Its brilliant colors of red, orange, and yellow set it apart from any other moon or
planet. Active volcanoes have been detected on Io, with some plumes extending up
to 200 mi (320 km) above the surface. 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. Close-up views taken in 1999 and 2000 showed that Io had more than 100
erupting volcanoes, gigantic lava flows and lava lakes, and towering, collapsing
mountains. The eruptions of Loki, the most powerful volcano in the solar system, can
be seen by Earth telescopes.

Observations by Galileo during 1998 revealed dozens of volcanic vents on Io where
lava is hotter than any surface temperatures recorded on any planetary body in our
solar system. At one such volcanic vent, known as Pillan Patera, two of the
spacecraft's instruments indicated that the lava temperature may have been 3,140°F.

In 1996, the Galileo spacecraft detected a huge iron core within Io that occupies half
the moon's diameter. Galileo also discovered evidence that Io has its own magnetic
field.