Which characteristic immediately sets Io apart from Europa, Ganymede, and Callisto?

Answer

Most volcanically active world observed.

Io is the most volcanically active world ever observed, featuring relentless output of magma and gas, contrasting sharply with its icy siblings that are famous for their frozen crusts.

Which characteristic immediately sets Io apart from Europa, Ganymede, and Callisto?
Jupiterplanetsolar systemmoonsIo