I am afraid that you are confusing a few items. Lets consider some of them  in turn.

Firstly, whatever effect Europa and Callipso might have on the orbit of Io, it could have an elliptical orbit anyway in their absence.  In fact, mathematically speaking a truly circular orbit would be vanshingly unlikely. No known object has anything of the type, and if it did, it would not stay that waay. Io just has an unusually eccentric orbit, like say, Mercury or Pluto.

The gravitational forces of the other moons might have a slight shepherding effect on Io, but the forces they exert have little directly to do with Io's heat generation. Those forces are far, far too small to cut a figure, compared in particular, to those from Jupiter.

Next, what strains Io's structure is not so much simple gravitation, but tidal force, the difference between the trajectories that independent particles would follow through the positions and velocities of the various parts of Io at various stages of its orbit through Jupiter's gravitational field. If those particles were unattached in free fall, there would be no heat generated and the tidal forces simply would separate them into a dispersing dust cloud, but in practice of course, that is not how things happen. Instead their free-fall trajectories are very stronly disturbed, which is how their kinetic energy gets converted into frictional and distortional forces that generate heat.

You say that "gravity is one of the four forces of nature and not an energy", and in some ways that is correct, though in general relativity it is seen as a distortion of space-time, and as such at best debatably even a force at all. But that is irrelevant. It is not gravity as such that supplies the energy, so to speak. It is the fact that different particles on different trajectories through a gravitational field have their trajectories continuously diverted from free fall because they are fastened together.  This derives energy from the kinetic consequences of one of the most fundamental equations of all : F=MA.

Does that help? I am not sure I have been very coherent!

Sorry about that!

Jon

If you've ever kneaded a ball of dough vigorously, you'll know that some of the KE you put into changing its shape is converted into heat and it gets warm.  Tidal forces produced by a gravitational gradient through a rotating body produce tides on Earth which are mostly dissipated as heat, and in a few places are partly converted into electricity in tidal power stations.  On Io, the kneading produces internal heat.  Tidal forces obey an inverse cube law, so, being close to Jupiter, they're pretty big on Io.  This process robs Io, Earth and every other orbiting body of KE and gravitational PE, so they slowly spiral inwards

Io, one of the four brightest satellites orbiting Jupiter, has the hottest region in the Solar System on it barring the Sun, although, the average temperature of its parent planet is only -153 degrees Celsius (-243 degrees Farenheit). It is the most volcanically active body and one of the most mysterious bodies in the Solar System.

First of all, Io is hot and volcanic not because of its elliptical orbit but because of Tidal forces, forces caused because each of the particles of Io have to be forced to follow its orbit, in the gravitational field of other objects, in both trajectory and speed when they would have had different trajectories and speeds if left unbounded. These tidal interactions happen mainly between Io, Jupiter, Ganymede and Europa. In the simplest terms, tidal interactions are the same forces which cause tides on Earth. These forces, at any time, cause the region of Io closest to and the region farthest from Jupiter to swell up (expand away from the center of Io) and the region which is in between (neither too close nor too far from Jupiter) to press down (compress towards the center of Io).

Now, the period of rotation and revolution of Io is almost the same. So, just like our Moon, almost the same side of it faces Jupiter. But the gravitational effect of Ganymede and Europa causes it to wobble a bit leading to slight changes in the side facing Jupiter in turn leading to change in that region of Io which is swelled up and which is pressed down. Due to tidal interactions the shape of Io distorts by as much as 100 m whereas the highest ocean tides on Earth only reach about 18 m. This shifting deformation over Io generates a lot of heat by losing energy as what are known as hysteresis losses just like those in rubber, when it is deformed and reformed repeatedly. Hysteresis losses are losses that result from dissipation of energy as heat when bonds in a material deform and then regain the shape. So, the energy transfer isn't from kinetic energy of the rigid body (Io, in this case) to heat energy but from gravitational potential energy of Jupiter-Io-Ganymede-Europa system to electrostatic potential energy of the intermolecular bonds in Io to heat energy.

Io still has so many peculiar features that an Io Volcanic Probe Mission has been proposed to investigate mysteries like how does Io support its high magma temperatures without being almost totally molten and how does it maintain a strong enough lithosphere to support mountains higher than the Mount Everest.

Ritesh Singh,

Indian Institute of Technology Kharagpur.