# How do the tidal forces warming moons theories hold when [..]

 P: 190 How do the “tidal forces warming moons” theories hold when apart from heating from expansion, there may be also cooling from contraction? I can understand a temporary heating, from the tital forces exerted on the moon but wouldn't there be cooling as well eventually when particles "give in" to contraction? Wouldn't they eventually net a unchanging whole body temperature? i.e. How can Europa's oceans be warmed by that and how can Io's crust be melted by that?
 P: 743 Well, can you make your hands colder by vigorously rubbing them together?
 Mentor P: 12,015 The heat generated by the tidal forces is dissipated mostly, if not completely, by thermal radiation emitted by the moon. But still, this requires that the temperature of the moon increases until the amount of heat lost through thermal radiation is equal to the heat generated by all methods inside the moon. So the net effect is a warmer moon.
P: 190
How do the tidal forces warming moons theories hold when [..]

 Quote by Bandersnatch Well, can you make your hands colder by vigorously rubbing them together?
But where does the energy come from? There's a human rubbing there. I thought an orbit recycles it on turning. Does it actually drop slightly from orbit?
 P: 900 let me use an analogy to explain this. you stretch a piece of rubber. due to internal friction it heats up. when it relaxes back, it does not cool back down! this is because the heat has dissipated. the stretching of a planet is exactly the same - a shear force on a viscoelastic material. you can think of it as squeezing and stretching a tennis ball filled with jelly.
P: 190
 Quote by chill_factor let me use an analogy to explain this. you stretch a piece of rubber. due to internal friction it heats up. when it relaxes back, it does not cool back down! this is because the heat has dissipated. the stretching of a planet is exactly the same - a shear force on a viscoelastic material. you can think of it as squeezing and stretching a tennis ball filled with jelly.
Yes but what "gave in" to give that energy? The internal structure of the planet, the orbit?

[Because surely, tidal forces alone can not produce energy since an orbit is in its basic sense only recycling energy.]
Mentor
P: 12,015
 Quote by cdux Yes but what "gave in" to give that energy? The internal structure of the planet, the orbit? [Because surely, tidal forces alone can not produce energy since an orbit is in its basic sense only recycling energy.]
I believe it steals energy from the orbit of the Moon.
 P: 743 Yes, the energy comes from rotational and orbital kinetic energies of the system. As the tidal deformation heats up the satellites, their orbital and rotation periods change until the orbits fully circulate and the moons themselves get tidally locked. With multiple bodies as in the case of Jovian moons, the situation might get a bit more complicated, but it's still the case of orbital and rotational energy dissipation. Here's an excerpt from the abstract of C.F.Yoder's "How tidal heating in Io drives the Galilean orbital resonance locks"(http://adsabs.harvard.edu/abs/1979Natur.279..767Y) According to the proposed model, initially all three satellites{Io, Europa, Ganymede} are in orbits far from the 2:1 commensurabilities or the three body lock. The tide raised on Io damps down the free eccentricity; only modest tidal heating occurs. Subsequently the dissipative tide raised on Jupiter by Io causes Io's orbit to spiral outwards; Io approaches the 2:1 commensurability with Europa. Io's forced eccentricity increases rapidly to a critical value, and thereafter the resonant interaction causes Europa's orbit to expand at half that of Io's orbit. A fluid core is probably formed as the result of tidal heating. Finally Europa approaches the 2:1 commensurability, angular momentum is transferred from Europa's orbit to Ganymede's, and a steady state is attained.
 P: 190 Is it an inevitable fate of all theoretically isolated but naturally structured bodies to be heating up while dropping on their host (even if that heating up or drop is negligible for humanely observable periods)? Now I wonder if some 'unexplained deaths' of nearby planets were that phenomenon in a slower scale. Maybe a negligible heating up but a sizable drop of orbit over eons.
 Mentor P: 12,015 What do you mean by "drop of orbit"?
 P: 190 Giving up of gravitational potential energy. The distance times mass times g thing.
Mentor
P: 15,204
 Quote by cdux But where does the energy come from? There's a human rubbing there. I thought an orbit recycles it on turning. Does it actually drop slightly from orbit?
No, it's the other way around. Io, Europa, and Ganymede are slowly spiraling out from Jupiter. The orbital period of the fastest of them, Io, is over four Jovian days. This means they should be migrating outward, not inward.

So where does the energy come from? Ultimately, from Jupiter's rotation. Those moons are slowly slowing down Jupiter's rotation rate.
 P: 190 That also works, of course. Do some of them spiral out because of the local coincidence of the multitude of them and they would not if they were the only moons, or another process is afoot? Now I wonder how do they slow it down. Is it the complex system of the multiple of moons of would it happen with lone moons? And isn't the slowing of the rotational rate of the host relative to a moon's?

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