JWST Detects the Forbidden 1.707-micron SO Emission Band Complex above Io's Active Volcano Kanehekili Fluctus

We observed Io while in eclipse as part of our JWST-ERS program #1373, with NIRSpec/IFU in the G235H/F170LP and G395H/F290LP set-up on 15 November 2022. We detected thermal emission from several volcanoes. Kanehekili Fluctus was exceptionally bright, and Loki Patera had most likely entered a new brightening phase. Spectra at a resolving power R ≈ 2700 were recorded between 1.65 and 5.3 μm, and were matched by a combination of blackbody curves that showed that the highest temperature, ∼1200 K, for Kanehekili Fluctus originated from an area ∼0.25 km2 in size, and for Loki Patera this high temperature was confined to an area ∼0.06 km−2 in size. Lower temperatures, down to 300 K, cover areas of ∼2000 km2 for Kanehekili Fluctus, and ∼5000 km2 for Loki Patera. We further detected the a1Δ ⇒ X3Σ− 1.707 μm rovibronic forbidden SO emission band complex over the southern hemisphere, which peaked at the location of Kanehekili Fluctus. This is the first time this emission has been seen above an active volcano, and suggests that the origin of such emissions is ejection of SO molecules directly from the vent in an excited state, after having been equilibrated at temperatures of ∼1500 K below the surface, as was previously hypothesized.