A search for emission features in Io's extended cloud

Abstract

If sputtering provides the sodium in Io's extended cloud, then other elements abundant in Io's surface layer should also be present in this cloud. The paper presents spectra in the range from 3100 A to 8700 A of a portion of this cloud where Io's scattered continuum is weak, but where the sodium emission is still strong. Aluminum and calcium are found to be underabundant relative to sodium. Upper limits are set to some other cosmically abundant elements. In addition the 10,830 A feature was detected over various parts of the cloud, but it had an intensity comparable to that observed elsewhere in the night sky. There is no indication that helium emission brighter than 200 rayleighs occurs from the cloud itself.