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From: Sylvia Else <sylvia@email.invalid>
Newsgroups: sci.physics
Subject: Re: Europa and energy transfer
Date: Sat, 2 Nov 2024 12:52:10 +0800
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On 30-Oct-24 12:53 pm, Sylvia Else wrote:
> 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?

Perhaps I was naive to think anyone would address the essence of my 
post, rather than going off at massive tangent.

Sylvia.