¹Martin R. Lee,^1,2,3Luke Daly,¹Cameron Floyd,¹Pierre‐Etienne Martin
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13607]
¹School of Geographical & Earth Sciences, University of Glasgow, Glasgow, G12 8QQ UK
²Space Science and Technology Centre, School of Earth and Planetary Sciences, Curtin University, GPO BOC U1987, Perth, Western Australia, 6845 Australia
³Australian Centre for Microscopy and Microanalysis, The University of Sydney, Camperdown, New South Wales, 2006 Australia
Published by arrangement with John Wiley & Sons

The CM carbonaceous chondrites provide unique insights into the composition of the protoplanetary disk, and the accretion and geological history of their parent C‐complex asteroid(s). Of the hundreds of CMs that are available for study, the majority are finds and so may have been compromised by terrestrial weathering. Nineteen falls have been recovered between 1838 and 2020, and there is a hint of two temporal clusters: 1930–1942 and 2009–2020. Falls are considered preferable to finds to study because they should be near pristine, and here this assumption is tested by investigating their susceptibility to alteration before recovery and during curation. CMs falling on the land surface are prone to contamination by organic compounds from soil and vegetation. Where exposed to liquid water prior to collection, minerals including oldhamite can be dissolved and most fluid mobile elements leached. Within days of recovery, CMs adsorb water from the atmosphere and are commonly contaminated by airborne hydrocarbons. Interaction with atmospheric water and oxygen during curation over year to decadal timescales can produce Fe‐oxyhydroxides from Fe,Ni metal and gypsum from indigenous gypsum and oldhamite. Relationships between the petrologic (sub)types of pre‐1970 falls and their terrestrial age could be due to extensive but cryptic alteration during curation, but are more likely a sampling bias. The terrestrial history of a CM fall, including circumstances of its collection and conditions of its curation, must be taken into account before it is used to infer processes on C‐complex parent bodies such as Ryugu and Bennu.

¹L. J. Hicks,¹J. C. Bridges,²T. Noguchi,³A. Miyake,¹J. D. Piercy,¹S. H. Baker
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13611]
¹Space Research Centre, School of Physics & Astronomy, University of Leicester, Leicester, LE1 7RH UK
²Faculty of Arts and Science, Kyushu University, 744 Motooka, Nishi‐ku, Fukuoka, 819‐0395 Japan
³Division of Earth and Planetary Sciences, Kyoto University, Kitashirakawaoiwake‐cho, Kyoto, 606‐8502 Japan
Published by arrangement with John Wiley & Sons

Synchrotron Fe‐K X‐ray absorption spectroscopy and transmission electron microscopy have been used to investigate the mineralogy and Fe‐redox variations in the space‐weathered (SW) rims of asteroidal samples. This study focuses on the FIB lift‐out sections from five Itokawa grains, returned by the Hayabusa spacecraft, including samples RB‐QD04‐0063, RB‐QD04‐0080, RB‐CV‐0011, RB‐CV‐0089, and RB‐CV‐0148. Each of the samples featured partially amorphized SW rims, caused by irradiation damage from implanted low mass solar wind ions, and the impacting of micrometeorites. Using bright‐field and HAADF‐STEM imaging, vesicular blistering and nanophase Fe metal (npFe0) particles were observed within grain rims, and solar flare tracks were observed in the substrate host grain, confirming the presence of SW zones. We use Fe‐K XANES mapping to investigate Fe‐redox changes between the host mineral and the SW zones. All SW zones measured show some increases in the ferric‐ferrous ratio (Fe3+/ΣFe) relative to their respective host grains, likely the result of the implanted solar wind H+ ions reacting with the segregated ferrous Fe in the surface material.

¹Larry R. Nittler,¹Conel M. O’D. Alexander,^1,2Andrea Patzer,^1,3Maximilien J. Verdier‐Paoletti
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13618]
¹Earth and Planets Laboratory, Carnegie Institution of Washington, 5241 Broad Branch Rd NW, Washington, District of Columbia, 20015 USA
²Geosciences Center Göttingen, University of Göttingen, Goldschmidtstr. 1, 37077 Göttingen, Germany
³Institut de Minéralogie, de Physique des Matériaux, et de Cosmochimie (IMPMC), Sorbonne Université, Muséum national d’Histoire naturelle, UPMC Université Paris 06, UMR CNRS 7590, IRD, UMR 206, 75005 Paris, France
Published by arrangement with John Wiley & Sons

We report a NanoSIMS search for presolar grains in the CM chondrites Asuka (A) 12169 and A12236. We found 90 presolar O‐rich grains and 25 SiC grains in A12169, giving matrix‐normalized abundances of 275 (+55/−50, 1σ) ppm or, excluding an unusually large grain, 236 (+37/−34) ppm for O‐rich grains and 62 (+15/−12) ppm for SiC grains. For A12236, 18 presolar silicates and 6 SiCs indicate abundances of 58 (+18/−12) and 20 (+12/−8) ppm, respectively. The SiC abundances are in the typical range of primitive chondrites. The abundance of presolar O‐rich grains in A12169 is essentially identical to that in CO3.0 Dominion Range 08006, higher than in any other chondrites, while in A12236, it is higher than found in other CMs. These abundances provide further strong support that A12169 and A12236 are the least‐altered CMs as indicated by petrographic investigations. The similar abundances, isotopic distributions, silicate/oxide ratios, and grain sizes of the presolar O‐rich grains found here to those of presolar grains in highly primitive CO, CR, and ungrouped carbonaceous chondrites (CCs) indicate that the CM parent body(ies) accreted a similar population of presolar oxides and silicates in their matrices to those accreted by the parent bodies of the other CC groups. The lower abundances and larger grain sizes seen in some other CMs are thus most likely a result of parent‐body alteration and not heterogeneity in nebular precursors. Presolar silicates are unlikely to be present in high abundances in returned samples from asteroids Ryugu and Bennu since remote‐sensing data indicate that they have experienced substantial aqueous alteration.