Eight active volcanoes have been detected on Io, with some plumes
extending up to 320 kilometers (200 miles) above the surface. Over the
four-month interval between the Voyager 1 and 2 encounters, the active
volcanism appears to have continued. Seven of the volcanoes were
photographed by Voyager 2, and six were still erupting.
The relative smoothness of Io’s surface and its volcanic activity
suggest that it has the youngest surface of Jupiter’s moons. Its surface
is composed of large amounts of sulfur and sulfur dioxide frost, which
account for the primarily yellow-orange surface color. The volcanoes
seem to eject a sufficient amount of sulfur dioxide to form a
doughnut-shaped ring (torus) of ionized sulfur and oxygen atoms around
Jupiter near Io’s orbit. The Jovian magnetic field lines that go through
the torus allow particles to precipitate into the polar regions of
Jupiter, resulting in intense ultraviolet and visible auroras.