Because of its persistent red deposits, Pele is the most distinctive of Io's volcanoes (Plates 9a, b). Appropriately named after the Hawaiian goddess of volcanoes, Pele is the source of a giant plume more than 300 km high, as seen by Voyager (Strom et al., 1981). The reddish deposits laid down by the plume are ≈ 1200 km across and are rich in sulphur and sulphur dioxide (Geissler et al., 1999; Spencer et al., 2000a). Closer to the vent, dark pyroclastic material, most likely of silicate composition, streaks the surface (Strom et al., 1981; Geissler et al., 1999). SSI and the Hubble Space Telescope (Plate 9f) have shown that the Pele plume can exceed 400 km in height (Spencer et al., 1997b; McEwen et al., 1998a).

Voyager observed dramatic changes in the shape of the plume deposits between encounters when the deposits changed from a “cloven hoof” shape (almost certainly caused by a vent obstruction of some kind) to a circular appearance (Figures 1.3d, e). The change in the shape of the plume deposits took place in the four months between Voyager encounters, so the resurfacing for some period of time was relatively rapid. Although the Pele plume was not seen by Voyager 2, it may have changed to a more tenuous, gas-rich form – a “stealth” plume (Johnson et al., 1995).