¹C.M.Lisse et al. (>10)
The Astrophysical Journal 894, 116 Link to Article [DOI https://doi.org/10.3847/1538-4357/ab7b80]
¹ JHU-APL, 11100 Johns Hopkins Road, Laurel, MD 20723, USA; carey.lisse@jhuapl.edu, ron.vervack@jhuapl.edu

We report here time-domain infrared spectroscopy and optical photometry of the HD 145263 silica-rich circumstellar-disk system taken from 2003 through 2014. We find an F4V host star surrounded by a stable, massive 1022–1023 kg (MMoon to MMars) dust disk. No disk gas was detected, and the primary star was seen rotating with a rapid ~1.75 day period. After resolving a problem with previously reported observations, we find the silica, Mg-olivine, and Fe-pyroxene mineralogy of the dust disk to be stable throughout and very unusual compared to the ferromagnesian silicates typically found in primordial and debris disks. By comparison with mid-infrared spectral features of primitive solar system dust, we explore the possibility that HD 145263’s circumstellar dust mineralogy occurred with preferential destruction of Fe-bearing olivines, metal sulfides, and water ice in an initially comet-like mineral mix and their replacement by Fe-bearing pyroxenes, amorphous pyroxene, and silica. We reject models based on vaporizing optical stellar megaflares, aqueous alteration, or giant hypervelocity impacts as unable to produce the observed mineralogy. Scenarios involving unusually high Si abundances are at odds with the normal stellar absorption near-infrared feature strengths for Mg, Fe, and Si. Models involving intense space weathering of a thin surface patina via moderate (T < 1300 K) heating and energetic ion sputtering due to a stellar super-flare from the F4V primary are consistent with the observations. The space-weathered patina should be reddened, contain copious amounts of nanophase Fe, and should be transient on timescales of decades unless replenished.

¹Alexey Potapov,¹Cornelia Jäger,²Thomas Henning
The Astrophysical Journal 894, 110 Link to Article [DOI https://doi.org/10.3847/1538-4357/ab86b5]
¹Laboratory Astrophysics Group of the Max Planck Institute for Astronomy at the Friedrich Schiller University Jena, Institute of Solid State Physics, Helmholtzweg 3, D-07743 Jena, Germany; alexey.potapov@uni-jena.de
²Max Planck Institute for Astronomy, Königstuhl 17, D-69117 Heidelberg, Germany

The catalytic role of dust grain surfaces in the thermal reaction CO2 + 2NH3 → NH4+NH2COO− was recently demonstrated by our group. The rate coefficients for the reaction at 80 K on the surface of nanometer-sized carbon and silicate grains were measured to be up to three times higher compared to the reaction rate coefficients measured on KBr. In this study, the reaction was performed on carbon grains and on KBr in the extended temperature range of 50–80 K and with the addition of water ice. The reaction activation energy was found to be about three times lower on grains compared to the corresponding ice layer on KBr. Thus, the catalytic role of the dust grain surface in the studied reaction can be related to a reduction of the reaction barrier. Addition of water to NH3:CO2 ice on grains slowed the reaction down. At the H2O:CO2 ratio of 5:1, the reaction was not detected on the experimental timescale. This result calls into question the thermal formation of ammonium carbamate in dense molecular clouds and outer regions of protostellar and protoplanetary environments with dominating water ice mantle chemistry. However, it can still happen in inner regions of protostellar and protoplanetary environments in crystalline ices.

¹Florian Kirchschlager,¹M. J. Barlow,¹Franziska D. Schmidt
The Astrophysical Journal 893, 70 Link to Article [DOI https://doi.org/10.3847/1538-4357/ab7db8]
¹Department of Physics and Astronomy, University College London, Gower Street, London WC1E 6BT, UK; f.kirchschlager@ucl.ac.uk

Core-collapse supernovae can condense large masses of dust post-explosion. However, sputtering and grain–grain collisions during the subsequent passage of the dust through the reverse shock can potentially destroy a significant fraction of the newly formed dust before it can reach the interstellar medium. Here we show that in oxygen-rich supernova remnants like Cassiopeia A, the penetration and trapping within silicate grains of the same impinging ions of oxygen, silicon, and magnesium that are responsible for grain surface sputtering can significantly reduce the net loss of grain material. We model conditions representative of dusty clumps (density contrast of χ = 100) passing through the reverse shock in the oxygen-rich Cassiopeia A remnant and find that, compared to cases where the effect is neglected as well as facilitating the formation of grains larger than those that had originally condensed, ion trapping increases the surviving masses of silicate dust by factors of up to two to four, depending on initial grain radii. For higher density contrasts (χ gsim 180), we find that the effect of gas accretion on the surface of dust grains surpasses ion trapping, and the survival rate increases to ~55% of the initial dust mass for χ = 256.

