Why don't the inner moons of Jupiter have tidally-induced volcanism?

  • Volcanism on Io is caused by the fact that it is tidally heated. There are four moons that are closer to Jupiter than Io with higher eccentricities, yet they don't seem to have any volcanism at their surface. Why isn't the tidal heating enough to generate volcanism on any other moons of Jupiter?

    There are no spherical moons between Jupiter and Io. All the satellites inbetween are asteroids, the largest of them are Amalthea and Thebe having diameters of about 250 km (150 mi) and 110 km (70 mi). So there is no interior differentiation and thus any volcanism impossible.

  • Eric Jensen

    Eric Jensen Correct answer

    2 years ago

    It’s because they are much smaller than Io. Tidal forces are differential forces, that is, they result from the difference in gravitational pull on one side of a body compared to the other. When an object is small, the difference in distance to the two sides of it is necessarily small as well.

    According to Wikipedia, Amalthea, the largest of those four innermost moons has a long axis that is only 250 km, and the others are smaller yet. The strength of the tidal heating scales as the body’s radius to the 5th power, quite a strong dependence. The small mass plays a role, too, but the dependence on radius is stronger.

    For this same reason, small objects on Earth don’t feel tidal stretching from our Moon, yet the Earth as a whole does.

    On a related note, the inner three of those four satellites are inside Jupiter’s Roche limit, the orbital distance at which a purely self-gravitating body should be pulled apart by tidal forces. But they survive because most small bodies aren’t held together primarily by gravity (just as your own body isn’t) - the internal electromagnetic forces between atoms (which manifests macroscopically as the rigidity of the rock) is the main source of the bodies’ structural strength.

    To take this to a limit, artificial satellites around Earth don't experience measurable tidal heating, and don't disintegrate from tidal stresses, even though they're well inside the Roche limit.

    Re *"...as the rigidity of the rock"*: But the inner three includes Amalthea which *"consist of porous water ice with unknown amounts of other materials.)"*.

    Only inside for the fluid-body Roche limit?

    Water ice has rigidity - one surmises, in these specific instances, enough to withstand tidal forces. Presumably the moons for which that was not the case are all the ones that are no longer there.

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