G. Schmidt^a,b, F. Salvini^b  
Earth and Planetary Science Letters 118974 Link to Article [https://doi.org/10.1016/j.epsl.2024.118974]
^aIstituto di Astrofisica e Planetologia Spaziali (IAPS), INAF, Rome, Italy
^bGeoQuTe Lab, Department of Science, Roma Tre University, Rome, Italy
Copyright Elevier

Load on a planet’s lithosphere can often form a well-defined flexural bulge, including a permanent (or long-lasting) forebulge, which preserves important information on the force of the load and properties of the lithosphere itself. On Pluto, aspects of the outer ice shell (i.e. the lithosphere) have become increasingly ascertainable, as recent work using data from the New Horizons space probe has revealed evidence of ongoing surface cryovolcanism and a subsurface water ocean. However, the precise thickness and elasticity of the ice shell has yet to be fully established. Sputnik Planitia, one of the largest surface features on Pluto, is an elliptical depression that may have formed during an impact event and subsequently infilled with nitrogen ice. It is characterized by a smooth, radially asymmetrical, forebulge which has been retained in places along the border of the depression. However, the proportion of influence on the formation of the forebulge between the impact load and the load induced by the infill remains unknown. Here, we report results from the analysis of the forebulge of Sputnik Planitia to explore the characteristics of the ice shell and the nitrogen infill. By utilizing multiple Converging Monte Carlo (CMC) simulations within the material and environmental parameters of Pluto, the best fit flexure surface was able to replicate the topography of the flexure (including the forebulge) from ten profiles. Results show an ice shell thickness ranging from 65 to 90 km, with an average of 78 km. The density of the ice shell is 50 kg/m³ less than the density of the subsurface water ocean. We demonstrate that if the formation of the forebulge occurs solely from the nitrogen ice infill load, the infill must reach >18 km of thickness. Furthermore, a southeast-northwest central load symmetry may have been produced by an impacting object with a southeast-northwest trajectory.

Kees Erica R. JAWIN^1,2, Timothy J. MCCOY¹, Lisette E. MELENDEZ^1,3, Catherine M. CORRIGAN¹ , Kevin RIGHTER^4,8, and Harold C. CONNOLLY Jr^5,6,7 
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.14263]
¹Department of Mineral Sciences, National Museum of Natural History, Smithsonian Institution, Washington, DC, USA
²Center for Earth and Planetary Studies, National Air and Space Museum, Smithsonian Institution, Washington, DC, USA
³Department of Earth, Atmospheric, and Planetary Sciences, Purdue University, West Lafayette, Indiana, USA
⁴NASA Johnson Space Center, Houston, Texas, USA
⁵Lunar and Planetary Laboratory, University of Arizona, Tucson, Arizona, USA
⁶Department of Geology, School of Earth and Environment, Rowan University, Glassboro, New Jersey, USA
⁷Department of Earth and Planetary Science, American Museum of Natural History, New York, New York, USA
⁸Department of Earth and Environmental Sciences, University of Rochester, Rochester, New York, USA Sciences

Orbital observations of Bennu revealed a surface covered in boulders that are most similar among meteorites in our collections to aqueously altered carbonaceous chondrites, and initial analyses of the returned Bennu sample have begun to reveal insights into Bennu’s origins. We identified a suite of paired CM2 chondrite meteorites that have a finely layered texture and bear a striking similarity, although at a different scale, to rugged, layered boulders on Bennu. We investigated the nature and potential origin of this layered texture by performing a petrofabric analysis on samples MET 00431, 00434, and 00435. We developed a micro-geospatial mapping framework that is more commonly used for landscape-scale investigations. Our results reveal a pervasive fracture network that exhibits a similar orientation to flattened particles dominated by tochilinite–cronstedtite intergrowths (TCI). We propose that their petrofabrics originated from a low-energy impact on the parent body that occurred after the main period of aqueous alteration halted. The impactdeformed TCI (which formed during earlier aqueous alteration) and generated the fractures. We propose that the sample from Bennu may contain particles with similar layered textures to these meteorites which, if present, would likewise indicate the dominant role of impacts and aqueous alteration on Bennu’s parent body.

