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Volcanic activity on Jupiter moons

So I recently heard in the TV that the volcanic activity on many of the moons of Jupiter were caused by the gravitational pull from Jupiter. If this is true then does Jupiter not loose some energy or the moons(slower orbiting and such)? Because if it doesn't then we've kind of got us a Perpetual motion machine Confused
Gravitation is intrinsic to matter, it's not something that is lost, unless matter is lost. Likewise it isn't gained without an increase in matter. What's happening there, if gravity is to blame for the eruptions, is that the moon (and Jupiter) have patches of high and low density (matter isn't uniform throughout), so different parts are affected by gravity differently, causing some shifts and, apparently, heat. These shifts cause ruptures and boom, volcano.

As it is, the orbits of the planets are not perfect. I'm not sure what the decay rates are, as I've never looked into it. The influence of Jupiter on its moon doesn't really change the overall mass system of Jupiter and its moons orbiting the sun, they basically act as a single unit... but it could, perhaps, cause a slight wobble?

I'm certain others will have better explanations, and I may even be wrong, but that is my basic, outdated understanding Razz
The 'perpetual motion machine' may finally grind to a halt when the moons become tidal-locked.

The Earth's moon is tidal-locked: the same side always faces the Earth, because it is the (slightly) heavier side. It probably used to rotate at one point, but finally stopped moving because of the gravitational pull.

The tides on Earth? Every time the tide goes in or out, it's causing friction, and slowing down our spin relative to the moon. Eventually (very long time) this friction will cause the Earth to stop spinning relative to the moon, and the same side of the Earth will face the moon at all times.
(It will still rotate relative to the Sun for a while though. But the Sun also affects the tides and would eventually -- given enough time -- make the same face of the Earth face the Sun at all times. Since the heavier side would be the side facing the moon already, that's the same side that would eventually end up locked towards the sun as well. When it finally stabilized, there would be a perpetual solar eclipse. The Sun will probably explode before that happens though.)
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