First Detection of [SI] in Near-IR JWST Observations of Io in Eclipse, and Comparison With SO Emissions, Evolving Volcanic Eruptions, and Prior UV HST-STIS [SI] Emissions

We observed Io with the James Webb Space Telescope (JWST) NIRSpec/Integral Field Unit (1.0–5.3 μm, (Formula presented.)) in August 2023 while the satellite was in eclipse. Thermal emission from Kanehekili Fluctus is consistent with the cooling of lava flows after a vigorous eruption in November 2022. At Loki Patera, after a new brightening event was detected in November 2022, the lava lake was in a quiescent state, as expected from previous analyses. We mapped the SO emission band at 1.707 μm, and detected, for the first time, [SI] emission lines at 1.082 and 1.131 μm. The SO emissions are concentrated above Kanehekili Fluctus, and in two regions in the northern hemisphere. The disk-averaged brightness is 14.5 kR. The emissions are sourced from SO molecules ejected from 1,500 to 1,700 K vents in an excited state, with a typical SO column density above the northern hemisphere of (Formula presented.) (Formula presented.). Sulfur emissions are distributed homogeneously across a band in the northern hemisphere. The disk-averaged total brightness is 5.6 kR, versus 9.65 kR in the north. The emissions are produced through direct electron impact excitation by (Formula presented.) 4 eV electrons in the torus (density 2,500 (Formula presented.)) penetrating the atmosphere, and require the atmosphere to be hot ((Formula presented.) 1,700 K) to populate the upper levels before excitation. The sulfur column density over the northern hemisphere is (Formula presented.) (Formula presented.). These same parameters can explain recent 0.7725- (Formula presented.) m observations, as well as the 147.9-nm multiplet emissions observed with HST-STIS (Formula presented.) 20 years earlier. This suggests a quite stable system over decades-long timescales.