The first talk I attended at the Lunar and Planetary Science Conference in Houston was my one icy satellite talk for the day: Amy Barr, with an explanation for why Ganymede and Callisto look so different (note: links to LPSC talk titles will generally take you to PDF formatted conference abstracts). They are the two largest moons of Jupiter, similar in size (both about 5,000 kilometers in diameter) and density (both nearly double the density of water). Yet Ganymede has a youthful surface, criscrossed by grooved terrain that has disrupted craters, while Callisto has an ancient surface where nothing but cratering and "mass wasting" processes seem to have been going on for eons.
Employing a computer model, Barr and her coworkers looked at how the two moons would have experienced the Late Heavy Bombardment differently. The Late Heavy Bombardment is a controversial topic in planetary science. Evidence from the Apollo missions can be interpreted to suggest that all the large basins on the Moon formed during a relatively brief period about 600 million years after the Moon formed. New work modeling how the whole solar system formed suggests a way for this to happen: Uranus and Neptune initially formed closer to the Sun, and later migrated outward into a disk of icy planetesimals, which would have been scattered all over the solar system, many flying inward to crash on all the unsuspecting planets and moons.
So if the Late Heavy Bombardment happened, Amy showed that Ganymede and Callisto would have had quite different experiences. Both would have gotten hit much harder than the Moon: for every gram of material that hit the Moon, 40 grams would have hit Callisto, and 80 would have hit Ganymede (because it is deeper in Jupiter's gravity well). The stuff that hit Callisto would have been coming in at 15 kilometers per second, while the stuff that hit Ganymede would have come in faster, at 20 kilometers per second. Energy is proportional to the mass times the square of the velocity, so this seemingly small difference in velocity makes a big difference. The end result is that Ganymede gets hit so much more, and so much harder, that more than 50% of the ice in its mantle gets melted, which is enough to allow the metal and rock components to sink to the center, allowing the moon to differentiate into internal layers. Callisto doesn't ever reach that 50% threshold, so remains undifferentiated and geologically dead. It's a very neat explanation -- if the Late Heavy Bombardment happened, that is!
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