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 \lambda 1479 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 \pm 8 % and the forbidden transitions 38 \pm 8 % , on average , to the total signal of the S i \lambda 1479 multiplets .
Using the optically thick and thin transitions of S i \lambda 1479 observed near the limbs of Io , we derive a tangential atmospheric sulfur column abundance of 3.6 \times 10 ^ { 12 } cm ^ { -2 } \hskip { 3.0 pt } < \hskip { 3.0 pt } { \cal N } _ { s } \hskip { 3.0 pt } < 1.7 \times 10 ^ { 13 } cm ^ { -2 } , which is independent of electron temperature and density .
A low density SO _ { 2 } atmosphere , { \cal N } _ { so _ { 2 } } \sim 5-10 \times 10 ^ { 15 } cm ^ { -2 } , consistent with that inferred from other recent observations , is most consistent with these bounds .