From: sylvia@email.invalid

NASA has a mission to the Jovian system, to study Europa. That moon is
interesting because it appears to have liquid water under an icy
surface. The heat need to keep the water liquid comes from the
stretching and compression Europa experiences during its orbit around
Jupiter, the orbit not been exactly circular.

So much, so simple.

Some thought made me realise that although the tidal forces on Europa
mean that it is not exactly spherical, its two bulges cannot remain
perfectly aligned with Jupiter, because Europa's angular velocity
relative to Jupiter is higher at periapsis than at apoapsis. The result
is that the nearer bulge is sometimes ahead, and sometimes behind,
relative to Europa's orbital motion, resulting in a net force backwards
along the orbit, or forward along the orbit.

Again, certainly stuff that's already well known.

As far as I can see, the energy that is being dissipated as heat inside
Europa has to come from changes to Europa's orbit. Further, if Europa
were either perfectly rigid, or perfectly elastic, there would be no
energy transfer, and consequently no change to the orbit.

It would make no difference if Jupiter itself were perfectly rigid, so
the transfer cannot involve tides on Jupiter generated by Europa.

So the existence of the orbital energy transfer depends on Europa being
neither perfectly rigid nor perfectly elastic.

What escapes me is the mechanism.

Any thoughts?

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