Spatially resolved observations of the forbidden SO a1Δ→X3Σ− rovibronic transition on Io during an eclipse and a volcanic eruption at Ra Patera

We report observations of Io that were conducted on UT 12 November 2002 with the NIRSPEC spectrometer, coupled to the adaptive optics system, on the 10-m Keck II telescope. We detected a bright eruption in the Ra Patera area, with a color (H-K′ band) temperature of 1031±110 K over an effective area of 1.5±0.2 km2. The eruption was associated with a hot plume, which revealed itself through SO emission at a rate of (9±3)×1025 photonss−1, about 10-15% of Io's total SO flux at the time. The rotational temperature was 700±150 K. No significant SO emission was received from Io's northern hemisphere (north of Ra Patera/Loki); roughly 50% of the total SO emission came from the equatorial region (including Ra Patera, Janus and Loki), and ∼40% came from the south. The rotational temperatures typically measured between 600 and 1000 K. The emissions are most likely produced by SO molecules ejected out of volcanic vents in the excited a1Δ state. Our narrow band images that span the SO emission band suggest that a source near Io's south pole, Nemea, may be a source of some of the southern SO emissions.