^1,2Bidong Zhang,³Yangting Lin,¹Desmond E.Moser,²Paul H.Warren,³Jialong Hao,¹Ivan R.Barker,¹Sean R.Shieh,¹Audrey Bouvierd
Earth and Planetary Science Letters 569, 117046 Link to Article [https://doi.org/10.1016/j.epsl.2021.117046]
¹The University of Western Ontario, Department of Earth Sciences, London, Ontario N6A 3K7, Canada
²Department of Earth, Planetary, and Space Sciences, University of California, Los Angeles, CA 90095, USA
³Key Laboratory of Earth and Planetary Physics, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 100029, China
⁴Universität Bayreuth, Bayerisches Geoinstitut, Bayreuth 95447, Germany
Copyright Elsevier

The lunar Mg-suite magmatic rocks are commonly thought to represent mafic intrusions into the anorthositic flotation crust of the lunar magma ocean (LMO). Their geochronology is, therefore, important for constraining evolution models of the LMO. Petrogenetic models of the Mg-suite hold that their parent magmas were derived from primary LMO sources (Mg-cumulates, An-rich plagioclase, and melts enriched in KREEP—potassium, rare earth elements, and phosphorus). Previous radiogenic isotopic age interpretations of Mg-suite and putatively older, related ferroan anorthosites (FANs) overlap over a 200-million-year interval. The Apollo 78238 norite is an exemplary Mg-suite rock, with a relict coarse igneous texture modified by shock metamorphism. In-situ secondary ion mass spectrometry U-Pb analyses of zircon and baddeleyite in 78238 yield discordant arrays, attributed to recent impact metamorphism, with upper intercepts that constrain its crystallization age. The four oldest baddeleyite analyses give a weighted mean 207Pb/206Pb age of 4332 ± 18 Ma (2σ, MSWD = 0.06, P = 0.98), which is interpreted as the crystallization age of the norite. The overlap of the baddeleyite age with previously reported Sm-Nd and Pb-Pb mineral isochron ages for 78238 (Edmunson et al., 2009) supports a moderately fast cooling of the norite. Moreover, it is distinguishably younger than the most precisely dated sample of FAN (Apollo 60025), measured at 4360 ± 3 Ma by Sm-Nd and Pb-Pb mineral isochrons (Borg et al., 2011). Together with the baddeleyite 207Pb/206Pb age of Apollo Mg-suite troctolite 76535 at 4328 ± 8 Ma (White et al., 2020), the chronological record of the 78238 norite indicates a significant Mg-suite magmatic event at 4.33 Ga and a lower age limit on LMO differentiation.

¹Thomas Kenkmann
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13657]
¹Institute of Earth and Environmental Sciences, Geology, Albert-Ludwigs-Universität Freiburg, Albertstraße 23b, Freiburg im Breisgau, 79104 Germany
Published by arrangement with John Wiley & Sons

The number of newly discovered and confirmed impact structures on earth is growing continuously. In this review paper, the main attributes of 198 confirmed impact structures and 10 further structures, for which final confirmation based on the identification of shock features is not yet entirely satisfying, are presented. The impact craters are compared statistically, with regard to their morphology, structure, and status of erosion or burial. The size– and age–frequency distributions of terrestrial impact structures are presented. Additional aspects concern target petrography and shock effects found in the craters. Based on the discovery statistics of presently known crater structures, an estimate can be made of the number of craters that await discovery. The paper is complementary to the recently published atlas of terrestrial impact structures by Gottwald et al. (2020).

