The Dual Sources of Ioʹs Sodium Clouds

Ioʹs sodium clouds result mostly from a combination of two atmospheric escape processes at Io. Neutralization of Na+ and/or NaX+ pickup ions produces the “stream” and the “jet” and results in a rectangular-shaped sodium nebula around Jupiter. Atmospheric sputtering of Na by plasma torus ions produces the “banana cloud” near Io and a diamond-shaped sodium nebula. Charge exchange of thermal Na+ with Na in Ioʹs atmosphere does not appear to be a major atmospheric ejection process. The total ejection rate of sodium from Io varied from 3×1026 to 25×1026 atoms/s over seven years of observations. Our results provide further evidence that Ioʹs atmospheric escape is driven from collisionally thick regions of the atmosphere rather than from the exosphere.