Interior of Io
This
cutaway model illustrates the probable internal structure of
Io. Based on gravity-field measurements taken by the Galileo
spacecraft during a close flyby of the moon, scientists have
determined that Io is differentiated. Differentiation is the
gravitational separation of material in the interior of a
planet or moon according to density. While in a molten
state, dense materials such as iron will sink to the center
of a planetary body to form the core, and less dense
elements such as silicon and oxygen will rise toward the
surface.
Io is believed to have a dense core composed of iron and iron sulfide (shown in gray). The radius of the core is approximately 900 km (560 miles) which extends about halfway to the surface. It is likely that the core formed either from internal heating processes during the early stages of the moon's formation, or as a result of the ongoing tidal heating that drives the volcanic activity at the surface. Surrounding the core is a mantle of partially molten rock (shown in yellow-orange), which is overlain by a relatively thin, rock crust (shown in brown).
Illustration Credit: NASA/JPL