T. Mark Harrison^a, Bidong Zhang^a,^b, Andrew F. Parisi^a, Elizabeth A. Bell^a 
Earth and Planetary Science Letters 118933 Link to Article [https://doi.org/10.1016/j.epsl.2024.118943]
^aState Department of Earth, Planetary and Space Sciences, University of California Los Angeles, Los Angeles, CA 90095, USA
^bDepartment of Earth, Environmental and Planetary Sciences, Rice University, Houston, TX 77005, USA
Copyright Elevier

Investigations of Apollo-returned samples radically altered our understanding of lunar history which has important implications for terrestrial habitability and Solar System evolution. Radiometric dating of those samples inspired the hypothesis that Moon experienced a Late Heavy Bombardment (LHB) at ∼3.9 Ga. The LHB concept has come under several recent challenges, including the concern that ⁴⁰Ar/³⁹Ar step-heating dates of Apollo impactites had been misinterpreted. Ultraviolet laser ablation (UVLAMP) ⁴⁰Ar/³⁹Ar dates – with their capacity for much higher spatial resolution and thus potential to avoid dating near-ubiquitous clasts in impact melt rocks – should in principle provide more interpretable results. Here we compare new ion microprobe ²⁰⁷Pb/²⁰⁶Pb accessory mineral dates for two Apollo 17 impactites for which UVLAMP ⁴⁰Ar/³⁹Ar dates had been previously obtained. Our results are consistent with a single accessory phase growth event for each sample, though the two samples yielded statistically different mean ages of ca. 3.974±0.013 and 3.928±0.003 Ga. Both can reasonably be interpreted as dating an impact event, but the ²⁰⁷Pb/²⁰⁶Pb dates are older than the associated ⁴⁰Ar/³⁹Ar dates by several hundred million years. We interpret that the age differences result from subsequent thermal disturbances. The discordancy between impact ages inferred from lunar impactites using two different radiometric systems suggests caution in acceptance of the LHB hypothesis without the benefit of both larger lunar datasets and more multichronometric studies. Even with such information, our capacity to know the lunar bombardment history is likely limited by compositional and thermal effects which appear to restrict growth of impact-produced accessory minerals to a small fraction of the lunar surface. Using currently available datasets, the LHB hypothesis may be effectively untestable.

Uta Beyersdorf-Kuis^a,b, Ulrich Ott^a,b,c, Mario Trieloff^a
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2024.09.012]
^aMax-Planck Institute for Chemistry, Hahn-Meitner-Weg 1, D-55128 Mainz, Germany
^bUniversity of Heidelberg, Im Neuenheimer Feld 234-236, D-69120 Heidelberg, Germany
^cInstitute for Nuclear Research Atomki, Bem tér 18/c, H-4026 Debrecen, Hungary
^dKlaus-Tschira-Labor für Kosmochemie, D-69120 Heidelberg, Germany de Lorraine, CNRS, CRPG, UMR 7358, Nancy, 
Copyright Elsevier

Excesses of cosmic-ray produced nuclei in individual components of meteorites indicate “pre-irradiation”, either in the surface region of their parent bodies or as free-floating small particles in the early Solar System. We expand on our earlier work (Beyersdorf-Kuis et al., 2015) and report a study of cosmic-ray produced He and Ne in chondrules and “matrix” (i.e., matrix-dominated) material of several CR2 and CV meteorites as well as the highly primitive, unique, carbonaceous chondrite Acfer 094. In accordance with previous work, no evidence for pre-irradiation was found for CV3 Allende, while for CV3 Vigarano evidence for pre-irradiation is marginal at best. Also, the single chondrule from unique Acfer 094 that we studied has a cosmic ray exposure indistinguishable from the one we found for “matrix” material. Chondrules from Acfer 082 (CV) exhibit both excesses and deficits relative to “matrix”, which points to pre-irradiation of not only chondrules, but also matrix material. A similar case may be Renazzo (CR2), where, however, the identification is complicated by the presence of abundant pre-solar Ne-E. A large number of chondrules (ten) were studied from CR2 El Djouf 001, which yielded slightly variable, small but consistent, excesses relative to “matrix”, corresponding to “nominal” (i.e., irradiation by galactic cosmic rays in 4π geometry) excess ages of 1 to 2 Ma. Modelling suggests contributions from irradiation in the parent body regolith by solar cosmic rays (SCR) as well as galactic cosmic rays (GCR), where the latter dominates. Reevaluating the large variations previously identified in chondrules from QUE 99177, we suggest either a very different regolith history compared to that of El Djouf 001 or, more likely, pre-irradiation by, primarily, GCR in the early solar system as suggested previously. The case of solar-wind-rich NWA 852 (CR2) shows similarity to El Djouf 001 except for a much larger size of the effects. We suggest that the situation may be common among meteorites with a regolith origin. With independent information on the cosmic ray exposure age, which could be obtained by the study of cosmic-ray produced radionuclides, the individual parent body contributions may be disentangled, providing constraints on regolith dynamics.

