Jupiter's moon Io is volcanic. It is too small for its volcanism to be
explained by residual heat of formation or radioactive decay, unless the
moon is not millions of years old.
The volcanoes on Io are powered by tidal heating. Io is close to
Jupiter, so it is strongly affected by Jupiter's gravity. The other
moons of Jupiter exert their own gravitational forces. The resulting
tides raise and lower Io's surface by about 100 m, generating
frictional heat that drives the volcanoes.
