What primarily dictates the compositional difference between Io and the other three major moons?

Answer

The ice line temperature gradient

The distance from the hot, young Jupiter determined where water ice could condense. Io formed too close where it was hot, resulting in a rocky/sulfur composition, while outer moons incorporated ice.

What primarily dictates the compositional difference between Io and the other three major moons?
SpaceOriginJupiterGalilean moonsmoons