No longer two peas in a pod. Some source of heat--possibly a storm of impacts eons ago--led to the resurfacing of Ganymede (left) but left Callisto (right) largely unchanged.

Jupiter Moon Whipped Into Shape by a Beating

Dick writes about Earth and planetary science for Science magazine.

Odd couples don't get much odder than Jupiter's major moons, Ganymede and Callisto. Although their size and makeup are similar, Ganymede has become a proper moon with its own magnetic field, whereas Callisto has remained a bland ball of ice and rock. Now scientists think they have figured out why.

For 30 years, researchers have wondered what process could have got enough heat into Ganymede to drive its geological evolution without setting off Callisto as well. In search of a heat source that could discriminate between the two moons, planetary scientists Amy Barr and Robin Canup of the Southwest Research Institute in Boulder, Colorado, considered the late heavy bombardment (LHB). That's a hypothesized storm of comets and (in the inner solar system) asteroids that many planetary scientists think pummeled the solar system 3.9 billion years ago.

When Barr and Canup simulated the effects of LHB's impacts on Ganymede and Callisto, they found plenty of discrimination. Jupiter's powerful gravity would have accelerated incoming comets and drawn more of them near the planet, the researchers note. Ganymede, being closer to Jupiter, would have suffered twice as many impacts as Callisto and at higher velocities. So Ganymede would have received 3.5 times more energy than Callisto did.

In their modeling, that was more than enough heat to begin melting the ice in Ganymede's natal mixture of ice and rock. The thaw would have allowed the moon's rock to start sinking through the increasingly slushy interior. Then the sinking rock would have given up its gravitational energy as heat, accelerating Ganymede's separation into layers. Eventually, enough heat from radioactive decay would have built up to separate the rock's iron into a molten core, drive a magnetic field, and perhaps form Ganymede's grooved surface geology.

Shortchanged on impact energy, Callisto would not have melted enough to achieve "runaway" heating during separation, leaving it cold and without a core. With a negligible heat source below it, Callisto's surface would be geologically dead for eons, the team reports online this week in Nature Geoscience.

"It's a classic good-science paper," says planetary geologist James Head III of Brown University. It also lends support to the reality of LHB. In particular, Barr says, the work "fits nicely with the Nice model," which shows how Jupiter and Saturn could have stirred up an LHB while migrating outward through the solar system (Science, 17 July 2009, p. 262). But researchers agree that much analysis remains to be done, and another mission to the Jupiter system would be nice.

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