Galileo Observations of Volcanic Plumes on Io

Co-analysis of Galileo imaging observations of the gas and dust components of Io’s plumes suggests that the deposits produced by Prometheus-type plumes are made up of pyroclastic particles that are entrained with the gas flow. Peletype plumes, in contrast, deposit material that could be condensed from the gas phase. Introduction: Many outstanding questions remain to be answered about Io’s spectacular volcanic plumes [1]. Voyager and Galileo imaging observations have established that there are two distinct classes of plumes on Io [2]: giant plumes like Pele that vent sulfur-rich gases from the interior of the moon and spray-paint the surface with enormous red rings, and more numerous smaller plumes like Prometheus that are produced when hot flows of silicate lava impinge on volatile surface ices of SO2. Both classes of plumes possess gas and dust components, although the density of dust and gas varies from one plume to another. The fallout from plumes contributes significantly to Io’s resurfacing. Still poorly known are the temporal and spatial variability of plumes, the relationship between the plumes and the surface changes they produce, and the nature and source of the particulates present in plumes of both classes (in particular, whether the dust is condensed from the gas or simply entrained by the flow). Here we begin to address these questions through a systematic study of the dimensions and opacities of dust plumes in the complete set of Galileo images, together with comparisons of the dust plumes seen in daylight to the auroral glows seen in eclipse and the record of surface changes. Sunlit Observations: Galileo acquired many low-resolution imaging observations of the dust component of the plumes as seen in reflected sunlight during daytime observations. All but one of these depict the small, Prometheus-type plumes; Pele was barely discernable to Galileo’s imaging system, which lacked sensitivity at ultraviolet wavelengths. These 66 distinct measurements (corrected for foreshortening) yield a mean height for the Prometheus-type plumes of 82 +/15 km. Pillan, Ra and Thor reached over 100 km in height, but none of the smaller plumes exceeded 118 km. Repeated observations showed no temporal variability in dust column height outside of measurement uncertainty. The dust load varied considerably from plume to plume: no dust columns were seen at Acala and Culann, and the opacity of stealthy Marduk is an order of magnitude less than that of similarly sized Zamama (Figure 1).