Evidence for an Ionic Pathway in Oxygen and Sulfur Atoms Escaping Io

Interactions between Io’s atmosphere and the plasma torus can produce jets of energetic neutral atoms (ENAs) through either dissociative recombination or charge exchange. Sodium and potassium are thus far the only species with ENA jets that have been observed and characterized. Analogous processes may produce ENA jets for oxygen, sulfur, and chlorine, as well. We offer results from an observational campaign to search for these jets using high-resolution far-ultraviolet spectra from HST’s Cosmic Origins Spectrograph. After correcting for the moon’s Doppler shift, all Io observations with the G130M grating are synthesized to produce a single spectrum. Unblended, isolated lines of neutral oxygen, sulfur, and chlorine are identified, and we co-add these to produce a reference line spread function for each species. This reference is then compared with two observations offset from Io’s disk, targeting atmospheric escape in the extended clouds. Off-disk spectra were obtained with pointing just beyond Io’s orbit where Io was near its maximum elongation from Jupiter. Fast neutral jets leaving Io are oriented with motion tangential to the plasma torus: parallel to the line-of-sight when Io is receding and antiparallel when Io is approaching. These off-disk observations indeed reveal distinct Doppler shifts in the line spread functions of neutral atoms. On each side of Io’s orbit, the oxygen gas velocity is shifted towards the bulk flow velocity of the torus. On the receding side, peak emissions occur near the ~57 km/s velocity that the torus sweeps past Io. Doppler-shifted structure is also observed in sulfur, particularly on the receding side. However, since S emissions are weaker than O in this band, a sulfur gas velocity cannot be accurately determined. Dissociative recombination of NaCl⁺ and KCl⁺ is also expected to yield chlorine jets, but chlorine emission off-disk is too faint to confirm. This experiment shows that Io’s neutrals are far reaching; these atomic jets will bombard adjacent satellites and populate diffuse clouds that likely extend to hundreds of Jovian radii.