DPS 2001 meeting, November 2001
Session 24. Io, Tori, and Satellite Atmospheres
Oral, Chairs: I. de Pater, W. Smythe, Wednesday, November 28, 2001, 5:00-6:40pm, Regency E

## [24.01] Io in Eclipse: Keck LWS Images and the Detection of the Forbidden SO a1 \Delta arrow X3 \Sigma- Rovibronic Transition at 1.7 micron.

I. de Pater, H. G. Roe, J. R. Graham (UC Berkeley), D. F. Strobel (JHU), P. Bernath (Univ. of Waterloo), F. Marchis (UC Berkeley), B. Macintosh (IGPP/LLNL)

On UT 24 September 1999 we observed Io going into eclipse with the LWS (7-12 micron) system on the Keck I and, simultaneously, with NIRSPEC (1-2.5 micron) on the Keck II telescope. We obtained diffraction limited images (~ 0.25") at ~ 3.8, 5, 7.8 and 11.7 micron both in and out of eclipse. Loki and the Janus-Kanehikili group are clearly visible on the disk. With NIRSPEC we took spectra between 1.6 and 2.5 micron. The combined spectrum suggests that the volcano Loki contains a small (~ 2 km2) hot spot at ~ 960 K, a 50 km2 region at 640 K, and progressively larger areas at lower temperatures (400, 200 and 150 K), i.e., clearly an active volcano with outflowing cooling lava flows.

During the eclipse a bright emission band was discovered near 1.7 micron. This band has been identified with the a1 \Delta arrow X3 \Sigma- forbidden transition of SO. We interpret the observed emission rate of ~ 2 \times 1027 photons/s to be caused by SO molecules in the excited state directly ejected from the vent at a thermodynamic quenching temperature of ~ 1500 K, and SO/SO2 abundance ratio of ~ 0.1. The shape of the SO emission band suggests a rotational temperature of ~ 1000 K.