The auroral emissions appear to be related to material from Io that spirals along magnetic field lines to fall into Jupiter's atmosphere. Voyager also saw cloud-top lightning bolts, similar to superbolts in Earth's high atmosphere. That was also not observed by Pioneers 10 and 11, and Voyager scientists believe they are witnessing large temporal or spatial changes in the ionosphere of Jupiter. Relative abundance of helium to hydrogen is about 11 percent by volume. Many more are suspected.
Volcanism is associated with heating of Io by tidal pumping.
Europa and Ganymede, two large satellites nearby, perturb Io in its orbit and Jupiter pulls Io back again. Voyager 1 measured the temperature of a large hot spot on Io associated with a volcanic feature. Europa displayed a large number of intersecting linear features in the distant, low-resolution photos from Voyager 1. Scientists at first believed the features might be deep cracks, caused by crustal rifting or tectonic processes. There is a possibility that Europa may be internally active due to tidal heating at a level one-tenth or less that of Io.