^1,2Sheng Gou,^1,3Zongyu Yue,^1,2,3Kaichang Di,¹Wenhui Wan,¹Zhaoqin Liu,^1,2Bin Liu,¹Man Peng,¹Yexin Wang,⁴Zhiping He,⁴Rui Xu
Earth and Planetary Science Letters 535, 116117 Link to Article [https://doi.org/10.1016/j.epsl.2020.116117]
¹State Key Laboratory of Remote Sensing Science, Aerospace Information Research Institute, Chinese Academy of Sciences, Beijing 100101, China
²State Key Laboratory of Lunar and Planetary Sciences, Macau University of Science and Technology, Macau, China
³CAS Center for Excellence in Comparative Planetology, Hefei 230026, China
⁴dKey Laboratory of Space Active Opto-Electronics Technology, Shanghai Institute of Technical Physics, Chinese Academy of Sciences, Shanghai 200083, China
Copyright Elsevier

Space weathering introduces confounding effects on visible and near infrared reflectance spectra of airless bodies, which considerably darkens the reflectance, reddens the continuum slope and suppresses absorption features. It’s mainly attributed to the gradual formation and accumulation of submicroscopic metallic iron (SMFe) on regolith grains. In situ spectral measurements from Chang’e-4 rover provide a unique opportunity to investigate the space weathering effects on the intact lunar farside regolith. SMFe abundance at the landing site, which is 0.32±0.06 wt.%, is retrieved from in situ measured reflectance spectra by using Hapke model. The derived Is/FeO maturity index (82±15) indicates the Finsen crater ejecta-sourced regolith is mature, which is consistent with the geologic background that it had experienced about 3.7 Ga space weathering.

¹K.K.Larsen,¹D.Wielandt,¹M.Schiller,^1,2A.N.Krot,¹M.Bizzarro
Earth and Planetary Science Letters 535, 116088 Link to Article [https://doi.org/10.1016/j.epsl.2020.116088]
¹Centre for Star and Planet Formation, Natural History Museum of Denmark, University of Copenhagen, Copenhagen DK-1350, Denmark
²Hawai’i Institute of Geophysics and Planetology, University of Hawai’i at Manoa, HI 96822, USA
Copyright Elsevier

Refractory inclusions [Ca-Al-rich Inclusions (CAIs) and Amoeboid Olivine Aggregates (AOAs)] in primitive meteorites are the oldest Solar System solids. They formed in the hot inner protoplanetary disk and, as such, provide insights into the earliest disk dynamics and physicochemical processing of the dust and gas that accreted to form the Sun and its planetary system. Using the short-lived 26Al to 26Mg decay system, we show that bulk refractory inclusions in CV (Vigarano-type) and CR (Renazzo-type) carbonaceous chondrites captured at least two distinct 26Al-rich (26Al/27Al ratios of ∼5 × 10−5) populations of refractory inclusions characterized by different initial 26Mg/24Mg isotope compositions (μ26Mg*0). Another 26Al-poor CAI records an even larger μ26Mg*0 deficit. This suggests that formation of refractory inclusions was punctuated and recurrent, possibly associated with episodic outbursts from the accreting proto-Sun lasting as short as <8000 yr. Our results support a model in which refractory inclusions formed close to the hot proto-Sun and were subsequently redistributed to the outer disk, beyond the orbit of Jupiter, plausibly via stellar outflows with progressively decreasing transport efficiency. We show that the magnesium isotope signatures in refractory inclusions mirrors the presolar grain record, demonstrating a mutual exclusivity between 26Al enrichments and large nucleosynthetic Mg isotope effects. This suggests that refractory inclusions formed by incomplete thermal processing of presolar dust, thereby inheriting a diluted signature of their isotope systematics. As such, they record snapshots in the progressive sublimation of isotopically anomalous presolar carriers through selective thermal processing of young dust components from the proto-Solar molecular cloud. We infer that 26Al-rich refractory inclusions incorporated 26Al-rich dust which formed <5 Myr prior to our Sun, whereas 26Al-poor inclusions (such as FUN- and PLAC-type CAIs) incorporated >10 Myr old dust.

