Sulfur in vacuum: Sublimation effects on frozen melts, and applications to Io's surface and torus

Abstract

A form of sulfur that is white at room temperature, shows almost no color change on cooling, and is fluffy in texture has been found in experiments on the effects of vacuum sublimation on solid sulfur. The white sulfur is a residual skin that forms on frozen sulfur in vacuum by differential evaporation of molecular species in the solid. S 8 ring sulfur is the dominant sublimation phase lost to the vacuum sink, and polymeric sulfur is the dominant residual phase. The microtexture of the fluffy sulfur layer is skeletal with an organized structure of filamentary components constructed of chains and clumps of submicron polyhedra. The layer is very porous (∼98%) and attains a thickness of ∼0.5 mm after 800 hr at 10 −7 Torr (∼10 −10 atm), and does not thicken much thereafter. Its color changes from that of the original melt freeze—yellow, tan, or brown depending on the prefreeze melt temperature—to white at room temperature. The UV/VIS reflectance spectrum (0.35 to 0.70 μm) of the original sulfur is greatly modified by formation of the vacuum surface layer: the blue absorption band edge moves toward the UV resulting in an increase in reflectivity in the range 0.42−0.46 μm as much as 400% and the UV reflectivity below 0.40 μm is reduced to one-third its original level to as low as 2%. Initially the changing band-edge position remains temperature sensitive, as in unmodified sulfur, shifting to shorter wavelengths with decreasing temperature, and returning to its precooled wavelength with temperature recovery; but once vacuum “maturity” is reached the temperature-induced excursion range of the absorption edge is reduced by an order of magnitude and is mostly in the UV whereas for ordinary sulfur (S 8) it is mostly in the blue. The sublimation rate from fresh frozen sulfur at initial exposure to high vacuum (∼10 −7 Torr) is ∼3 × 10 15 S cm −2 sec −1 at 300°K, increases steeply with temperature, decreases with higher vacuum pressure, and decreases with vacuum exposure time reaching an equilibrium flux of ∼3 × 10 14 S cm −2 sec −1 after ∼1200 hr. For fresh frozen sulfur evaporating at ∼300°K and ∼10 −7 Torr there occur significant spectral, color, and albedo effects in as little as 10 hr; samples become uniformly whitened within ∼100 hr, and progressive whitening and change in surface spectral properties continue for at least 1200 hr. This vacuum sulfur should exist in large quantity on Jupiter's satellite Io if there is solid free sulfur there that has solidified from a melt. A sulfur volcanism model for Io based on these findings is outlined. Color and spectra of different sulfur areas of Io may indicate relative crystallization age and cooling history. Concepts to be developed from this work on vacuum sulfur may help in understanding properties of Io's surface such as composition, texture, adsorbtivity, thermal inertia, photometry, and posteclipse brightening. The inferred flux of subliming sulfur from hotspots on Io is consistent with estimated turnover rates of the surface and is sufficient to supply the requisite sulfur to the Io plasma torus.