The Abundance of Atomic Sulfur in the Atmosphere of Io

Observations with the Space Telescope Imaging Spectrograph aboard the HubbleSpace Telescope have been used to constrain the atomic sulfur column density inIo's atmosphere. The SI 1479 dipole allowed and forbidden transition multipletshave been resolved for the first time at Io, enabling the study of bothoptically thick and thin transitions from a single atomic species. The allowedtransitions contribute 62 +/- 8% and the forbidden transitions 38 +/- 8%, onaverage, to the total signal of the SI 1479 multiplets. Using the opticallythick and thin transitions of SI 1479 observed near the limbs of Io, we derivea tangential atmospheric sulfur column abundance of 3.6*10^12 cm^-2 < N_s <1.7*10^13 cm^-2, which is independent of electron temperature and density. Alow density SO_2 atmosphere, N_so2 ~ 5-10*10^15 cm^-2, consistent with thatinferred from other recent observations, is most consistent with these bounds.Comment: 12 pages, 6 figures, accepted for publication in the Ap