Sodium Emission from Io: Implications

The surface of lo may be covered with a layer of ammonia ice containing trace amounts of sodium potassium, and calcium; and atmospheric nitrogen could be formed as a photochemical product of ammonia photolysis. Intense sporadic sodium emission from Io can be excited by collisions involving vib~ationally excited n.itrogen molecules. These metastable molecules may be formed by electron impact, with electrons energized by an auroral mechanism. In order to account for the intensity ratio of the sodium doublet, it is necessary to invoke scattering in a thick gaseous envelope ejected by Io. Subject headings: abundances Jupiter satellites