¹C.J.Renggli,¹S.Klemme,²A.Morlok,¹J.Berndt,²I.Weber,²H.Hiesinger,³P.L.King
Earth and Planetary Science Letters 593, 117647 Link to Article [https://doi.org/10.1016/j.epsl.2022.117647]
¹Institut für Mineralogie, Westfälische Wilhelms-Universität Münster, Münster, 48149, Germany
²Institut für Planetologie, Westfälische Wilhelms-Universität Münster, Münster, 48149, Germany
³Research School of Earth Sciences, The Australian National University, Canberra, 2601, Australia
Copyright Elsevier

The surface of Mercury is enriched in sulfur, with up to 4 wt.% detected by the NASA MESSENGER mission, and has been challenging to understand in the context of other terrestrial planets. We posit, that magmatic S was mobilized as a gas phase in volcanic and impact processes near the surface, exposing silicates to a hot S-rich gas at reducing conditions and allowing conditions for rapid gas-solid reactions. Here, we present novel experiments on the reaction of Mercury composition glasses with reduced S-rich gas, forming Ca- and Mg-sulfides. The reaction products provide porous and fragile materials that create previously enigmatic hollows on Mercury. Our model predicts that the gas-solid reaction forms Ca-Mg-Fe-Ti-sulfide assemblages with SiO2 and aluminosilicates, distinct from formation as magmatic minerals. The ESA/JAXA BepiColombo mission to Mercury will allow this hypothesis to be tested.

¹Christopher J. Cline II,²Mark J. Cintala
Meteoritics & Planetary Science (in Press) Open Access Link to Article [https://doi.org/10.1111/maps.13886]
¹Jacobs Technology, NASA Johnson Space Center, Astromaterials Research and Exploration Science, Mail Code X13, 2101 NASA Parkway, Houston, Texas, 77058 USA
²NASA Johnson Space Center, Astromaterials Research and Exploration Science, Mail Code X13, 2101 NASA Parkway, Houston, Texas, 77058 USA
Published by arrangement with John Wiley & Sons

The dimensions of relatively small-scale impact craters are undoubtedly sensitive to the physical properties of the target. Studying gravity-controlled crater formation at the laboratory scale often relies on cohesionless, granular materials, which, by their nature, make it difficult to separate the individual contributions to this process from all of the relevant target properties. Here, we conduct a suite of impact experiments to isolate and evaluate the effects of density, porosity, and internal friction on impact crater morphometry. Each made from one of four different granular materials, targets were impacted vertically with 4.76 mm aluminum projectiles at an average speed of ~1.55 km s−1. Two different methods were used to load these materials into the target bucket (pouring and sieving), resulting in targets that varied in bulk density and internal friction. The experimental results indicate that depth–diameter ratios of the craters are largely influenced by the loading method of the target material and are sensitive to the friction and porosity of the targets. Sieved targets (relatively higher density, lower porosity, and higher friction angle) produce craters that are markedly shallower, have notably smaller volumes, and exhibit a flat-floored morphology, with some possessing small central mounds. Flat-floored craters are typically attributed to a strength-layered target; in these experiments, however, they were produced in cohesionless targets. This study demonstrates that a flat floor is not necessarily diagnostic of strength layering in a target and, in some instances, might be the consequence of greater shear strengths in granular materials with high coefficients of static friction.