The Pioneer 10 and 11 Jupiter probes have provided several opportunities for observation of the Jovian satellites J I through J V. From these data, a tenuous atmosphere of hydrogen for Io has been identified. In the present work, an additional short-wavelength (less than 800 A) emission associated with an extended cloud centered on Io is reported and interpreted as arising from the radiative decay of excited atomic ions. Characteristic X-rays produced by electron bombardment of Io's surface may also contribute to the signal. Emission features associated with Amalthea (J V) and Europa (J II) are also observed. Signals apparently associated with J V occur in the long-wavelength channel, while emissions were observed in the short-wavelength channel during J II observations. The data of the long-wavelength channel are interpreted as arising from atomic hydrogen Lyman-alpha emission in all cases. The source species for the short-wavelength emissions cannot at this time be unambiguously determined, but the wavelength range of the signals is well established.
LUNAR AND PLANETARY EXPLORATION
Colloquium on Jupiter: Studies of the interior, atmosphere, magnetosphere and satellites; May 19-21, 1975; Tucson, AZ