Mercury’s Hollows : A Potential Signature of the Sulfur Exosphere-Subsurface Transport

Mercury’s surface that interacts directly with solar particles and micro-meteoroids, resulting in a surface-bound exosphere composed of planetary atoms (Leblanc & Johnson 2003, Berezhnoy & Klumov 2008). Various elements, such as Na, K, Ca, and Mg, have been detected on Mercury's surface and in its exosphere (McClintock et al. 2018). However, while sulfur has been identified on Mercury's surface (Nittler et al. 2020), it has not been observed in the exosphere (Leblanc et al. 2023). Despite this, numerous studies predict that sulfur, like other volatiles, should be released through micro-meteoroid impact vaporization and photon-stimulated desorption (Sprague et al. 1995, Berezhnoy & Klumov 2008, Schaible et al. 2020). Mercury's surface can reach temperatures up to ~700 K on the dayside, allowing atoms to thermally desorb from the surface. The temperature gradient in Mercury's porous regolith should promote Knudsen diffusion in the subsurface. Repeated adsorption and desorption could lead to the migration of volatiles within the regolith, creating a long-term reservoir (Verkercke et al. 2024).

Recent research suggests that geological formations known as hollows, primarily observed in craters, might be formed by the local accumulation of subsurface sulfur (Phillips et al. 2021, Barraud et al. 2023). These features could be maintained by the migration of atomic sulfur in the exosphere and/or diffusion processes in the subsurface. In fact, subsurface volatile reservoirs have been proposed to explain the origin of certain geological features. Calcium sulfide has been suggested as a candidate to account for the presence of low-reflectance material, including hollows (Barraud et al. 2023). The correlation of these features with impact craters indicates that the volatiles forming these structures are buried and require impacts to be exposed (Thomas et al. 2014, Blewett et al. 2016, Phillips et al. 2021).

The variability of sulfur in Mercury's exosphere and subsurface was recently examined using an exospheric global model (EGM) (Verkercke et al. 2025). This model uses a Monte-Carlo approach to predict the ejection of surface atoms (Leblanc & Johnson 2010) through four processes: photon-stimulated desorption (PSD), solar wind sputtering (SWS), micro-meteoroid impact vaporization (MMIV), and thermal-stimulated desorption (TSD). Verkercke et al. (2025) predicted that sulfur primarily accumulates at the cold poles, which are a result of Mercury's 3:2 spin-orbit rotation, particularly in areas with high calcium surface abundance. However, this study did not correlate the total sulfur quantity in Mercury's regolith with hollows. Since the material forming hollows is believed to be buried in the subsurface, this study focuses on the volatiles accumulated in the regolith and shielded from ejection processes. Using a 3-D EGM with the same assumptions as Verkercke et al. (2025), this work aims to analyze the amount of sulfur as a function of depth in the regolith and compare the spatial distribution of the sulfur reservoir with the hollows' spatial distribution reported by Thomas et al. (2014) and Blewett et al. (2016). Additionally, a similar analysis is conducted for sodium on Mercury to compare the reservoir formation processes between the two volatile species. Our results indicate a similar spatial distribution of the sulfur reservoir and hollows, with no such correlation found for sodium, emphasizing the role of hot and cold longitudes in the formation of these reservoirs.

References

 

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