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Subject: Re: Europa and energy transfer
Date: Wed, 30 Oct 2024 15:05:55 -0700
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On 10/29/24 21:53, 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?

I am thinking that the standard model for Earth is that supposedly
in the 1800s Lord Kelvin did some calculations.

The outer layers of the Earth have an insulating effect on the inner 
layers of the earth.

Then the inner layers even can act as more insulation.

And further down.

And so on.

He calculated that it would take a hot interior literally 10s to
even 100 million years for a hot Earth interior to cool down because
the Earth could retain heat and insulate successive layers so well.

When everything was put together, however, his calculations were not
wrong.

The standard model of geologic time in the present however is that the
Earth is billions of years old.

How is this discrepancy resolved?

I am thinking that the standard modern view is that radiation from
the slow radioactive decay of uranium, potassium, thorium, and other
elements deep in the interior is added to the Earth's insulating
effect to keep the interior of the Earth and other inner solar system
bodies heated.

I am thinking that Io is supposed to be rather seriously affected
in its composition and internal dynamics by tidal forces.  I was
thinking however that Ganymede and Callisto were also supposed to
have liquid water mantles.

It might be that there is a combination of insulation, internal
radiation, and tidal forces going on for the different moons
of Jupiter.