Observations of Amalthea by the Galileo spacecraft, in late 2002, showed that the moon has a very low density, indicating that it may be a loosely packed pile of rubble. The gaps between solid chunks likely take up more of the moon's total volume than the solid pieces, and even the chunks are probably material that is not dense enough to fit some theories about the origin of Jupiter's moons. Amalthea now seems more likely to be mostly rock with perhaps a little ice, rather than a denser mix of rock and iron. Its irregular shape and low density suggest it has been broken into many pieces that cling together from the pull of each other's gravity, mixed with empty spaces where the pieces don't fit tightly together. Galileo's discovery supports the idea that the inner moons of Jupiter have undergone intense bombardment and breakup. Amalthea does not have quite enough mass to pull itself together into a consolidated, spherical body like Earth's Moon or Jupiter's four largest moons. The density estimate, obtained from Galileo's flyby, extends an emerging pattern of finding irregularly shaped moons and asteroids to be porous rubble piles. What's more of a surprise is the density estimate is so low that even the solid parts of Amalthea are apparently less dense than Io. One model for the formation of Jupiter's moons suggests moons closer to the planet would be made of denser material than those farther out. This is based on a theory that the infant Jupiter, like a weaker version of the early Sun, would have emitted enough heat to prevent volatile, low-density material from condensing and being incorporated into the closer moons. Jupiter's four largest moons fit this model, with the innermost of them, Io, also the densest, made mainly of rock and iron. See also rubble-pile asteroid.
Related entries• Jupiter, moons
• Amalthea in mythology (Encyclopedia of History)
Related category• PLANETS AND MOONS
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