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5 December 2013 11:26 am ,
Vol. 342 ,
Dyslexia, a learning disability that hinders reading, hasn't been associated with deficits in vision, hearing, or...
Exotic, elusive, and dangerous, snakes have fascinated humankind for millennia. They can be hard to find, yet their...
Researchers have sequenced and analyzed the first two snake genomes, which represent two evolutionary extremes. The...
Snake venoms are remarkably complex mixtures that can stun or kill prey within minutes. But more and more researchers...
At age 30, Dutch biologist Freek Vonk has built up a respectable career as a snake scientist. But in his home country,...
Since arriving on the island of Guam in the 1940s, the brown tree snake ( Boiga irregularis ) has extirpated native...
An animal rights group known as the Nonhuman Rights Project filed lawsuits in three New York courts this week in an...
Researchers have been hot on the trail of the elusive Denisovans, a type of ancient human known only by their DNA and...
- 5 December 2013 11:26 am , Vol. 342 , #6163
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Jupiter Moon Whipped Into Shape by a Beating
25 January 2010 (All day)
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.