Location Made Twin Moons Different [Ganymede, Callisto]

Scientists have long wondered why Jupiter's once-twin moons, Ganymede and Callisto, came to be so different. Ganymede, the larger of the two, has a rocky core and scars from geologic trauma spread across its face. Callisto's center is a mix of ice and rock. Its ancient surface is heavily pocked with impact craters. What happened? Comets, says Southwest Research Institute's Amy Barr, lead author of a paper published in this week's Nature Geoscience that shows how differences in the frequency and strength of comet impacts explains the moons' distinctive histories... Ganymede, located about 500,000 miles closer to giant Jupiter than Callisto, bore the brunt of its parent's heavy hands. Jupiter's extreme gravity (a 150-pound person would weigh 355 pounds on Jupiter) tugged more comets toward Ganymede and caused them to crash at higher speeds than it did for Callisto. Ganymede got so hot that its ices melted, leaving rocks to pool in the moon's core like chocolate chips at the bottom of a container of melted ice cream.

"Finally the two dumb neighbors measured their horses that they couldn't tell apart, and found the perfect solution -- the white horse was two and a half hands taller than the black horse."

The assumption is, these twin moons which aren't that much alike were formed at the same time, and that is the source of the mystification.




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Newfound Moons Tell Secrets of Solar System
by Henry Fountain
August 12, 2003
The fact that most of the satellites' orbits are retrograde and eccentric speaks volumes about their origins: They had to have come from elsewhere, and been captured by the planets at some point. If they formed at the same time as the planets, from the spinning nebular disk, their orbits would be nearly circular and in the same direction as the planets' rotation, like the "regular" moons... In the case of the irregular satellites, they could not have shifted from an orbit around the Sun to an orbit around one of the giant planets without slowing down -- through friction in an atmosphere, perhaps; the influence of gravity; or a collision with another object... But there are two other possibilities for capture, Dr. Nesvorny said. One is that rapid growth of the core led to a corresponding increase in gravity, enough to pull down a nearby object. The other is that captured objects were a result of a collision between two planetesimals, the force of the collision being enough to dissipate the energy of at least one of them. Either of these two theories may be a more likely explanation for the satellites of Uranus and Neptune, which formed differently from Jupiter and Saturn, without the large amounts of gas.

Retrograde satellites lose momentum to the parent body and slowly spiral inward, which puts an upper limit (possibly not considered by these researchers) on the length of time the retrograde moons have spent as satellites, and obviously, will spend as satellites.

Craters on Vesta and Ceres could hold key to Jupiter's age

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