¹G.Munaretto,¹A.Lucchetti,¹M.Pajola,¹G.Cremonese,^1,2M.Massironi
Icarus (in Press) Link to Article [https://doi.org/10.1016/j.icarus.2022.115284]
¹INAF, Osservatorio Astronomico di Padova, Vicolo dell’Osservatorio, Italy
²Department of Geosciences, University of Padova, Padova, Italy
Copyright Elsevier

The origin and formation mechanism of Mercury’s hollows, which are bright, often haloed, small, shallow, irregular, rimless and flat floored depressions, represent one of the major open science questions regarding the Hermean surface and the processes shaping it morphology. In this work, we perform a photometric modelling of multiangular and multiband images of Tyagaraja and Canova craters’ hollows to investigate the physical properties of their reflecting material. Thanks to such observations, we demonstrate that we can derive a better topographic correction when compared to the one obtained from the global photometric models of Mercury. Indeed, our parameters, which result from the inversion of the Hapke and Kaasalainen-Shkuratov models, can be useful for both future spectrophotometric analyses of Mercury and laboratory experiments aiming to identify hollows analogue materials. The analysis of our estimated model parameters imply that the Tyagaraja and Canova hollow walls are more backscattering and smoother than the crater floors, in agreement with independent phase ratio analyses. Our results suggest that the hollow forming material is made of roundish particles or particles with a high density of scattering centers, such as holes, vesicles or fractures, consistent with the release of volatiles as part of the hollows’ formation mechanism.

^1,2,3ShiYong Liao,²Birger Schmitz
Icarus (in Press) Link to Article [https://doi.org/10.1016/j.icarus.2022.115285]
¹Purple Mountain Observatory, Chinese Academy of Sciences, Nanjing, China
²Astrogeobiology Laboratory, Department of Physics, Lund University, Lund, Sweden
³CAS Center for Excellence in Comparative Planetology, Hefei, China
Copyright Elsevier

The breakup of the L-chondrite parent body (LCPB) in the asteroid belt at 466 Ma ago is the largest asteroid breakup documented in Earth’s geological record for the past ca. three billion years. Recovery of abundant macroscopic fossil L chondrites in mid-Ordovician marine sediments as well as reconstructions of the flux of micrometeoritic chrome spinel through the ages have given much new information on the precise timing of the breakup and its effects on Earth. In the present study, we focus on the flux of large micrometeorites to Earth shortly (in the 2 Ma time interval) before the LCPB breakup (pre-LCPB), which may be crucial for understanding the dynamical evolution of the asteroid belt leading up to the breakup. We present chrome-spinel data (32–355 μm grain size) from two mid-Ordovician limestone sections in Sweden (Kinnekulle and Öland, 300 km apart) and one section in western Russia (Lynna River), ca. 1100 km from Kinnekulle. One aim is also to test the level of reproducibility of chrome-spinel flux reconstructions between different sites.

Between 300 and 600 kg of limestone were collected from each section in the stratigraphic interval corresponding to ca. 2 Ma before up to immediately before the LCPB breakup. The relations between H, L and LL meteorites from Kinnekulle (38.7 ± 6.3%, 33.2 ± 6.1% and 28.1 ± 5.8%), Öland (46.0 ± 5.6%, 31.2 ± 3.1% and 24.5 ± 4.8%) and Lynna River (38.2 ± 5.5%, 32.8 ± 5.3% and 29.0 ± 5.1%) sections are indistinguishable from each other within uncertainties, revealing a globally homogeneous influx of H, L and LL meteorites. This gives support for the validity of previous reconstructions for the meteorite flux based on chrome spinel reconstructions for fifteen time windows through the Phanerozoic.

All the pre-LCPB samples from the three regions show a collective dominance of H-chondritic grains (42 ± 3%) over L (31 ± 3%) and LL grains (27 ± 3%), largely similar to the Phanerozoic background flux. Intriguingly, the presence of background concentrations of L-chondritic material also in the pre-LCPB flux demonstrates that the idea of a largely intact LCPB still existing before the final breakup may be far from reality. Apparently, a substantial amount of equilibrated chondritic material from deep levels of an L-chondritic body reached Earth even before the inferred catastrophic disruption at 466 Ma ago. This would concur with a “rubble pile” structure of the L-chondrite parent body and exposure of abundant deep-seated material as a result of earlier disruption and re-accretion events. The LL-chondritic contribution in the pre-LCPB flux is higher than at other Phanerozoic time windows, including the Cambrian. This anomalously enhanced flux thus cannot be ascribed to the Neoproterozoic breakup of the large LL-chondritic Flora asteroid. Previously observed high concentrations of chrome-spinel grains of achondritic origin can be reproduced only in our samples from immediately (< 1 Ma) before the LCPB breakup. We speculate that this, together with the high LL percentage before the breakup, may be explained by dynamic perturbations, possibly in the near-Earth region, leading up to the LCPB event.