Io
Io's surface is radically different from any other body in the
solar system. It came as a very big surprise to the Voyager
scientists on the first encounter. They had expected to see
impact craters like those on the other terrestrial bodies, and
from their number per unit area to estimate the age of Io's
surface. But there are very few, if any, impact craters on Io.
The surface is very young and active.