite in the outer part of the CAI via a fluid with within the stability range of nepheline; (2) a later-stage fluid, with elevated that could preferentially stabilize sodalite, penetrated further into the CAI interior, replacing anorthite with sodalite. The lack of a nepheline-sodalite replacement relationship indicates that the conditions and fluid chemistry were suitable for nepheline and/or sodalite to be stable. Together with other Fe-rich secondary phases, fayalitic olivine may have recorded an increase in pH and of the fluid. These changes were probably induced by the extensive alteration of the outer part of the CAI by feldspathoids. The observed alteration microstructures are consistent with a coupled dissolution-precipitation alteration mechanism. The fluid alteration was also responsible for the formation of Na- and Ca-rich halos in the matrix surrounding the CAI.

We compared ALNH-04 with other CAIs and chondrules showing alkali-halogen-(iron) zoning sequences in Allende, and found that the observed zoning structures are consistent with the two-stage fluid-assisted metasomatic process mentioned above. The different distribution patterns of nepheline and sodalite in plagioclase-rich CAIs, chondrules, and melilite-rich CAIs may be explained by different chemical potential gradients in SiO2 in the fluid. Precipitation of nepheline and sodalite may require a higher SiO2 activity compared to grossular and dmisteinbergite (±secondary anorthite), which controlled the formation location of sodalite during the second fluid alteration event. Fluids with different compositions may be produced by fluid percolation along different directions and pathways, changing convection patterns, or release of water from a differentiated asteroidal interior.

^1,2Jörg Fritz,³Ansgar Greshake,⁴Mariana Klementova,⁵Richard Wirth,⁴Lukas Palatinus,⁶Reidar G. Trønnes,^3,7,8Vera Assis Fernandes,⁹Ute Böttger,¹⁰Ludovic Ferrière
American Mineralogist 105, 1704–1711 Link to Article [http://www.minsocam.org/msa/ammin/toc/2020/Abstracts/AM105P1704.pdf]
¹Zentrum für Rieskrater und Impaktforschung, Nördlingen, Vordere Gerbergasse 3, D-86720 Nördlingen,
Germany. ORCID 0000-0002-6333-4775
²Saalbau Weltraum Projekt, Liebigstraße 6, D-64646 Heppenheim, Germany
³Museum für Naturkunde Berlin, Invalidenstrasse 43, D-10115 Berlin, Germany. ORCID 0000-0001-6475-9751
⁴Institute of Physics of the Czech Academy of Science, v.v.i., Na Slovance 2, 182 21 Prague, Czech Republic. † ORCID 0000-0002-8987-8164
⁵Helmholtz-Zentrum Potsdam–Deutsches GeoForschungsZentrum, Sektion 3.5 Grenzflächen-Geochemie, Telegrafenberg, D-14473 Potsdam, Germany
⁶Natural History Museum and Centre for Earth Evolution and Dynamics (CEED), University of Oslo, N-0315 Oslo,
Norway. ORCID 0000-0002-4458-5624
⁷Department of Earth and Environmental Sciences, University of Manchester, Williamson Building, Oxford Road, M13 9PL Manchester, U.K.
ORCID 0000-0003-0848-9229
⁸Instituto Dom Luiz (IDL), Faculdade de Ciências, Universidade de Lisboa, Campo Grande, 1749-016, Lisboa, Portugal
⁹Institut für Optische Sensorsysteme, Deutsches Zentrum für Luft und Raumfahrt Berlin, Rutherfordstrasse 2, D-12489 Berlin, Germany
¹⁰Natural History Museum, Burgring 7, A-1010 Vienna, Austria. ORCID 0000-0002-9082-6230
Copyright: The Mineralogical Society of America

We report on the occurrence of a new high-pressure Ca-Al-silicate in localized shock melt pockets found in the feldspatic lunar meteorite Oued Awlitis 001 and discuss the implications of our discovery.
The new mineral crystallized as tiny, micrometer-sized, acicular grains in shock melt pockets of roughly anorthitic bulk composition. Transmission electron microscopy based three-dimensional electron diffraction (3D ED) reveals that the CaAl4Si2O11 crystals are identical to the calcium aluminum silicate (CAS) phase first reported from static pressure experiments. The new mineral has a hexagonal structure, with a space group of P63/mmc and lattice parameters of a = 5.42(1) Å; c = 12.70(3) Å; V = 323(4) Å3;Z = 2. This is the first time 3D ED was applied to structure determination of an extraterrestrial mineral.
The International Mineralogical Association (IMA) has approved this naturally formed CAS phase as the new mineral “donwilhelmsite” [CaAl4Si2O11], honoring the U.S. lunar geologist Don E. Wilhelms.
On the Moon, donwilhelmsite can form from the primordial feldspathic crust during impact cratering events. In the feldspatic lunar meteorite Oued Awlitis 001, needles of donwilhelmsite crystallized in ~200 mm sized shock melt pockets of anorthositic-like chemical composition. These melt pockets quenched within milliseconds during declining shock pressures. Shock melt pockets in meteorites serve as natural crucibles mimicking the conditions expected in the Earth’s mantle. Donwilhelmsite forms in the Earth’s mantle during deep recycling of aluminous crustal materials, and is a key host for Al and Ca of subducted sediments in most of the transition zone and the uppermost lower mantle (460–700 km). Donwilhelmsite bridges the gap between kyanite and the Ca-component of clinopyroxene at low pressures and the Al-rich Ca-ferrite phase and Ca-perovskite at high-pressures. In ascending buoyant mantle plumes, at about 460 km depth, donwilhelmsite is expected to break down into minerals such as garnet, kyanite, and clinopyroxene. This process may trigger minor partial melting, releasing
a range of incompatible minor and trace elements and contributing to the enriched mantle (EM1 and EM2) components associated with subducted sedimentary lithologies.

^1,2Ryoichi Nakada,²Tomohiro Usui,³Masashi Ushioda,⁴Yoshio Takahashi
American Mineralogist 105, 1695–1703 Link to Article [http://www.minsocam.org/msa/ammin/toc/2020/Abstracts/AM105P1695.pdf]
¹Kochi Institute for Core Sample Research, Japan Agency for Marine-Earth Science and Technology (JAMSTEC), Monobe 200, Nankoku, Kochi 783-8502, Japan
²Earth-Life Science Institute, Tokyo Institute of Technology, Meguro, Tokyo 152-8550, Japan
³Geological Survey of Japan, National Institute of Advanced Industrial Science and Technology (AIST), Central 7, 1-1-1 Higashi, Tsukuba, Ibaraki 305-8567, Japan
⁴Department of Earth and Planetary Science, The University of Tokyo, Hongo 7-3-1, Bunkyo, Tokyo 113-0033, Japan
Copyright: The Mineralogical Society of America