^1,2,3C.Cartier,⁴O.Namur,⁵ L.R.Nittler,⁵S.Z.Weider,⁶E.Crapster-Pregont,⁶A.Vorburger,⁶E.A.Franck,¹B.Charlier
Earth and Planetary Science Letters 534, 116108 Link to Article [https://doi.org/10.1016/j.epsl.2020.116108]
¹Département de Géologie, Université de Liège, 4000, Sart Tilman, Belgium
²Laboratoire Magmas et Volcans, Université Blaise Pascal, Clermont-Ferrand, 63038, France
³CRPG, CNRS, Université de Lorraine, UMR 7358, Vandoeuvre-lès-Nancy, 54501, France
⁴Department of Earth and Environmental Sciences, KU Leuven, Leuven, 3001, Belgium
⁵Carnegie Institution of Washington, Department of Terrestrial Magnetism, Washington, DC 20015, USA
⁶Department of Earth and Planetary Sciences, American Museum of Natural History, New York, NY 10024, USA
Copyright Elsevier

In this study we investigate the likeliness of the existence of an iron sulfide layer (FeS matte) at the core-mantle boundary (CMB) of Mercury by comparing new chemical surface data obtained by the X-ray Spectrometer onboard the MESSENGER spacecraft with geochemical models supported by high-pressure experiments under reducing conditions. We present a new data set consisting of 233 Ti/Si measurements, which combined with Al/Si data show that Mercury’s surface has a slightly subchondritic Ti/Al ratio of 0.035 ± 0.008. Multiphase equilibria experiments show that at the conditions of Mercury’s core formation, Ti is chalcophile but not siderophile, making Ti a useful tracer of sulfide melt formation. We parameterize and use our partitioning data in a model to calculate the relative depletion of Ti in the bulk silicate fraction of Mercury as a function of a putative FeS layer thickness. By comparing the model results and surface elemental data we show that Mercury most likely does not have a FeS layer, and in case it would have one, it would only be a few kilometers thick (<13km). We also show that Mercury’s metallic Fe(Si) core cannot contain more than ∼1.5 wt.% sulfur and that the formation of this core under reducing conditions is responsible for the slightly subchondritic Ti/Al ratio of Mercury’s surface.

^1,2Brendan A. Haas,^1,2Christine Floss,³Rhonda M. Stroud,^1,2Ryan C. Ogliore
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13438]
¹Laboratory for Space Sciences, Washington University, St. Louis, Missouri, 63130 USA
²Physics Department, Washington University, St. Louis, Missouri, 63130 USA
³Materials Science and Technology Division, Naval Research Laboratory, 4555 Overlook Ave. SW, Washington, District of Columbia, 20375 USA
Published by arrangement with John Wiley & Sons

Aluminum foils from the Stardust cometary dust collector contain impact craters formed during the spacecraft’s encounter with comet 81P/Wild 2 and retain residues that are among the few unambiguously cometary samples available for laboratory study. Our study investigates four micron‐scale (1.8–5.2 μm) and six submicron (220–380 nm) diameter craters to better characterize the fine (<1 μm) component of comet Wild 2. We perform initial crater identification with scanning electron microscopy, prepare the samples for further analysis with a focused ion beam, and analyze the cross sections of the impact craters with transmission electron microscopy (TEM). All of the craters are dominated by combinations of silicate and iron sulfide residues. Two micron‐scale craters had subregions that are consistent with spinel and taenite impactors, indicating that the micron‐scale craters have a refractory component. Four submicron craters contained amorphous residue layers composed of silicate and sulfide impactors. The lack of refractory materials in the submicron craters suggests that refractory material abundances may differentiate Wild 2 dust on the scale of several hundred nanometers from larger particles on the scale of a micron. The submicron craters are enriched in moderately volatile elements (S, Zn) when normalized to Si and CI chondrite abundances, suggesting that, if these craters are representative of the Wild 2 fine component, the Wild 2 fines were not formed by high‐temperature condensation. This distinguishes the comet’s fine component from the large terminal particles in Stardust aerogel tracks which mostly formed in high‐temperature events.

