Observations of the Pele Plume (Io) with the Hubble Space Telescope

J.R. Spencer (Lowell Obs.), G.E. Ballester (U.Michigan), P. Sartoretti (Inst. Astrophys. Paris), A.S. McEwen (U. Arizona), J.T. Clarke (U. Michigan), M. McGrath (STScI)

In July 1996, with the Hubble Space Telescope, we observed the Pele plume silhouetted against Jupiter at a wavelength of 0.27, the first definitive observation of an Io plume from Earth. The plume height, 420 tex2html_wrap_inline15 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 before the July 1995 detection 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, where it was not seen. The wavelength dependence of the optical depth in the 1996 observations can be matched by a plume of fine dust, with minimum mass of 10 tex2html_wrap_inline17 g and maximum particle size of 0.08 tex2html_wrap_inline19 m, or by a plume of SO tex2html_wrap_inline21 gas with a column density of tex2html_wrap_inline23 cm tex2html_wrap_inline25 and total mass of 10 tex2html_wrap_inline27 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.