Re: Is Titan a saturnian Europa/Ganymede?

From: Jack (merde44spinks_at_yahoo.com)
Date: 07/06/04 

Date: Tue, 06 Jul 2004 01:50:41 GMT

Henry Spencer wrote:
> In article <fdVFc.6903$JR4.3306@attbi_s54>,
> Jack <merde44spinks@yahoo.com> wrote:
>
>>My memory was a little fuzzy on this, I forgot that the reason
>>Io is heated is because of its orbital resonance with Europa, which
>>causes it to slow down or speed up at different parts of it's orbit.
>
>
> Well, kind of sort of. The more usual way of saying this is that the
> resonance with Europa is what keeps Io's orbital eccentricity non-zero,
> and it's the eccentricity that causes the changes in both orbital rate and
> orbital radius, shifting the tidal bulges around and causing internal
> heating. (You can't change the speed of an orbiting object without also
> changing the shape of the orbit.)

It's described the way I described it twice in my reference. It seems
to me that you are confusing cause and effect. The cause of the tidal
heating is the change in orbital speed caused by the resonances
of Io with other moons of Jupiter. This causes a spot on Io which
faces Jupiter to oscillate slightly, because Io's rotational period
doesn't change at all, but it's orbital speed does. This results
in a *temporary* eccentricity of its orbit, but since a speed-up
is balanced by a slow-down later, the overall eccentricity is zero.
It's this time-dependent change in which part of the moon is
being pulled strongest by Jupiter that results in the heating.
And the time-dependent eccentricity, like the tidal heating, is a
result, not a cause, and the overall eccentricity of Io is zero.

>
>>Both Io and Titan have synchronous orbits...
>
>
> Uh, no, synchronous *orbits* are another story; both moons are far above
> the altitude required for that. They both have synchronous *rotations*,
> rotating once per orbit.

Yes, but I wasn't trying to be pedantic, just noting in the general
sense that the rotational period and the orbit are synchronized.

[snip]
>>I don't know if Titan's orbit is affected by other moons or not.
>
>
> Not substantially. Saturn only has one big moon, Titan itself. The
> others are much too small to have the sort of effects seen at Jupiter.
>
>
>>But if it is, the effect would not have to be nearly as great
>>to cause methane volcanism because the temperatures involved
>>are much lower than for sulfur volcanism.
>
>
> Again, it's not the effect of the other moons that causes the heating,
> it's the orbital eccentricity. The role of the other moons at Jupiter is
> to *maintain* Io's orbital eccentricity, which otherwise would quickly be
> reduced to zero.

You've got this mixed up. See above. Io's eccentricity is zero. The
interaction with the moons causes a *time-dependendant, temporary*
eccentricity which over a complete orbit averages to zero. The
eccentricity is not the cause, it's an effect.

> In Titan's case, it simply doesn't see sufficiently drastic tidal effects
> to circularize its orbit rapidly. So its rather lower level of internal
> heating will persist without other moons pumping up its orbit. However,
> it doesn't get enough heating to cause runaway melting. Its *interior*
> isn't methane. Any methane volcanism would be a local thing, not the
> result of an entirely molten interior like the Io volcanism.

Titan doesn't have to be made entirely of methane, just like Io is not
entirely made of sulfur. Methane boils at -161 C here on earth, higher
in the denser atmosphere of Titan. The surface of Titan averages
-178 C. The heat generated by pressure alone would be enough to create
methane volcanoes, but even a small effect from forces similar to
what's involved at Io could help.
Jack

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