Outbursts on Io

With better data now available for thermal anomalies, Io's outbursts have again become a promising avenue for future progress. The outburst data are plotted in Fig. 1. Most of the data lie above the boiling point of sulfur and, therefore, require silicate lava. Recently a detailed analysis has been carried out on the 1990 event [see Veeder et al., 1994a,b; Blaney et al., 1994]. Such large outbursts (i.e., 10 W or larger) are estimated to occur about 6% of the time [Blaney et al., 1994]. The uncertainty in this estimate, however, is large. In the 1990 event the source area increased at a rate of 1.5 x 10 ms. If this growth continued unabated it would equal the whole surface of Io in 8.5 years. With a 6% frequency of occurrence, this becomes 142 years. If one assumes that all of Io's thermal anomalies are due to similar flows but in various stages of cooling, then the spreading rate taken together with the heat flow constrains the average flow thickness to be 1.9 m. The corresponding effusion rate is 3 x 10 ms. This is huge by terrestrial standards. However, recent modeling of the 1800-01 Hualalai flow on the island of Hawaii finds an effusion rate of 10 ms [Baloga and Spudis, 1992]. Also there are examples larger effusion-rate flows on the Moon [see Hulme and Fielder, 1977]. Therefore Io's outbursts fall within the bounds of our experience. These examples may serve as useful guides for developing an understanding Io's flows.