¹Winfried H. Schwarz,¹Michael Hanel,¹Mario Trieloff
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13437]
¹Klaus‐Tschira‐Labor für Kosmochemie, Institut für Geowissenschaften, Universität Heidelberg, Im Neuenheimer Feld 234‐236, D‐69120 Heidelberg, Germany
Published by arrangement with John Wiley & Sons

In situ U‐Pb measurements on zircons of the Ries impact crater are presented for three samples from the quarry at Polsingen. The U‐Pb data of most zircons plot along a discordia line, leading to an upper intercept of Carboniferous age (331 ± 32 Ma [2σ]). Four zircons define a concordia age of 313.2 ± 4.4 Ma (2σ). This age most probably represents the age of a granite from the basement target rocks. From granular textured zircon grains (including baddeleyite and anatase/Fe‐rich phases, first identified in the Ries crater), most probably recrystallized after impact (13 analyses, 4 grains), a concordia age of 14.89 ± 0.34 Ma (2σ) and an error weighted mean ²⁰⁶Pb*/²³⁸U age of Ma 14.63 ± 0.43 (2σ) is derived. Including the youngest concordant ages of five porous textured zircon grains (24 spot analyses), a concordia age of 14.75 ± 0.22 Ma (2σ) and a mean ²⁰⁶Pb*/²³⁸U age of 14.71 ± 0.26 Ma (2σ) can be calculated. These results are consistent with previously published ⁴⁰Ar/³⁹Ar ages of impact glasses and feldspar. Our results demonstrate that even for relatively young impact craters, reliable U‐Pb ages can be obtained using in situ zircon dating by SIMS. Frequently the texture of impact shocked zircon grains is explained by decomposition at high temperatures and recrystallization to a granular texture. This is most probably the case for the observed granular zircon grains having baddeleyite/anatase/Fe‐rich phases. We also observe non‐baddeleyite/anatase/Fe‐rich phase bearing zircons. For these domains, reset to crater age is more frequently for high U,Th contents. We tentatively explain the higher susceptibility to impact resetting of high U,Th domains by enhanced Pb loss and mobilization due to higher diffusivity within former metamict domains that were impact metamorphosed more easily into porous as well as granular textures during decomposition and recrystallization, possibly supported by Pb loss during postimpact cooling and/or hydrothermal activity.

^1,2Toni Schulz,³Pavel P. Povinec,⁴Ludovic Ferrière,^5,6A. J. Timothy Jull,³Andrej Kováčik,³Ivan Sýkora,²Jonas Tusch,²Carsten Münker,⁴Dan Topa,^1,4Christian Koeberl
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13435]
¹Department of Lithospheric Research, University Vienna, Althanstrasse 14, 1090 Vienna, Austria
²Institut für Geologie und Mineralogie, Universität zu Köln, Zülpicher Strasse 49b, 50674 Köln, Germany
³Faculty of Mathematics, Physics and Informatics, Department of Nuclear Physics and Biophysics, Comenius University, SK‐84248 Bratislava, Slovakia
⁴Natural History Museum, Burgring 7, 1010 Vienna, Austria
⁵Department of Geosciences, University of Arizona, Tucson, Arizona, 85721 USA
⁶Hungarian Academy of Sciences, Institute for Nuclear Research, ICER, 4026 Debrecen, Hungary
⁷MicroStep‐MIS, 84104 Bratislava, Slovakia
Published by arrangement with John Wiley & Sons

