[zeugma]

Io is frictionally heated by the gravitational changes of its elliptical orbit close to massive Jupiter; that energy never "runs out."

There is no such thing as perpetual motion. The energy has to come from somewhere. In this particular case, I vote Jupiter. I would imagine it could probably be calculated how much rotational energy Io steals from Jupiter with each orbit.

[SengirV]

Correct, there is no perpetual motion. But a couple billions years of gravity induced friction goes a LONG way.

[editor-surveyor]

>> “ I would imagine it could probably be calculated how much rotational energy Io steals from Jupiter with each orbit.” <<

.
Exactly as much as jupiter ‘steals’ back.