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 ftp.lowell.edu, pub/spencer,
and get file grlpele97.ps.Z. 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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