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mwebster745 t1_j2bwqwt wrote

Actually the film is interesting in subtly addressing the magnetosphere thing. A moon around a gas giant would be at risk of being fried by radiation focused by the planet's own magnetosphere. This is the challenge with Europa, and why the Europa clipper mission won't ever actually orbit Europa itself. In avatar you repeatedly see areas of massive magnetic flux, generally where the 'spirit trees' are. You can tell from large rock rings standing on end, following the contouring of magnetic field lines. This is also close to the 'flying mountains' which are suspended by such a magnetic field. The new movie was a bit different, but the original mentioned 'unobtanium' being mined which was a natural room temperature superconductor causing the magnetic fields. Generally small planets and moons solidify and a solid (ish) core like Mars doesn't produce a strong magnetic field like earth has. A small moon like Pandora flying free would solidify and lose a magnetic field, then have it's atmosphere stripped by solar wind, again like Mars, but an exo-moon would have massive gravitational tidal flexing to keep it's core active just like Io and to a lower degree Europa.

So kinda, outside of the magic 'unobtanium' it could exist

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The_Solar_Oracle t1_j2bzhwh wrote

Pandora is not actually so small that its core would necessarily be solid. Jupiter's largest moon, Ganymede, is known to have its own magnetic field brought about by a partially molten core, and it's only 2.5% the mass of Earth! By contrast, Pandora is said to be 72% the mass of Earth, which makes it significantly more massive than Mars and slightly less massive than Venus. What really determines whether or not a celestial body's core stays molten on its own probably boils down to a favorable amount of transuranic elements in addition to size. After all, half of Earth's own interior energy can be attributed to the decay of radioisotopes and the occasional natural fission reactor. The presence of tectonic activity might also be important, which could explain some differences between Earth and Venus in regards to their interior activity.

It can also take a very long time for stellar winds to strip away another celestial body's atmosphere, and there are means to replenish any losses like volcanic outgassing.

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ICLazeru t1_j2cpfly wrote

In the case of a moon orbiting a much more massive body, it's also possible that the gravitational tide of the larger body keeps the core molten. This would only work for so long as Pandora had rotational momentum of course, and over time the body would become tidally locked with the same side always facing its larger partner.

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The_Solar_Oracle t1_j2cqtx9 wrote

Pandora is probably tidally locked, but the kind of tidal forces responsible for prolonged geological activity for moons like Io also require special resonance orbits with other satellites. Europa and Ganymede have a 2:1 and 4:1 orbital resonance with Io respectively, and these resonances maintain Io's orbital eccentricity. Without this elegant dance of moons, the same tidal forces that physically distort Io would also circularize its orbit.

Interestingly, though, Pandora is also reliably depicted as being fairly close to Polyphemus (a planet that is itself slightly less massive than Jupiter). While there doesn't appear to be a canon figure available on the internet in regards to its orbital period (and, thus, lengths of its days), there is no doubt in my mind that James Cameron has figures to this end for use by the production team.

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ICLazeru t1_j2cpllo wrote

Why would the mountains be floating rather than just drifting toward the magnetic pole?

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