Saturn's icy moon Iapetus has long baffled scientists with its unusual walnut shape.
Now a team of researchers says they have an explanation: The satellite's surface froze during its infancy, locking the moon's shape at a time when it was spinning much faster than it is now. 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.
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.
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