The Tissint meteorite fell on July 18, 2011 in Morocco and was quickly recovered, allowing the investigation of a new unaltered sample from Mars. We report new high‐field strength and highly siderophile element (HSE) data, Sr‐Nd‐Hf‐W‐Os isotope analyses, and data for cosmogenic nuclides in order to examine the history of the Tissint meteorite, from its source composition and crystallization to its irradiation history. We present high‐field strength element compositions that are typical for depleted Martian basalts (0.174 ppm Nb, 17.4 ppm Zr, 0.7352 ppm Hf, and 0.0444 ppm W), and, together with an extended literature data set for shergottites, help to reevaluate Mars’ tectonic evolution in comparison to that of the early Earth. HSE contents (0.07 ppb Re, 0.92 ppb Os, 2.55 ppb Ir, and 7.87 ppb Pt) vary significantly in comparison to literature data, reflecting significant sample inhomogeneity. Isotope data for Os and W (¹⁸⁷Os/¹⁸⁸Os = 0.1289 ± 15 and an ε¹⁸²W = +1.41 ± 0.46) are both indistinguishable from literature data. An internal Lu‐Hf isochron for Tissint defines a crystallization age of 665 ± 74 Ma. Considering only Sm‐Nd and Lu‐Hf chronometry, we obtain, using our and literature values, a best estimate for the age of Tissint of 582 ± 18 Ma (MSWD = 3.2). Cosmogenic radionuclides analyzed in the Tissint meteorite are typical for a recent fall. Tissint’s pre‐atmospheric radius was estimated to be 22 ± 2 cm, resulting in an estimated total mass of 130 ± 40 kg. Our cosmic‐ray exposure age of 0.9 ± 0.2 Ma is consistent with earlier estimations and exposure ages for other shergottites in general.

¹A. J. G. Jurewicz et al. (>10)
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13439]
¹Center for Meteorite Studies, Arizona State University, m/c 6004, Tempe, Arizona, 85287 USA
Published by arrangement with John Wiley & Sons

NASA’s Genesis Mission returned solar wind (SW) to the Earth for analysis to derive the composition of the solar photosphere from solar material. SW analyses control the precision of the derived solar compositions, but their ultimate accuracy is limited by the theoretical or empirical models of fractionation due to SW formation. Mg isotopes are “ground truth” for these models since, except for CAIs, planetary materials have a uniform Mg isotopic composition (within ≤1‰) so any significant isotopic fractionation of SW Mg is primarily that of SW formation and subsequent acceleration through the corona. This study analyzed Mg isotopes in a bulk SW diamond‐like carbon (DLC) film on silicon collector returned by the Genesis Mission. A novel data reduction technique was required to account for variable ion yield and instrumental mass fractionation (IMF) in the DLC. The resulting SW Mg fractionation relative to the DSM‐3 laboratory standard was (−14.4‰, −30.2‰) ± (4.1‰, 5.5‰), where the uncertainty is 2ơ SE of the data combined with a 2.5‰ (total) error in the IMF determination. Two of the SW fractionation models considered generally agreed with our data. Their possible ramifications are discussed for O isotopes based on the CAI nebular composition of McKeegan et al. (2011).

¹A.‐J. Soini,¹I. T. Kukkonen,¹T. Kohout,²A. Luttinen
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13441]
¹Department of Geosciences and Geography, University of Helsinki, PO Box 64, FI‐00014 Helsinki, Finland
²Finnish Museum of Natural History, University of Helsinki, PO Box 44, FI‐00014 Helsinki, Finland
Published by arrangement with John Wiley & Sons

We report direct measurements of thermal diffusivity and conductivity at room temperature for 38 meteorite samples of 36 different meteorites including mostly chondrites, and thus almost triple the number of meteorites for which thermal conductivity is directly measured. Additionally, we measured porosity for 34 of these samples. Thermal properties were measured using an optical infrared scanning method on samples of cm‐sizes with a flat, sawn surface. A database compiled from our measurements and literature data suggests that thermal diffusivities and conductivities at room temperature vary largely among samples even of the same petrologic and chemical type and overlap among, for example, different ordinary chondrite classes. Measured conductivities of ordinary chondrites vary from 0.4 to 5.1 W m⁻¹ K⁻¹. On average, enstatite chondrites show much higher values (2.33–5.51 W m⁻¹ K⁻¹) and carbonaceous chondrites lower values (0.5–2.55 W m⁻¹ K⁻¹). Mineral composition (silicates versus iron‐nickel) and porosity control conductivity. Porosity shows (linear) negative correlation with conductivity. Variable conductivity is attributed to heterogeneity in mineral composition and porosity by intra‐ and intergranular voids and cracks, which are important in the scale of typical meteorite samples. The effect of porosity may be even more significant for thermal properties than that of the metal content in chondrites.

