IOS FAST SODIUM - IMPLICATIONS FOR MOLECULAR AND ATOMIC ATMOSPHERIC ESCAPE

Recent observational evidence for sodium-bearing molecular ions in the Io plasma torus has strong implications for the nature of Io's atmosp here (Schneider et al. 1991, Science 253, 1394-1397). We use a Monte C arlo model of fast-sodium production to analyze high-resolution ground -based images of sodium emission. We find the observations can be expl ained if a significant fraction of Io's exobase is molecular, possibly including a sodium-bearing molecule. Total sodium loss rates from Io imply a collisionally thick atmosphere. Most of the images indicate si gnificant slow-down of the corotating plasma near Io. (C) 1994 Academi c Press, Inc.