The Pele Plume (Io): Observations with the Hubble Space Telescope

J. Spencer, P. Sartoretti, G. Ballester, A. McEwen, J. Clarke, and M. McGrath

Submitted to Geophysical Research Letters April 3rd, 1997, revised July 10.

Revised version available in compressed postscript form, including figures from here, or if that doesn't work, try anonymous ftp to, pub/spencer, and get file File size is 179 kbytes

Abstract (submitted version):

In July 1996, with the Hubble Space Telescope (HST), we observed the Pele plume silhouetted against Jupiter at a wavelength of 0.27 microns, the first definitive observation of an Io plume from Earth. The plume height, 420 +/- 40 km, was greater than any plume observed by Voyager. We also obtained a tentative detection of the plume in reflected sunlight, at the same wavelength, in July 1995. A non-detection of the plume 21 hours earlier suggests that it is capable of very rapid time variations. The 1996 images showed that the plume had significantly smaller optical depth at 0.34 and 0.41\mic, where it was not detected. The wavelength dependence of the optical depth in the 1996 observations can be matched by a plume of fine dust, with minimum mass of 1e9 g and maximum particle size of 0.08 microns, or by a plume of SO2 gas with a column density of 3.7e17 /cm2 and total mass of 1.1e11 g. Either of these possibilities, or a combination of them, is quantitatively consistent with earlier Voyager and HST observations of Io. Our models of dust scattering suggest, however, that early Voyager imaging estimates of the mass of the Loki plume (Collins 1981) may have been much too large.
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