What primary materials characterize the chemical makeup of Io's surface, unlike the other Galilean moons?

Answer

Rock and molten sulfur.

Unlike the other Galilean moons composed largely of rock mixed with water ice, Io is almost entirely rock and molten sulfur, having melted out volatiles that would otherwise form ice crusts.

What primary materials characterize the chemical makeup of Io's surface, unlike the other Galilean moons?
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