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.