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

Bright geological formations called hollows were observed at Mercury and are mostly found in impact craters. While their composition is unknown, subsurface sulfur could be responsible for their formation. Mercury's porous surface favors the subsurface diffusion of returning exospheric atoms. With time, diffusing atoms could form reservoirs in Mercury's subsurface. We use a 3-D Exospheric Global Model (EGM) including subsurface diffusion to study the evolution of sulfur and sodium in Mercury's subsurface. We identify similar spatial distributions of the sulfur reservoir and of the hollows, but no relationship in the case of sodium. EGM predicts that the longitudinal distribution of hollows correlates with the peak subsurface S column density, ∼60° west of each “hot longitude.” Deeper reservoirs at Mercury's hot poles are predicted, potentially explaining the larger hollows found in these regions. The subsurface diffusion could continuously replenish the sulfur reservoir that could be responsible for the formation of hollows.