¹J.Gattacceca et al. (>10)
Earth & Plantetary Science Letters 569, 117049 Link to Article [https://doi.org/10.1016/j.epsl.2021.117049]
¹CNRS, Aix Marseille Univ, IRD, INRAE, CEREGE, 13545 Aix-en-Provence, France
Copyright Elsevier

Glassy ejecta are associated to a limited number of impact craters, and yet hold key information about hypervelocity impact processes. Here we report on the discovery of a ∼650 km2 impact glass strewnfield in the Central Depression of the Atacama Desert. These cm-sized splash-form objects, that we refer to as atacamaites, are essentially composed of a dacitic glass formed by high-temperature melting of local magmatic rocks, with the addition of a variable iron meteorite contamination, 5 wt.% on average. The most likely nature for the impactor is the IIAB iron group. The fission-track plateau method, on two samples, yielded a mean formation age of Ma. No associated impact crater has been discovered so far, suggesting it may be a relatively small, km-sized crater. The glassy nature, aerodynamic shapes, elevated formation temperature, and low water content are reminiscent of tektites. However, their small size, heterogeneity, oxidation state, significant contamination by the impactor, and likely more proximal provenance distinguish them from tektites. Atacamaites have no equivalent among the few known terrestrial ejected impact glasses, and increase the intriguing diversity of such products that we propose to name “tektoids”.

^1,2Guillaume Florin,¹Béatrice Luais,^2,3Olivier Alard,²Tracy Rushmer
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13698]
¹CNRS, CRPG, Université de Lorraine, Nancy, F-5400 France
²Department of Earth and Planetary Sciences, Macquarie University, Sydney, New South Wales, 2109 Australia
³Géosciences Montpellier, UMR 5243, CNRS & Université Montpellier, Montpellier, 34095 France
Published by arrangement with John Wiley & Sons

We analyzed the highly siderophile element (HSE) contents and bulk Ge isotopic compositions of large metal grains in the CB chondrites Bencubbin (CBa), Gujba (CBa), and HaH 237 (CBb). Our results suggest that the large grains were formed by the aggregation of smaller condensed grains, and the two Benccubinite groups are distinguishable based on their bulk metal δ74/70Ge mass-dependent isotopic values of 0.99 ± 0.30‰ (CBa) and −0.65 ± 0.10‰ (CBb). Based on our observations of these three samples, the isotopic compositions of metal in CBa chondrites are best explained by condensation at slow cooling rates in the center of an impact plume, whereas the metal in CBb chondrites formed under fast cooling rates along the plume edges. We also analyzed the Ge contents and isotopic compositions of the core, intermediate, and rim fractions of two Gujba metal grains, which were separated by sequential digestion. These results show a gradual decrease in δ74/70Ge and [Ge] from core to rim. We suggest that these δ74Ge zonations result from near-equilibrium condensation and evaporation processes in a heterogeneous plume. We propose a model for their formation in which (1) small grains (to become grain cores) condensed at equilibrium; (2) these grains were transported to a warmer region of the plume where they reached temperatures lower than that of Fe-Ni condensation, but high enough for the rapid evaporation of Ge; (3) Ge evaporation followed by slow cooling enriched the grains in heavy Ge isotopes and the surrounding gas in light Ge isotopes; and (4) equilibrium recondensation of metal from the gas and around the small grains formed the light Ge isotopic zonations observed in grain rims.

¹K. D. Burgess,¹R. M. Stroud
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13692]
¹U.S. Naval Research Laboratory, Washington, District of Columbia, 20375 USA
Published by arrangement with John Wiley & Sons

The return of samples from S-type asteroid 25143 Itokawa have enabled significant improvements in our understanding of space weathering on asteroids and the link between S-type asteroids and ordinary chondrites. We report on three new particles, providing details on space weathering of adjacent grains within a particle and several different phases. The features we observe are consistent with formation via irradiation from the solar wind, as opposed to micrometeoroid bombardment. We also see differences in the degree of weathering for grains collected from the two different touchdown locations on the asteroid. Continued analysis of new grains from Itokawa allow us to draw a more complete picture of the variety of space weathering features and the processes that lead to their formation on Itokawa and other airless bodies.

