![]() Small bodies such as those four innermost moons of Jupiter are too small to pull themselves into a roundish shape. Tidal heating results in part from a tension between the tidal forces exerted by the planet that tends to pull a moon out of round and the moon's self-gravitational force that tends to pull a moon into a roundish shape. Compare that with Io's mean radius of 1821.6 km. The largest of the four, Amalthea, has a mean radius of 83.5 km. The reason they don't exhibit volcanism is because they are too small. ![]() The other three have lower eccentricities. Only one of those innermost moons (Thebe) has an eccentricity higher than that of Io. 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. 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. 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. The small mass plays a role, too, but the dependence on radius is stronger.įor this same reason, small objects on Earth don’t feel tidal stretching from our Moon, yet the Earth as a whole does. The strength of the tidal heating scales as the body’s radius to the 5th power, quite a strong dependence. ![]() When an object is small, the difference in distance to the two sides of it is necessarily small as well.Īccording 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. 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. However, Io does not display plate tectonics: the volcanoes are more or less randomly distributed and there are no correlated belts of mountains and trenches (subduction zones) such as on Earth.It’s because they are much smaller than Io. That means the processes of eruption are the same-melted materials at depth working their way up to the surface through cracks and buoyancy forces. Volcanoes on Io look virtually identical to volcanoes on the Earth. Io will continue to be a very interesting object for a long time, and for that reason it is going to be studied for years to come. ![]() Thus, the ultimate source of Io’s volcanism is not decreasing with time. The elliptical orbit results from the combined gravitational effects of Jupiter and the other major Galilean satellites outside Io’s orbit (Europa, Ganymede and Callisto). Io's tidal heating is continuous as it orbits around Jupiter in an elliptical orbit. Once most of those elements have decayed there will be much less tectonic and volcanic activity on the Earth. Today, Earth’s internal energy is mostly coming from the decay of radioactive elements from primordial heating. However, Io has more energy to get rid of than the Earth, therefore there is much more volcanic activity on its surface than there is on Earth. Aside from this, the volcanic processes are fairly similar. Io's volcanism is mainly driven by tidal heating, while Earth’s volcanism is driven by internal heat.
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