We propose that the observed enrichment of sulfur at the surface of Mercury (up to 4 wt.%) is the product of silicate sulfidation reactions with a S-rich reduced volcanogenic gas phase. Here, we present new experiments on the sulfidation behavior of olivine, diopside, and anorthite. We investigate these reaction products, and those of sulfidized glasses with Mercury compositions previously reported, by mid-IR reflectance spectroscopy. We investigate both the reacted bulk materials as powders, as well as cross-sections of the reaction products by in-situ micro-IR spectroscopy. The mid-IR spectra confirm the presence of predicted reaction products including quartz. The mid-IR reflectance of sulfide reaction products, such as CaS (oldhamite) or MgS (niningerite), is insufficient to be observed in the complex run products. However, the ESA/JAXA BepiColombo mission to Mercury will be able to test our hypothesis by investigating correlated abundances of sulfides with other reaction products such as quartz.

To support the data analysis for the MErcury Radiometer and Thermal Infrared Imaging Spectrometer (MERTIS) instrument on the ESA-JAXA BepiColombo mission, we have measured the thermal infrared emissivity of finely grained silicates (<25 μm grain size) at different temperatures under vacuum to simulate the daytime conditions on the surface of Mercury. The silicates were selected to represent the mineralogy of Mercury as closely as possible (Helbert et al., 2007; Namur and Charlier, 2017; Vander Kaaden et al., 2017). The set includes one olivine (a Mg-rich forsterite), three pyroxenes (diopside, enstatite, and hypersthene), five feldspars (plagioclase group; anorthite, labradorite, andesine, oligoclase, and K-feldspar; microcline) and a feldspathoid (nepheline). The emissivity measurements for each mineral was carried out within the MERTIS spectral range of 7-14 μm with temperatures increasing from 100 C up to 500 C under vacuum (~0.1 mbar). The relationships between the spectral parameters such as the Christiansen Feature (CF) position, first Reststrahlen band (RB1) position, RB1 emissivity, and RB spectral contrast and temperature were investigated for all silicates. The study shows that the RB1 position shifts to longer wavelengths, RB1 emissivity decreases, and RB spectral contrast increases with increasing temperatures for all silicates studied. We apply the plot of CF vs RB1 as a tool to discriminate the major silicate groups such as feldspars, pyroxenes, and olivine, regardless of the temperatures at which they were measured. The CF vs RB1 plot can facilitate the first order discrimination of the mineralogy of Mercury’s surface with MERTIS. Moreover, this approach can be more widely used to map the igneous surface mineralogy of silicate targets such as the Moon, Mars, and S-type asteroids in the 7-14 μm spectral region with remote sensing from orbit and ground-based telescope observations.

Global mapping of the nature and distribution of volatiles such as sulfides on Mercury’s surface is essential for understanding the thermal evolution of the planet. The surface exposure of these sulfides over extreme day-night temperature cycles (176 days; 450 degC to -170 degC) on Mercury leads to thermal weathering of these sulfide compounds. It has been seen that among the proposed sulfides on Mercury (MgS, FeS, CaS, CrS, TiS, NaS, and MnS), CaS showed relatively stable and distinctive spectral features in the thermal infrared region (TIR; 7-14 μm) when studied under the simulated Mercury day conditions for temperatures ranging from 100 degC up to 500 degC under vacuum (0.1 mbar) (Varatharajan et al., 2019). In this study, we re-investigated the stability of CaS and its spectral emissivity spectral behavior. We exposed the sample for four consecutive Earth days simulating Mercury day cycles and measured the TIR spectra of CaS for temperatures up to 500 degC (with steps of 100 degC) every day. This time the spectral analysis is coupled and supported by XRD diffraction on the fresh and temperature-processed sample, showing the mineralogical evolution with temperature. We confirm that CaS is a stable compound and therefore it would remain stable on Mercury’s surface regardless of investigated peak surface temperatures. This study further implies that, for the hollows dominated by the sublimation of sulfides on Mercury (Blewett et al., 2013; Helbert et al., 2013a; Vilas et al., 2016), CaS could be the last of the sulfides that could be mapped on Mercury as other sulfides were lost by thermal decomposition, leaving behind hollows. This could make CaS an important tracer for other sulfides, which might be lost in the hollow-forming process and supports the detection of CaS within hollows by MESSENGER (Vilas et al., 2016). The emissivity spectra reported here are significant for the detection and mapping of CaS associated with hollows and pyroclastics using the Mercury Radiometer and Thermal Imaging Spectrometer (MERTIS) datasets.