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From: Physfitfreak <physfitfreak@gmail.com>
Newsgroups: sci.physics
Subject: Re: Europa and energy transfer
Date: Sat, 2 Nov 2024 00:14:11 -0500
Organization: Modern Human
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On 11/1/24 23:52, Sylvia Else wrote:
> 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.


Yes. You were naive to think sci.physics is a classroom.