Kees C. WELTEN¹, Marc W. CAFFEE² , Monika E. KRESS^1,7, Marlene D. GISCARD⁴ , A. J. Timothy JULL⁴, Ralph P. HARVEY⁵ , and John SCHUTT^5,6
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.14264]
¹Space Sciences Laboratory, University of California, Berkeley, California, USA
²Department of Physics, Purdue University, West Lafayette, Indiana, USA
³Department of Physics & Astronomy, San Jose State University, San Jose, California, USA
⁴NSF Arizona AMS Laboratory, University of Arizona, Tucson, Arizona, USA
⁵Department of Geology, Case Western Reserve University, Cleveland, Ohio, USA
⁶Geology & Geophysics, University of Utah, Salt Lake City, Utah, USA
⁷Stanford University, Stanford, California, USA

The US Antarctic Search for Meteorites (ANSMET) discovered a dense cluster of 88 ordinary chondrites with a total mass of more than 100 kg on a blue ice area (BIA) of 1.6 × 0.3 km² near the Otway Massif, Grosvenor Mountains, Antarctica. The larger masses (weighing up to 29 kg) were found at one end of an oval-shaped pattern and the smaller masses (50–200 g) at the other end. We measured concentrations of the cosmogenic radionuclides ¹⁰Be (half-life—1.36 × 10⁶ year) and ³⁶Cl (3.01 × 10⁵ year) in the metal fraction of 17 H chondrites, including 14 fragments of this cluster, to verify the hypothesis that this meteorite cluster on the Otway Massif BIA represents a meteorite strewn field produced by the atmospheric breakup of a single meteoroid. The ¹⁰Be and ³⁶Cl concentrations confirm that 10 out of 14 H chondrites from different locations within this small area are paired fragments of the same meteorite fall, while the four other H chondrites represent two additional—smaller—falls. The radionuclides suggest a pre-atmospheric mass of 200–400 kg for the large pairing group, suggesting that 25%–50% of the meteoroid survived atmospheric entry. Based on the distribution of the paired H chondrites and evidence of their common cosmic-ray exposure history in space, we conclude that most of the 88 meteorites within this small area represent a meteorite strewn field. The small size of the strewn field suggests that the meteoroid entered at a steep angle (>60°), while the low amount of fusion crust on most meteorite surfaces most likely indicates atmospheric break up at low altitude, while additional fragmentation of a large surviving fragment may have occurred during impact on the ice. This well-documented strewn field provides a good opportunity to apply model simulations of the atmospheric fragmentation of this object as a function of entry angle, velocity, and meteoroid strength. Cosmogenic ¹⁴C analyses in two members of the Otway Massif pairing group yield a terrestrial age of 15.5 ± 1.5 kyr, which represents the time elapsed since this meteorite fell on Earth. The excellent preservation of an Antarctic meteorite strewn field suggests that the Otway Massif BIA represents a relatively stagnant blue ice field.

R. J. CURTIS¹, T. J. WARREN¹, K. A. SHIRLEY¹, D. A. PAIGE², and N. E. BOWLES¹
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.14266]
¹Atmospheric, Oceanic and Planetary Physics, Department of Physics, University of Oxford, Oxford, UK
²Department of Earth and Space Sciences, University of California, Los Angeles, California, USA

