What observation suggests the Galilean moons formed in situ (at their current general locations) within the disk?

Answer

The existence of the 4:2:1 orbital resonance linking the inner three moons.

This stable resonance is a hallmark of systems that evolve together simultaneously within a single, dissipating disk, as opposed to bodies being added randomly over time.

What observation suggests the Galilean moons formed in situ (at their current general locations) within the disk?
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