  A thermophysical model was developed to test the viability of 57 candidate
hollow-forming volatiles within the hollow-formation model framework of Blewett
et al. (2013). We find that the thermophysical properties of elemental sulfur
(S) combined with the abundance of S on Mercury, make it the most likely
hollow-forming volatile explored in this study. We propose a novel model for
hollow formation in which a subsurface heat source drives sulfur-rich systems
that deposit volatiles (importantly, S) in the near-surface at night within a
"sulfur permafrost zone", and daytime solar heating drives sublimation to form
hollows.