^1,2A. Losiak,³A. Jõeleht,³J. Plado,⁴M. Szyszka,³K. Kirsimäe,⁵E. M. Wild,⁵P. Steier,²C. M. Belcher,¹A. M. Jazwa,³R. Helde
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13431]
¹Planetary Geology Lab, Institute of Geological Sciences, Polish Academy of Sciences, Warsaw, Poland
²wildFIRE Lab, Hatherly Laboratories, University of Exeter, Exeter, UK
³Department of Geology, University of Tartu, Tartu, Estonia
⁴Institute of Geology, Adam Mickiewicz University in Poznań, Poznań, Poland
⁵VERA Laboratory, Faculty of Physics—Isotope Physics, University of Vienna, Vienna, Austria
Published by arrangement with John Wiley & Sons

The Ilumetsa site, in Estonia, consists of two round, rimmed structures that are 725 m apart. The structures are listed as proven impact craters in the Impact Earth database, despite lack of commonly accepted, unequivocal proof of extraterrestrial collision identified at this location. We excavated trenches though the Ilumetsa Large and Ilumetsa Small structures and found small pieces of charcoal within the putative proximal ejecta in both structures, in a similar geological setting as previously identified charcoal in Kaali (Losiak et al. 2016) and Morasko craters (see Szokaluk et al. 2019). Our ¹⁴C dating of charcoal allowed us to conclude that these crater‐like features formed simultaneously between 7170 and 7000 cal. years bp, about 7 ka after deglaciation of this area. A ground penetrating radar survey of the nearby bog shows that no additional Ilumetsa structures bigger than 40 m exist. Geochemical studies of the ejecta and a search using a metal detector did not reveal any clear indication of extraterrestrial material. This suggests Ilumetsa may have been formed by an impact of stony‐iron or stony body, which got significantly weathered in a wet‐temperate climate. The mystery of the formation of the structures at Ilumetsa remains; however, due to significant circumstantial evidence discussed herein, we are confident to call it a “probable” impact site.

¹Billy P. Glass,²Luigi Folco,²Matteo Masotta,^2,3Fabrizio Campanale
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13433]
¹Department of Geological Sciences, University of Delaware, Newark, Delaware, 19716 USA
²Dipartimento di Scienze della Terra, Università di Pisa, Via S. Maria 53, 56126 Pisa, Italy
³Center for Nanotechnology Innovation@NEST, Istituto Italiano di Tecnologia (IIT), Piazza San Silvestro 12, 56127 Pisa, Italy
Published by arrangement with John Wiley & Sons

We examined 16 white opaque inclusions exposed on two polished slices of a Muong Nong‐type Australasian tektite from Muong Phin, Laos. The inclusions usually consist of a core, surrounded by a froth layer, and a quartz neoblast layer. The cores are composed primarily of a mixture of silica glass, coesite, and quartz in varying proportions. A thin (up to ~4 μm) layer of SiO₂‐poor glass enriched in FeO, MgO, CaO, Al₂O₃, and TiO₂ is observed as a bright halo in backscattered electron images around the quartz neoblasts and in places contains μm‐sized crystals, which may be Fe,Mg‐rich spinel. The distribution and textural relationships between the coesite‐bearing inclusions and the tektite matrix point to an in situ formation of the coesite due to an impact, rather than to infall, from a nearby impact, into tektite melt produced by the aerial burst of a bolide. The quartz neoblasts probably formed by crystallization of silica melt squeezed out of the inclusion core during the development of the froth layer. The bright halo may be the result of silica diffusing from the adjacent tektite melt into the growing quartz neoblasts. We propose that the survival of coesite was possible due to the froth layer that acted as a heat sink during bubble expansion and then as a thermal insulator.