The Abundance of Atomic Sulfur in the Atmosphere of Io

Abstract

Observations with the Space Telescope Imaging Spectrograph aboard the Hubble Space Telescope have been used to constrain the atomic sulfur column density in Io's atmosphere. The S I λ1479 dipole allowed and forbidden transition multiplets have been resolved for the first time at Io, enabling the study of both optically thick and thin transitions from a single atomic species. The allowed transitions contribute 62%+/-8% and the forbidden transitions 38%+/-8%, on average, to the total signal of the S I λ1479 multiplets. Using the optically thick and thin transitions of S I λ1479 observed near the limbs of Io, we derive a tangential atmospheric sulfur column abundance of 3.6×10¹²cm^{- 2}<N_S<1.7×10¹³ cm^-2, which is independent of electron temperature and density. A low-density SO₂ atmosphere, N_SO2~(5-10)×10¹⁵ cm^-2, consistent with that inferred from other recent observations, is most consistent with these bounds.