Enigmatic Hollows Formed on Mercury through Gas-Rock Reactions

Schematic drawing of hollows formation through sulfidation of Mercury’s surface, secondary sulfide remobilization, and finally collapse. Credit: Renggli et al., 2022.

One of the most surprising findings from NASA’s MESSENGER mission was that Mercury is not depleted in volatile elements and compounds, despite its proximity to the Sun and small size. Interestingly, there is more evidence for the existence of substantial water ice in permanently shadowed craters on Mercury than on the Moon. For example, cold trap areas first identified by telescopic observations on Mercury in the 1990s are interpreted to be nearly 100% occupied by water ice. In contrast, analogous areas on the Moon observed by the Lunar Reconnaissance Orbiter over the last decade are patchier, with an estimated <12% surface water ice, according to studies by Ariel Deutsch (NASA Ames Research Center). MESSENGER discovered another class of enigmatic volatile-related features called hollows — shallow, flat-floored, steep-sided, rimless depressions generally found in or around much smaller impact craters. Hollows appear to form through sublimation of a moderately volatile material, which has been hypothesized to be either graphite (C) or sulfides.

A new study by Christian Renggli (Institut für Mineralogie, Westfälische Wilhelms-Universität Münster) and co-authors describes new sulfide synthesis experiments that test a novel formation mechanism for the hollows. In these experiments, reactions between S2 gas and synthetic mercurian-composition glasses at low pressure and increasing temperature form Fe-, Mg-, Ca-, and Ti-bearing sulfides. Such materials could be produced on Mercury through degassing of a reduced S-rich gas during volcanic eruptions, followed by a sulfidation reaction with the surface rocks and regolith, and are predicted to be porous and fragile. Thus, they could potentially be removed by heat, solar irradiation, and disturbance by repeated impacts that stir and mix regolith (impact gardening) to produce the enigmatic pits we call the hollows. This hypothesis can be tested in the future by the mid-infrared (MIR) spectrometer onboard the ESA/JAXA BepiColombo mission because of distinct mineralogical predictions of associated CaS, MgS, FeS, quartz, and aluminosilicates. READ MORE