and might still contain molten material similar to the
magnesium-rich lavas that erupted on Earth more than
1.5 billion years ago. Volcanoes
on Io are spread
around the moon in a
much more random fashion
than on Earth, where the distribution of volcanism
reflects tectonic patterns.
Io probably formed at about the same time as the moons
of the other giant planets. Io’s current volcanic activity
suggests that its entire mass has recycled—turned inside
out—more than once in the past, leading to chemical differentiation.
Volatiles such
as water and carbon dioxide
probably escaped into
space long ago, while most
of the heavier materials
sank to the interior to form a core. Sulfur and various sulfur
compounds, aided by silicate magmas, are constantly being
recycled, forming the complex surface seen today on Io.
Farther out in the Solar System, there is a type of volcanism
different from that on Earth. Cryovolcanism is