Musings of a Political Scientist

Jupiter and Moons

Jupiter and Moons by Bryan Brandenburg

Jupiter and Moons

Jupiter has at least 63 moons. The four large moons, known as the “Galilean moons”, are Io, Europa, Ganymede and Callisto.

Galilean moons

The orbits of Io, Europa, and Ganymede, the largest moon in the solar system, form a pattern known as a Laplace resonance; for every four orbits that Io makes around Jupiter, Europa makes exactly two orbits and Ganymede makes exactly one. This resonance causes the gravitational effects of the three moons to distort their orbits into elliptical shapes, since each moon receives an extra tug from its neighbors at the same point in every orbit it makes.

The tidal force from Jupiter, on the other hand, works to circularize their orbits. This constant tug of war causes regular flexing of the three moons’ shapes, with Jupiter’s gravity stretching the moons more strongly during the portion of their orbits that are closest to it and allowing them to spring back to more spherical shapes when they’re farther away. This flexing causes tidal heating of the three moons’ cores. This is seen most dramatically in Io’s extraordinary volcanic activity, and to a somewhat less dramatic extent in the geologically young surface of Europa indicating recent resurfacing.

Classification of Jupiter’s moons

Before the discoveries of the Voyager missions, Jupiter’s moons were arranged neatly into four groups of four. Since then, the large number of new small outer moons has complicated this picture. There are now thought to be six main groups, although some are more distinct than others. A basic division is between the eight inner regular moons with nearly circular orbits near the plane of Jupiter’s equator, which are believed to have formed with Jupiter, and an unknown number of small irregular moons, with elliptical and inclined orbits, which are believed to be captured asteroids or fragments of captured asteroids. It is thought that the groups of outer moons may each have a common origin, perhaps as a larger moon or captured body that broke up.

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