Saturn's "Walnut Moon" Mystery Solved

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But "if the heat comes from outside, the surface is going to be very warm and should have relaxed," she said. "So we had to find a heat source in the interior."

Eventually, her team realized that the decay of short-lived radioactive isotopes could provide just enough heat. The energy, the scientists say, kept the interior of the moon at about the temperature of liquid water during the geologically brief time the crust was forming and the moon was losing its spin.

Scientists know how common these isotopes were in the early solar system from studies of meteorites and comet-dust samples recently returned to Earth by the Stardust mission. (Related: "Stardust's Comet Clues Reveal Early Solar System" [December 15, 2006].)

Applying that to Iapetus allowed Castillo's team to produce a model that explained Iapetus' current bulge.

As a bonus, the scientists were also able to determine the moon's age—"roughly" 4.564 billion years old, about the same time Earth was formed.

But Iapetus isn't coughing up all its secrets. The enigmatic moon also has a ridge of enormous mountains running much of the way along its equator, atop the bulge.

These appear to have formed after the crust froze, when the interior of the planet contracted for some reason, Castillo said.

This would have caused the crust to be squeezed, as though in a vice, creating mountains.

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