^1,2,3C.Cartier,⁴O.Namur,⁵ L.R.Nittler,⁵S.Z.Weider,⁶E.Crapster-Pregont,⁶A.Vorburger,⁶E.A.Franck,¹B.Charlier
Earth and Planetary Science Letters 534, 116108 Link to Article [https://doi.org/10.1016/j.epsl.2020.116108]
¹Département de Géologie, Université de Liège, 4000, Sart Tilman, Belgium
²Laboratoire Magmas et Volcans, Université Blaise Pascal, Clermont-Ferrand, 63038, France
³CRPG, CNRS, Université de Lorraine, UMR 7358, Vandoeuvre-lès-Nancy, 54501, France
⁴Department of Earth and Environmental Sciences, KU Leuven, Leuven, 3001, Belgium
⁵Carnegie Institution of Washington, Department of Terrestrial Magnetism, Washington, DC 20015, USA
⁶Department of Earth and Planetary Sciences, American Museum of Natural History, New York, NY 10024, USA
Copyright Elsevier

In this study we investigate the likeliness of the existence of an iron sulfide layer (FeS matte) at the core-mantle boundary (CMB) of Mercury by comparing new chemical surface data obtained by the X-ray Spectrometer onboard the MESSENGER spacecraft with geochemical models supported by high-pressure experiments under reducing conditions. We present a new data set consisting of 233 Ti/Si measurements, which combined with Al/Si data show that Mercury’s surface has a slightly subchondritic Ti/Al ratio of 0.035 ± 0.008. Multiphase equilibria experiments show that at the conditions of Mercury’s core formation, Ti is chalcophile but not siderophile, making Ti a useful tracer of sulfide melt formation. We parameterize and use our partitioning data in a model to calculate the relative depletion of Ti in the bulk silicate fraction of Mercury as a function of a putative FeS layer thickness. By comparing the model results and surface elemental data we show that Mercury most likely does not have a FeS layer, and in case it would have one, it would only be a few kilometers thick (<13km). We also show that Mercury’s metallic Fe(Si) core cannot contain more than ∼1.5 wt.% sulfur and that the formation of this core under reducing conditions is responsible for the slightly subchondritic Ti/Al ratio of Mercury’s surface.

^1,2Brendan A. Haas,^1,2Christine Floss,³Rhonda M. Stroud,^1,2Ryan C. Ogliore
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13438]
¹Laboratory for Space Sciences, Washington University, St. Louis, Missouri, 63130 USA
²Physics Department, Washington University, St. Louis, Missouri, 63130 USA
³Materials Science and Technology Division, Naval Research Laboratory, 4555 Overlook Ave. SW, Washington, District of Columbia, 20375 USA
Published by arrangement with John Wiley & Sons

Aluminum foils from the Stardust cometary dust collector contain impact craters formed during the spacecraft’s encounter with comet 81P/Wild 2 and retain residues that are among the few unambiguously cometary samples available for laboratory study. Our study investigates four micron‐scale (1.8–5.2 μm) and six submicron (220–380 nm) diameter craters to better characterize the fine (<1 μm) component of comet Wild 2. We perform initial crater identification with scanning electron microscopy, prepare the samples for further analysis with a focused ion beam, and analyze the cross sections of the impact craters with transmission electron microscopy (TEM). All of the craters are dominated by combinations of silicate and iron sulfide residues. Two micron‐scale craters had subregions that are consistent with spinel and taenite impactors, indicating that the micron‐scale craters have a refractory component. Four submicron craters contained amorphous residue layers composed of silicate and sulfide impactors. The lack of refractory materials in the submicron craters suggests that refractory material abundances may differentiate Wild 2 dust on the scale of several hundred nanometers from larger particles on the scale of a micron. The submicron craters are enriched in moderately volatile elements (S, Zn) when normalized to Si and CI chondrite abundances, suggesting that, if these craters are representative of the Wild 2 fine component, the Wild 2 fines were not formed by high‐temperature condensation. This distinguishes the comet’s fine component from the large terminal particles in Stardust aerogel tracks which mostly formed in high‐temperature events.