Could a planet have a moon that itself had a smaller moon?

A former subsatellite would help explain some of the mysteries of Iapetus, one of Saturn's moons. For starters, Iapetus is not a sphere—it's a bit squished. And its flattened shape implies that Iapetus once spun very quickly, completing a rotation in 16 hours. It now takes 79 days. So what put on the brakes?

Maybe it was a onetime moonlet of Iapetus, explained Kevin Walsh of the Southwest Research Institute in Boulder, Colorado, at a recent planetary science meeting in France. The subsatellite would have spiraled slowly away from Iapetus until Saturn grabbed it.

But not before its outward drift sapped rotational energy from Iapetus and slowed it down.

The moonlet could account for another feature of Iapetus, too. The moon has a tall ridge running along its equator, like a walnut's seam.

If the short-lived moonlet emerged from a debris disk, as Earth's moon did, the moonlet could have forced leftover debris onto Iapetus to form the walnut ridge.

The moonlet idea is still preliminary. But solving two mysteries with one hypothesis means that it's not so nutty.

—John Matson