Spatially Resolved Spectroscopy of Io's Pele Plume and SO ₂ Atmosphere

Abstract

We report on the first successful, spatially resolved spectroscopic observations of Io's SO ₂ atmosphere. Observations made with the Hubble Space Telescope Faint Object Spectrograph on 1 August 1996 using the 0.26 ^'' aperture have provided detections of SO ₂ gas in absorption in three locations on the Io disk: the Pele volcano, the Ra volcano, and T3 (a control region at latitude 45°S, longitude 300°W). The column densities of SO ₂ at these three locations have been determined by best-fit models of the geometric albedo and vary by a factor of five, with N_{SO 2}=3.25× 10 ¹⁶, 1.5× 10 ¹⁶, and 7×10 ¹⁵ cm ^-2 for Pele, Ra, and T3, respectively. Thus, SO ₂ gas is found to be present, and collisionally "thick" ( N≳6×10 ¹⁴ cm ^-2), in all three locations. The factor of five difference in column densities among the three targets provides the first direct evidence that the Io atmosphere is spatially inhomogeneous. Models of the SO ₂ gas band absorption at different temperatures give best-fit models with temperatures of T=280 (Pele), 150 (Ra), and 200 K (T3). Addition of SO to the models in the amounts N_SO ∼2.5×10 ¹⁵ (Pele), 5×10 ¹⁴ (Ra), and 1.5×10 ¹⁵ cm ^-2 (T3) provides improved (χ ²) fits to the data for all three locations and gives reasonably good agreement with the previous detection of SO in the Io atmosphere at an abundance ∼0.1 times that of SO ₂. We set an upper limit of 2×10 ¹⁴ cm ^-2 on the abundance of CS ₂. Observations with the HST WFPC2 obtained on 24 July 1996, 7 days earlier than our FOS spectra, showed an active plume over the Pele volcano. If Pele was still active on 1 August, our results imply that the regions of highest SO ₂ gas density on Io may be associated with active volcanic plumes and not sublimation from the visibly bright SO ₂ frost patches common on the surface of the satellite. As a result of the positive detection of atomic sulfur emission from the S I] 1900 Å multiplet from two of our three targets (∼3.6 kR, Pele; ∼1.6 kR, T3; <1.5 kR, Ra) our spectra also provide the first concurrent measurements of S, SO, and SO ₂ gases in the atmosphere and give a ratio of S/SO ₂ abundance of ∼0.003-0.007. The SO ₂ distribution we observe falls off much more slowly with latitude than the best available sublimation atmosphere models, but matches well the latitudinal distribution of the more realistic sublimation-driven atmosphere models that include hydrodynamic flow and photochemistry.