¹Masaaki Miyahara et al. (>10)
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.14271]
¹Graduate School of Advanced Science and Engineering, Hiroshima University, Higashi-Hiroshima, Japan
Published by arrangement with John Wiley & Sons

The surface morphology of regolith grains from the C-type asteroid Ryugu was studied in search of evidence of impact events on the asteroid. Scanning electron microscopy revealed that ~8% of C0105-042 Ryugu grains have a smooth surface on one side of the grains. One of these grains has striated linear grooves (striations) on its smooth surface. Transmission electron microscopy of the grain showed that a porous fine-grained Mg-Fe phyllosilicate assemblage, which is the main component of Ryugu grains, is compacted near the smooth surface. The smooth surface with striations closely resembles a slickenside, a characteristic texture found in terrestrial fault rocks formed by shear deformation. There is no evidence of melting/decomposition in the Mg-Fe phyllosilicates near the smooth surface, indicating that the shear heating temperature is less than ~1100 K. Assuming that the average length of the striations corresponds to the minimum displacement of the micro-fault, the shock pressure recorded in the C0105-042 Ryugu grain is estimated to be <~4.5 GPa by a fault mechanics calculation. The shock pressures of C0105-042, together with those of C0014 (~2 GPa) and C0055 (>~3.9 GPa) in previous studies suggest that the impact velocities recorded in these grains are < ~0.89–1.63 km s−1. Based on the impact velocities, these grains may record an impact event that occurred when asteroid Ryugu was in the orbit in Main Belt.

¹Kiran Shahood Almas,¹Richard D. Ash,¹Richard J. Walker
Meteoritics & Planetary Science (in Press) Open Access Link to Article [https://doi.org/10.1111/maps.14267]
¹Department of Geology, University of Maryland, College Park, Maryland, USA
Published by arrangenent with John Wiley & Sons

Over 20,000 meteorites have been recovered from hot deserts. The effects of hot desert weathering upon highly siderophile elements (HSE) have been little studied. We have investigated the effects of neutral to mildly acidic leaching of three L6-type ordinary chondrites of different weathering grades on HSE concentrations and Re-Os isotopic systematics. We have characterized the bulk sample HSE patterns of these meteorites and conducted leaching experiments with progressively longer leaching times to determine the possible effects of long-term residence in a desert. The most weathered sample (NWA 14239) displayed greater HSE concentration homogeneity than the other samples and released lower quantities of HSEs during leaching. Water leaching was milder than acetic acid and did not significantly modify the Re-Os isotopic systematics of the residue relative to the bulk sample of NWA 869. Short-term leachates of the less weathered samples (Viñales and NWA 869) were characterized by low 187Os/188Os ratios, indicating the preferential dissolution of early solar system–formed phases such as non-magnetic chondrules and matrix with low Re/Os that are no longer intact in the most weathered sample. Of the HSE, Pd is most resistant to both water and acetic acid leaching, with a maximum removal of ~5% Pd, while Re, Os, and Ir are most mobile with up to 40% removal.