Something I just learned, and it’s right on Wikipedia, is that all this tidal heating of Io and Europa is due to the stable interlocked orbits of Io, Europa, and Ganymede, with periods in the ratio 1:2:4 . ( Callisto is far enough away not to be locked in, and is effectively a “loner” . )
Any one of these moons, alone, would settle into a fixed circular orbit without any variable tidal forces, but all together they constantly tug each other back and forth in a stable but dissipative pattern.
Well, it makes sense to me!
Wikipedia is amazing for what you learn. I learned there that the singer formerly known as Terence Trent D’Arby breaks wind uncontrollably at the sight of monkeys. Good thing Io doesn’t look like a monkey.