Good answers! Great answers. My questioning of the models is that we don't know the interior structure or properties of the moons so we can't calculate the tidal heating, which is dissipative.
For Europa to have thin ice (5 km), we need something like 10^13 (10 trillion) watts being dissipated continuously out of this little satellite. It is difficult to see how this much power can be produced by secondary tidal effects from other Galilean satellites. If the ice is 25 km thick, the combination of tidal and radioactive heating seems much more credible. Some of the references noted by Larry Klaes in his submissions indicate that the ice may indeed be thicker than originally thought (or hoped for). But then we get back to the issue of how does Europa's surface refresh itself if the ice is so thick.
A qualitative "proof" of Gregg's description might be verifying the supposition that tidal heating, and thus ice thickness, varies appropriately from Europa to Callisto. By verifying I mean measurement.
This is such an interesting topic. It can be instructive to ask how much is known about the effects of tidal forces on the Earth. Our geophysicists only decided that the continents move in about 1968-1970. We still don't know why they move. Studying Europa and the other large outerplanetary moons helps us understand the Earth, and life.
Chris
Gary McMurtry wrote:
Thank you so much Gregg. This is the best explanation I've yet to read. BTW, a long time ago, back in 1996, I heard (via Fraser Fanale) that these moon interplays might vary over time, causing more or less heating of the Jovian moons by tidal friction. If so, then an Europa ocean could have experienced freeze-thaw cycles over time, with thicker ice crust formation or even pockets of "seas" forming as opposed to a planet-wide ocean, or even a total freeze.
Interesting, no?
Gary
Hi,
I'm going to chime in on the heating issue, since I beat the heck out of this problem once in a planetary science course once.
The tidal heating of Jovian moons is caused by a kind of "argument" between two factors. Each moon wants to orient itself (by matching its spin to it's orbital period) so that one side always faces Jupiter. The moon is actually slightly prolate, i.e. out of round, with the long axis pointed at Jupiter, because the force of gravity and the "centrifugal" force caused by its orbital motion are only in balance at the center of the moon. Some extra gravity pulls the Jupiter-facing side in and some extra centrifugal force pulls the other side out. This is the long way of saying that there should be a tidal bulge aligned with Jupiter. I say "should be" because of the other factors. There are actually three such factors: each of the three other moons. They too want to yank the long axis of each other, pulling it out of alignment with Jupiter. There is no single orbit/spin solution that satisfies the demands of Jupiter and the other three moons simultaneously, therefore each moon is being slightly "flexed", meaning that the long axis is being pulled by the other moons assymmetrically. The closest end to another moon is attracted more. This bending heats the moon. The farther out you go, the less force Jupiter exerts, so the moon is able to react more (by slight deviations in its ordinary rotation) to the other moons. Also, the closer you are to Jupiter, the more often and rapidly conjunctions occur. Io passes Europa more often and more quickly than Callisto passes Ganymede. Therefore, Callisto has the least heating and Io the most. The "illusion" or misconception about the heating of Io and Europa is that it is caused by Jupiter. This is true, but only indirectly. Jupiter, acting alone, provides a force that prevents tidal variation. If there was only one moon, it would orient to Jupiter, stay there, and freeze solid. It is the other moons that provide tidal variation which generates heating.
The period of tidal flexing is quite regular, since the three inner Galilean satellites orbit with periods that are multiples of Io's nearly exactly. Callisto is close, and in any case has the least tidal effect.
Gregg
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