Saturn's "Walnut Moon" Mystery Solved

Richard A. Lovett
for National Geographic News
July 18, 2007
The mystery of Saturn's icy moon Iapetus—which is shaped like a walnut when it should be more like a sphere—has finally been figured out, scientists say.

The answer: The satellite's crust froze solid when Iapetus was young, forming a rigid shell that forced the moon to retain its youthful shape.

Today, Iapetus is 20 miles (33 kilometers) wider at the equator than the poles.

Normally, that kind of distortion happens only if a moon is spinning rapidly, like a figure skater in a tight spin. But an Iapetus day is nearly 80 Earth days long, though it was once much shorter.

"You would expect a very fast-spinning moon to have this bulge, but not a slow-spinning moon," Dennis Matson, a scientist with NASA's Saturn-orbiting Cassini mission, said in a statement. (See a photo gallery of Cassini mission highlights.)

In a paper published in the online version of the journal Icarus, a team led by Julie Castillo of NASA's Jet Propulsion Laboratory has finally found an explanation for the moon's odd shape.

Short-lived radioactive elements, such as aluminum-26 and iron-60, could have provided enough heat to keep the moon's interior warm and squishy during its infancy.

This would have allowed the exterior to freeze solid, forcing the moon to keep its early shape even as its spin reduced and gravity tried to pull it into a sphere.

"Iapetus spun fast, froze young, and left behind a body with lasting curves," Castillo said in a statement.

Deep Heating

Simply knowing that the moon had frozen in a youthful shape wasn't enough. The key to solving the puzzle was finding what had provided the transient heat source that liquefied the satellite's interior—while keeping its surface cold and hard.

One possibility, Castillo told National Geographic News, was meteorite bombardment.

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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