Two classes of volcanic plumes on Io

Results of an analysis of the properties, source regions and deposits of volcanic plumes on Io are presented which suggest the presence of two plume types. Eruptions at the Aten Patera caldera in the south polar region and the Surt caldera in the far north, which were deduced to have taken place between the times of the Voyager 1 and Voyager 2 encounters from evidence of surface changes, are similar to the Pele eruption. These Pele-type eruptions are characterized by durations of from a few days to a few weeks, dark-red annular deposits of about 1400 km in diameter, temperatures of about 650 K and locations restricted to areas with large, silicate landforms. Smaller, more numerous eruptions of the Prometheus type were observed on both encounters, being characterized by durations in excess of several years, bright ringed deposits about 250 km in diameter, restriction to an equatorial location high in SO2 and temperatures less than 400 K. In addition, an intermediate type of feature was noted at either end of the linear feature Loki. Two separate mechanisms, involving SO2 and sulfur as driving sources, are proposed to account for the Pele- and Prometheus-type eruptions, respectively, and the discrete temperatures of the plumes are suggested to reflect the temperatures at which sulfur is a low-viscosity fluid.