A laboratory study was performed using the Visible Oxford Space Environment Goniometer in which the broadband (350–1250 nm) bidirectional reflectance distribution functions (BRDFs) of two representative Apollo regolith samples were measured, for two surface roughness profiles, across a range of viewing angles—reflectance: 0–70°, in steps of 5°; incidence: 15°, 30°, 45°, and 60°; and azimuthal: 0°, 45°, 90°, 135°, and 180°. The BRDF datasets were fitted using the Hapke BRDF model to (1) provide a method of comparison to other photometric studies of the lunar regolith and (2) to produce Hapke parameter values which can be used to extrapolate the BRDF to all angles. Importantly, the surface profiles of the samples were characterized using an Alicona 3D® instrument, allowing two of the free parameters within the Hapke model, φ and , which represent porosity and surface roughness, respectively, to be constrained. The study determined that, for , the 500–1000 μm size-scale is the most relevant for the BRDF. Thus, it deduced the following “best fit” Hapke parameters for each of the samples: Apollo 11 rough— = 0.315 ± 0.021,  = 0.261 ± 0.007, and  = 0.039 ± 0.005 (with  = 21.28° and φ = 0.41 ± 0.02); Apollo 11 smooth— = 0.281 ± 0.028,  = 0.238 ± 0.008, and  = 0.032 ± 0.006 (with  = 13.80° and φ = 0.60 ± 0.02); Apollo 16 rough— = 0.485 ± 0.155,  = 0.155 ± 0.083, and  = 0.135 ± 0.007 (with  = 21.69° and φ = 0.55 ± 0.02); Apollo 16 smooth— = 0.388 ± 0.057,  = 0.063 ± 0.033, and  = 0.221 ± 0.011 (with  = 14.27° and φ = 0.40 ± 0.02). Finally, updated hemispheric albedo functions were determined for the samples, which can be used to set laboratory measured visible scattering functions within thermal models.

^1,2Rojas J et al. (>10)
Nature Astronomy  Link to Article [DOI 10.1038/s41550-024-02364-y]
¹Univ. Paris-Saclay, CNRS/IN2P3, IJCLab, Orsay, France
²Earth and Planets Laboratory, Carnegie Institution for Science, Washington DC, USA

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

T. Mahlé^a, Y. Marrocchi^a, J. Neukampf^a, J. Villeneuve^a, E. Jacquet^b
Geochimica et Cosmochimica acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2024.08.027]
^aUniversité de Lorraine, CNRS, CRPG, UMR 7358, Nancy, France
^bInstitut de Minéralogie, de Physique des Matériaux et de Cosmochimie (IMPMC), Muséum national d’Histoire naturelle, Sorbonne Université, CNRS, CP52, 57 rue Cuvier, 75005 Paris, France
Copyright Elsevier

Among carbonaceous chondrites, the chondrules of CB and CH stand out by being dominated by skeletal barred olivine and cryptocrystalline textures. These non-porphyritic chondrules are thought to have formed within an impact-generated plume resulting from large-scale asteroidal collisions late in disk history. Porphyritic chondrules are also present, if rare, in CB and CH chondrites and might correspond to nebular objects formed earlier in the disk. We report on the mineralogy, petrology, and oxygen isotopic compositions of porphyritic chondrules in the Isheyevo CH/CB_b chondrite. These chondrules show minor element variations at both the chondrule and individual olivine grain scales, which are similar to those observed in other chondrites. In terms of oxygen isotopes, individual chondrules show contrasting behavior with either negligible, mass-dependent or mass-independent O-isotopic variations. They also display different average Δ¹⁷O, ranging from −6 ‰ to +4 ‰, anticorrelated with size, with most chondrules (8/13) showing Δ¹⁷O > 0 ‰. Our results show that porphyritic chondrules in CB (and CH) chondrites are of nebular origin and do not result from the collisional impact at the origin of other CB components. We propose that CB porphyritic chondrules originate from the chondritic impactor involved in the collision, similarly to hydrated matrix-rich clasts reported in Isheyevo. Altogether, this shows that two chondrule populations, formed by both nebular and planetary processes, co-exist in CB and CH chondrites. Isheyevo thus represents an archetypal chondrite lying at the transition between two dominant chondrule-forming regimes, nebular and impact-related.

^1,2P. J. Wozniakiewicz et al. (>10)
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.14251]
¹Physics and Astronomy, University of Kent, Canterbury, Kent, UK
²Earth Sciences Department, Natural History Museum, London, UK

The Kwajalein micrometeorite collection utilized high volume air samplers fitted with polycarbonate membrane filters to capture particles directly from the atmosphere at the Earth’s surface. This initial study focused on identifying cosmic spherule-like particles, conservatively categorizing them into four groups based on bulk compositional data: Group I exhibit a range of compositions designated terrestrial in origin; group II are Fe-rich and contain only additional O, S, and/or Ni; group III are silicate spherules with Mg-to-Si At% ratios less than 0.4; group IV are silicate spherules with Mg-to-Si At% ratios greater than 0.4. Spherules in groups I, II, and III have compositions that are also consistent with particles that are produced in great numbers by natural and/or anthropogenic terrestrial activities (e.g., volcanic microspherules, fly ash from coal fired power plants, etc.) and thus are assumed terrestrial in origin. Group IV spherules exhibit compositions closest to those of cosmic spherules identified in other collections and are, therefore, designated cosmic spherule candidates. Detailed analysis of seven group IV spherules found that whilst five exhibited morphology and compositions consistent with S-type cosmic spherules, two appear unique to this collection and could not be matched to either terrestrial or extraterrestrial spherules studied to date.

