Low sulfide concentration in Mercury’s smooth plains inhibits hollows

Abstract

MESSENGER (Mercury, Surface, Space Environment, Geochemistry, and Ranging) mission to Mercury led to the discovery of hollows. These geological landforms have no close counterpart on other airless silicate bodies. Multispectral images and geochemical measurements by MESSENGER suggest that hollows are formed by the loss of volatile-bearing minerals. We investigated the mineralogical composition of the hollows using near-ultraviolet to near-infrared spectra obtained by MESSENGER. We compared reflectance spectra of hollows with laboratory spectra of Mercury’s analogs: sulfides, chlorides, silicates, and graphite. The best candidates to reproduce the curvature of the hollow spectra are calcium sulfide, magnesium sulfide, and sodium sulfide. In addition, we performed spectral modeling with spectra obtained at the highest spectral and spatial resolution within the hollows. Our results show that the enrichment of sulfides in hollow material is up to two times higher than the sulfide concentration derived from chemical measurements of Mercury’s high-reflectance smooth plains. This result explains the small percentage of hollows found within these plains.

Mid‐Infrared Spectroscopy of Sulfidation Reaction Products and Implications for Sulfur on Mercury

“…We previously suggested that hollow forming regions on Mercury may be enriched in sulfides by sulfidation reactions (Renggli et al., 2022). This hypothesis aligns well with a recent investigation of the mineralogy of hollows using multispectral data from MESSENGER (Barraud et al., 2023). This study suggested sulfide (CaS, MgS, and Na 2 S) enrichment in the hollow forming regions, with higher abundances of sulfides in the bright halos, and lower abundances in the hollow floors.…”

How Does the Thermal Environment Affect the Exosphere/Surface Interface at Mercury?

“…Examination of the spectral properties of colocated pyroclastic vents and hollows within Tyagaraja crater by Lucchetti et al (2021) show that the volatile component is compositionally different between the two units, and that the vent deposits are well described by a combination of sulfides, whereas the hollows may be better represented by a combination of chloride minerals. Comparisons of laboratory spectra of heated sulfides (Helbert et al 2013b), silicates, chlorides, and graphite with MASCS spectra of various hollows, however, show that sulfides more closely match the spectral properties of hollows (Barraud et al 2023). In any case, the proposed minerals are comprised of elements that, with the exception of sulfur, have been detected in the exosphere.…”

“…Their morphology suggests that they formed via sublimation of volatiles through a number of mechanisms, including solar heating and heating via contact with magmatic materials or impact melt (Blewett et al 2011(Blewett et al , 2013(Blewett et al , 2018Thomas et al 2014Thomas et al , 2016Phillipps et al 2021). Examination and modeling of the MESSENGER MDIS color imaging and MASCS Vis-NIR observations, coupled with comparisons with laboratory spectral measurements, indicate that the volatile species involved in hollows formation are predominately sulfides, such as CaS, MgS, and NaS (e.g., Barraud et al 2023), though chlorides have also been considered (e.g., Lucchetti et al 2021). It has been postulated that these volatiles are either a layer that has been exposed to the surface and subsequently lost either due to sublimation, where sublimation has been halted through the formation of a capping lag deposit (e.g., Blewett et al 2018), or formed through the interactions of sulfur-rich gases with bedrock (e.g., Phillipps et al 2021;Renggli et al 2022).…”

SpuBase: Solar Wind Ion Sputter Database for Modeling Purposes

“…Wilson et al (1999) have shown, for olivine, that the volume of an irradiated surface might instead increase as the mineral is amorphized, counteracting in part the sputter erosion. The significant increase in sputter yield-and presumably erosion rates and volume changes as shown in Table 5-between sulfides and oxides would favor the theory of Mercury's hollows being composed of fast-eroding sulfides (Vilas et al 2016;Lucchetti et al 2018Lucchetti et al , 2021Barraud et al 2023). This is only the case if sputtering can outperform desorption and impact processes.…”