THE PELE PLUME (IO) - OBSERVATIONS WITH THE HUBBLE-SPACE-TELESCOPE

In July 1996, with the Hubble Space Telescope (HST), we observed the P ele plume silhouetted against Jupiter at a wavelength of 0.27 mu m, th e first definitive observation of an Io plume from Earth. The height, 420 +/- 40 km, was greater than any plume observed by Voyager. The plu me had significantly smaller optical depth at 0.34 and 0.41 mu m, wher e it was not detected. The wavelength dependence of the optical depth can be matched by a plume either of fine dust, with minimum mass of 1. 2 x 10(9) g and maximum particle size of 0.08 mu m, or of SO2 gas with a column density of 3.7 x 10(17) cm(-2) and total mass of 1.1 x 10(11 ) g. Our models suggest that early Voyager imaging estimates of the mi nimum mass of the Loki plume [Collins, 1981] may have been too large b y a factor of similar to 100. We may have detected the Pele plume in r eflected sunlight, at 0.27 mu m, in July 1995, but did not see it 21 h ours earlier, so the plume may be capable of rapid changes.