Phase transformations and the spectral reflectance of solid sulfur: Can metastable sulfur allotropes exist on Io?

The spectral reflectance of elemental sulfur that has solidified from a melt changes with time after the sulfur has solidified. This temporal variation arises as a result of phase transformations occuring within the solid. In a set of laboratory investigations, we find that variations in the thermal history of the sulfur samples profoundly affect the solid-state transformation rate and the corresponding spectral variation of freshly frozen sulfur. In particular, samples that were heated to 393 and 453 K for various lengths of time (up to 50 hr) and then solidified and aged at various temperatures (from 260 to 318 K) brighten at visible wavelengths on very different time scales. This temporal variation is thought to be due to differences in the amount and type of metastable allotropes present in the sulfur after solidification as well as to the physics of the phase transformation process itself. Our laboratory data have implications for the spectral variation and physical behavior of freshly solidified sulfur, if any exists, on Jupiter's satellite Io. Depending on its thermal history, molten sulfur on Io will initially solidify into a glassy solid or a monoclinic crystalline lattice; these forms may contain polymeric molecules as well as the more abundant S ₈ molecules. If freshly frozen sulfur on Io can lose heat rapidly and approach ambient dayside Io temperatures within a few hours, then our laboratory results imply that the metastable monoclinic or polymeric allotropes can be maintained on Io and will take years to convert to the stable orthorhombic crystalline form. We present cooling rate calculations that indicate that metastable allotropes can be preserved in small droplets of sulfur ejected during volcanic plume eruptions on Io. However, sulfur in large lakes or flows on Io might remain warm long enough for the conversion of monoclinic sulfur into orthorhombic sulfur to proceed, and we would expect rapid brightening (on the order of hours or days) in these areas after the liquid sulfur has solidified.