Satellite monitoring offers a powerful means to regularly characterize the mechanical and thermal states of active volcanoes. Satellite-measured surface inflation and radiant heat flux reflect the pressurization and discharge, respectively, of a magmatic system, suggesting that studying these parameters together may help in better understanding future eruptive activity. We incorporate InSAR surface displacement data and ATSR thermal radiance data into a probabilistic model for activity at Etna volcano in 1996--2000, when surface deformation and thermal radiance appeared to be related in a periodic fashion. The probabilistic approach addresses both the magnitude and timing of eruptive events, based upon a simple physical model of eruptions as renewal processes. We anticipate that this approach could serve as a framework for probabilistic assessment in eruption scenarios with persistent activity and multiple monitoring datastreams.