Surface Response of Mercury's Sulfides under Solar Wind Ion Irradiation

TL;DR

This study investigates how Mercury's surface sulfides respond to solar wind irradiation, revealing their radiation hardness and implications for sulfur detection in Mercury's exosphere, which is crucial for upcoming space missions.

Contribution

It provides the first experimental evidence that Mercury-relevant sulfides resist radiation-induced sulfur depletion, challenging previous assumptions about surface segregation processes.

Findings

Mercury-relevant sulfides show no detectable segregation after irradiation.

Sulfides with S-S spacing >3.2 Å resist sulfur depletion.

Radiation hardness may enhance sulfur detection in Mercury's exosphere.

Abstract

The MESSENGER mission revealed unexpectedly high sulfur content within Mercury's surface, deviating from the Lunar regolith, which was, until recently, considered a good Mercury analogue. Mercury's exposure to energetic space weathering processes such as meteoritic impact and solar-wind sputtering suggests this high sulfur concentration should be reflected in the suprathermal sulfur population of the Hermean exosphere. UV spectroscopy has not yet detected exospheric sulfur, a result attributed primarily to its low glow-factor. Future detection by BepiColombo's Mass Spectrum Analyzer depends on sulfur abundance in the exosphere. Radiation-induced segregation has been observed in the common sulfide troilite (FeS), a constituent mineral in returned Lunar samples, meteorites, and asteroids, where the resulting metal cap is expected to reduce sulfur ejection to Mercury's exosphere. In this work, we investigate the irradiation response of Mercury-relevant sulfides. Niningerite (MgS) and oldhamite (CaS) were irradiated with solar-wind speed 2 keV H$_2^+$ or 4 keV He$^+$, and in-situ compositional and chemical bond analysis as a function of fluence was performed using an XPS microprobe. Neither MgS nor CaS expressed detectable damage-induced segregation and instead reached metal-to-sulfur ratios close to bulk with irradiation. Based on this finding, structural information, and literature analyses, we infer that an S-S anionic spacing exceeding $\sim$3.2 \AA{} inhibits radiation-induced sulfur depletion and promotes stoichiometric sputtering. We therefore predict no cation (metal) surface segregation in Hermean sulfides and no reduction in suprathermal sulfur emission caused by metal cladding formation in TiS, CrS, and Ca-Mg sulfides. This radiation-hardness for Mercury-relevant sulfides is novel and unexpected, and should facilitate detection in Mercury's exosphere by the BepiColombo mission.