By the end of the Galileo mission, many characteristics of Io's volcanism had been identified for the first time. Processes were observed that continue to yield insight into the way volcanic activity helped shape the Earth and other terrestrial planets. This chapter summarizes the post-Galileo view of Io.

Volcanism and crustal structure

Galileo revealed a world dominated by silicate volcanism, putting to rest a major point of contention that dated back to Voyager. Galileo discovered many active volcanic centers on Io (Appendix 1), considerably more than were detected by Voyager. The global distribution of activity and volcanic features favors the heating of Io by tidal dissipation taking place, in large part, in the aesthenosphere – the partially molten upper mantle.

The upper few kilometers of Io's crust appear to be mostly mafic silicates interbedded with deposits of sulphur, sulphur dioxide, and silicate pyroclasts. The thickness of this volatile-rich layer probably varies from location to location but provides plenty of material that is easily mobilized by thermal interaction with mafic magma. Ultramafic magmas may be present, but the wide error bars ascribed to the data do not show this conclusively. If recent data analyses (Keszthelyi et al., 2004a, 2005a) are confirmed, ultramafic volcanism may not be present or widespread, and the presence of a global magma ocean (e.g., Keszthelyi et al., 1999) may not be required.