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Comets caused differences in Jupiter's moons

By Michelle Kessler, USA TODAY

Jupiter-moonx-large Jupiter's moon Ganymede likely endured a double blasting of comets 3.8 billion years ago, say planetary scientists, explaining perplexing differences with its twin moon, Callisto.

In the Nature Geoscience journal report led by Amy Barr of the Southwest Research Institute in Boulder, Colo., scientists how that Jupiter's gravity likely drew more, faster comets onto Ganymede, its closer moon, during a period called the "Late Heavy Bombardment" of the solar system.

Ganymede, some 3,270 miles wide, and Callisto, some 2,980 miles wide, are both icy moons of Jupiter and among the largest ones in the solar system. But Ganymede, which orbits some 665,000 miles from Jupiter, has a solid rocky core. Callisto, which travels on an orbit some 1,171,000 miles from the planet, does not. "The origin of the `Ganymede-Callisto dichotomy' has puzzled scientists since the Voyager era," three decades ago, says the study, because both moons formed from the same stuff, the same way, at the same time around the same planet.

In the study, the authors show the extra oomph of faster-traveling comets, accelerated by the closer proximity of Jupiter's heavy gravity, would have been sufficient to completely melt the ice of Ganymede after an impact. That would allow heavy elements to sink, forming a core inside the moon.

Callisto, however, doesn't receive quite as energetic a smacking around, leaving it a mixed ball of rock and ice. The difference supports earlier studies suggesting the comet disk that supplied much of the impact scars seen on the moon today must have weighed about 20 times as much as Earth.

By Dan Vergano
Photo: Cutaway views of the possible internal structures of Jupiter's moons. Ganymede is at the lower left, Callisto at the lower right, Io on the upper left, and Europa on the upper right. (NASA/JPL)

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