The Galileo mission to Jupiter has greatly advanced our understanding of Io. It is now clear that Io is a differentiated body consisting of a metallic core and a silicate mantle. The radius of the core is between 0.37 and 0.52 times Io's radius, depending on the unknown composition. The composition of the mantle is also not known precisely, but the best analogy to Io's composition are the L- and LL-chondrite meteorites. The bulk Fe-Si ratio is well below CI chondrites and Mars, but higher than the Moon. The physical state of the core is unknown, but it is likely liquid (due to the intensely heated mantle above). Though a differentiated crust of some sort is inevitable, we cannot constrain its thickness or composition.

The strong lithosphere required to support the observed mountains is a consequence of the large flux of molten rock to the surface, which rapidly buries older, cold flows. This melt segregation is responsible for nearly all of Io's very high surface heat flux, and most likely drives the sulfur volcanism that makes Io's surface so distinctive. Melt fractions in the mantle required to balance the observed heat flow are 10-20%, which is consistent with the bulk of the temperature estimates for surface flows, though there may be localized regions at higher temperature with higher melt fractions.

Since Io's heat flow is dominated by the relatively rapid process of melt segregation, the available heat flow data are most likely representative of the average heat production by tidal dissipation in Io's interior. Though measurements of the orbital evolution of Io are at present inconclusive, future measurements (e.g., from an orbiter) should establish its rate of migration and constrain the dissipation in Jupiter.

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