^a,bH. Li, ^bZ. Wang, ^cZ. Chen, ^bW. Tian, ^dW-R. Wang, ^bG. Zhang ^bL. Zhang
Geochimica et Cosmochimica acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2024.09.002]
^aCenter for High Pressure Science and Technology Advanced Research (HPSTAR), Beijing 100193, China

^bMOE Laboratory of Orogenic Belts and Crustal Evolution, School of Earth and Space Sciences, Peking University, Beijing 100871, China
^cLaboratory of Metallogeny and Mineral Assessment of MNR, Institute of Mineral Resources, Chinese Academy of Geological Sciences, Beijing 100037, China
^dKey Laboratory of Paleomagnetism and Tectonic Reconstruction of MNR, Institute of Geomechanics, Chinese Academy of Geological Sciences, Beijing 100081, China
Copyright Elsevier

Apatite is ubiquitous in lunar samples and has been used widely for estimating volatile abundances in the lunar interior. However, apatite compositional and isotopic variations within and between samples have resulted in varying and ambiguous results. Understanding apatite petrogenesis will help with both identifying the appropriate composition for volatile estimation and interpreting isotopic variations. Here we report a comprehensive petrogenetic investigation of apatite in Chang’E-5 (CE5) basaltic sample CE5C0800YJYX013GP. Apatite displays both intra-grain and inter-grain compositional variations with F and Cl contents falling in the ranges of 0.97–2.47 wt% and 0.24–1.09 wt%, respectively. These apatite compositions show relatively low F and high Cl characteristics in comparison to apatites of Apollo high-Ti and low-Ti mare basalts, but are similar to those reported for lunar meteorites LAP 04841 and MIL 05035. We discern three zoning profiles: fractional crystallization (FC)-dominated, degassing-induced and a third indicated by REE-enriched cores, which are interpreted as representing different generations of apatite. FC-dominated zoning is characterized with decreasing F and increasing Cl and S contents from core to rim; while the opposite is true for the degassing-induced zoning. Regardless of the zoning patterns, apatite Cl and S contents display positive correlations, with S contents up to ∼ 3000 ppm, much higher than previous reports for Apollo samples (up to ∼ 600 ppm). We demonstrate that the fractional crystallization model proposed by Boyce et al. (2014) in combination with H₂O degassing and high S contents in melt (likely at sulfide saturation) can explain these high Cl and S contents observed in CE5 apatite.

Based on the core composition of the FC-dominated zoning profile, which has the lowest incompatible element concentrations, bulk F, Cl and H₂O contents in the parental melt are estimated to be ∼ 72 ± 21, ∼43 ± 14 and ∼ 1576 ± 518 ppm, respectively. These estimates have lower F/Cl ratios than those measured in olivine-hosted melt inclusions from Apollo mare basalts. By adopting the petrogenetic model for CE5 basalt proposed by Su et al. (2022), i.e., 10 % partial melting of a hybrid mantle source, followed by ∼ 30–70 % fractional crystallization (∼50 % for our sample), we estimate the F, Cl, H₂O and S contents in the mantle source are in the ranges of ∼ 2.5–4.6, ∼0.7–1.4, ∼53–105 and ∼ 38–125 ppm, respectively, similar to estimates for both depleted Earth mantle and primitive lunar mantle. However, by adopting the model of Tian et al. (2021), 2–3 % partial melting of a mantle source composed of 86 PCS+2% TIRL (PCS, percent crystallized solid; TIRL, trapped instantaneous residual liquid), followed by 43–88 % fractional crystallization, these estimates will be 5–10 times lower. To be certain whether the relatively low F and high Cl characteristics of CE5 apatite imply an enriched mantle source requires further evaluation of the petrogenetic models for CE5 basalt.