^1,2Giovanni Pratesi,¹Vanni Moggi Cecchi,³Richard C. Greenwood,³Ian A. Franchi,³Samantha J. Hammond,⁴Mario Di Martino,^4,5Dario Barghini,⁵Carla Taricco,⁶Albino Carbognani,⁴Daniele Gardiol
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13695]
¹Dipartimento di Scienze della Terra, Università degli Studi di Firenze, Via G. La Pira 4, Florence, 50121 Italy
²INAF—Istituto di Astrofisica e Planetologia Spaziali, Via Fosso del Cavaliere 100, Rome, 00133 Italy
³Planetary and Space Sciences, The Open University, Milton Keynes, MK7 6AA UK
⁴INAF—Osservatorio Astrofisico di Torino, Via Osservatorio 20, Turin, 10025 Italy
⁵Dipartimento di Fisica, Università degli Studi di Torino, Via P. Giuria 1, Pino Torinese, 10125 Italy
⁶INAF—Osservatorio di Astrofisica e Scienza dello Spazio, Via Piero Gobetti 93/3, Bologna, 40129 Italy
Published by arrangement with John Wiley & Sons

The Cavezzo meteorite, which fell on January 1, 2020, is the first meteorite detected and recovered by the Italian PRISMA Fireball Network. Two specimens, weighing 3.12 g (specimen 1) and 52.19 g (specimen 2), were collected 3 days after the bolide was observed, thanks to an effective media campaign that encouraged the involvement of local people. The two specimens of this meteorite have not only completely different lithological characteristics but also a different geochemistry and oxygen isotopic composition as well. Specimen 1 is anomalous both for the textural–structural features, varying seamlessly from chondritic to “achondritic,” and a very unusual modal mineralogy—such as the relatively high amount of olivine (63.1 vol%), plagioclase (18.2 vol%), high-Ca pyroxene (10.3 vol%), and chlorapatite (2.1 vol%); and the unusually low content of low-Ca pyroxene (5.8 vol%), metal (0.1 vol%), and troilite (much lesser than 0.1 vol%)—although the compositional values for olivine (Fa 24.24 mol%) and low-Ca pyroxene (Fs 20.41 mol%) appear to be similar to those of the L chondrite group. Conversely, in specimen 2, not only the texture and the crystal chemistry but also the modal mineralogy (low-Ca pyroxene much more abundant than high-Ca pyroxene and occurrence of metal and sulfides) look like those of an ordinary L chondrite. The differences between the two specimens are also confirmed by geochemistry. The oxygen isotope composition of specimen 1 plots at the boundary between the H and L groups (δ17O‰ 3.250; δ18O‰ 4.736; Δ17O‰ 0.788) whereas specimen 2 plots at the boundary of the L and LL fields (δ17O‰ 3.737; δ18O‰ 4.957; Δ17O‰ 1.159). The bulk chemistry shows a different content of many minor and trace elements (including rare earth elements), such as a strong depletion of siderophile and chalcophile elements in specimen 1. The two specimens then do not contain fragments of each other, thus preventing us from classifying this “double face” meteorite as an ordinary chondrite breccia. In detail, specimen 1 can be considered a “xenolith” in which chondritic structure and igneous texture coexist without discontinuity, and therefore, it represents a previously unsampled portion of the L parent body. In summary, these findings support the classification of Cavezzo as an L5 anomalous chondrite.

¹Antara Sen et al. (>10)
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13699]
¹Department of Physics and Astronomy, Ithaca College, Ithaca, New York, 14850 USA
Published by arrangement with John Wiley & Sons

Data returned by the OSIRIS-REx (Origins, Spectral Interpretation, Resource Identification, and Security–Regolith Explorer) spacecraft have shown that asteroid (101955) Bennu has a globally low-albedo surface covered in boulders with diverse texture, color, and albedo properties, and an aqueously altered composition dominated by phyllosilicates. To test whether Bennu’s color and albedo diversity could be caused by texture and/or composition variations, we performed a laboratory-based study using simple two-component mixtures (called “mudpies”) of the phyllosilicate saponite and carbon-rich opaques. Each mudpie is prepared in four different textures: fine powder, coarse particles, sanded slab, and textured rock. We find that a sanded slab made from 90% saponite and 10% lampblack is a good analog for Bennu, and the color and albedo changes due to texture variations are substantial. At 550 nm, texture changes alone can create up to 36% brightness contrast, and in color measured as a 473 nm/847 nm ratio, texture changes can provide up to 18% color contrast. In comparison, Bennu shows approximately 25% albedo and <1% color contrasts from boulder type to boulder type. These findings suggest that if texture contributes to color on Bennu, the texture variations are typically more subtle than what we simulated in the laboratory. According to our study, the color and albedo properties of different boulder types on Bennu are consistent with different concentrations of carbon-rich opaques (and possibly consistent with variations in carbonate concentration). The variations within each boulder group are consistent with textural differences.