  In February 2003, March 2003 and January 2004 Pele plume transmission spectra
were obtained during Jupiter transit with Hubble's Space Telescope Imaging
Spectrograph (STIS), using the 0.1 arcsec long slit and the G230LB grating. The
STIS spectra covered the 2100-3100 A wavelength region and extended spatially
along Io's limb both northward of Pele. The S2 and SO2 absorption signatures
evident in the these data indicate that the gas signature at Pele was
temporally variable, and that an S2 absorption signature was present ~ 12 deg.
from the Pele vent near 6+/-5 S and 264 +/-15 W, suggesting the presence of
another S2 bearing plume on Io. Contemporaneous with the spectral data, UV and
visible-wavelength images of the plume were obtained in reflected sunlight with
the Advanced Camera for Surveys (ACS) prior to Jupiter transit. The dust
scattering recorded in these data provide an additional qualitative measure of
plume activity on Io, indicating that the degree of dust scattering over Pele
varied as a function of the date of observation, and that there were several
other dust bearing plumes active just prior to Jupiter transit. We present
constraints on the composition and variability of the gas abundances of the
Pele plume as well as the plumes detected by ACS and recorded within the STIS
data, as a function of time. We discuss the implications of these results for
thermochemical conditions at the plume vents, and our understanding of plume
eruption styles active on Io.
