An estimate of the equilibrium speciation of sulfur vapor over solid sulfur and implications for planetary atmospheres

Sulfur allotropes have been observed in planetary atmospheres and are believed to have been present in the ancient Earth atmosphere. The vapor pressures of sulfur allotropes, especially S-2, S-3, and S-4, are poorly known at typical atmospheric temperatures, but have generally been assumed to be high enough to represent significant gas phase abundances in model calculations. Here I present estimates of the speciation of the equilibrium vapor pressure over solid sulfur, which would seem to imply that the vapor pressures of most allotropes are too low at typical atmospheric temperatures for gas phase formation reactions to be important. However, consideration of the kinetics of condensation shows that gas phase sulfur allotrope reactions can be important under certain conditions. The implications for the mechanism of sulfur isotope mass-independent fractionation on early Earth are discussed. Implications for the presence of sulfur allotropes in Venusian clouds, the distribution of S-2 in the atmosphere of Jupiter following the comet Shoemaker-Levy 9 impacts, and the observation of S-2 in volcanic plumes on Io are also presented.