Detection of the forbidden SO [a.sup.1] [DELTA] [right arrow] [X.sup.3] [[SIGMA].sup.-] rovibronic transition on Io at 1.7 [micro]m

We report the discovery of the forbidden electronic [a.sup.1] [DELTA] [right arrow] [X.sup.3] [[SIGMA].sup.-] transition of the SO radical on Io at 1.7 [micro]m with the W. M. Keck II telescope on 24 September 1999 (UT), while the satellite was eclipsed by Jupiter. The shape of the SO emission band suggests a rotational temperature of ~1000 K; i.e., the gas is extremely hot. We interpret the observed emission rate of ~2 x [10.sup.27] photons [s.sup.-1] to be caused by SO molecules in the excited [a.sup.1] [DELTA] state being directly ejected from the vent at a thermodynamic quenching temperature of ~1500 K, assuming a SO/S[O.sub.2] abundance ratio of ~0.1 and a total venting rate of ~[10.sup.31] molecules [s.sup.-1] (Strobel and Wolven 2001, Astrophys. Space Sci. 277, 1-17). The shape of our complete (1.6-2.5 [micro]m) spectrum suggests that the volcano Loki contains a small (~2 [km.sup.2]) hot spot at 960 [+ or -] 12 K, as well as a larger (~50 [km.sup.2]) area at 640 [+ or -] 5 K. Key Words: Io; infrared observations; volcanism; atmosphere composition; atmosphere structure.