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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.