Just imagine the frustration of Gian Domenico Cassini after discovering Saturn's Iapetus in 1671. He saw the moon just fine whenever it was west of the planet, but on the eastern side it was a no show. Cassini finally puzzled it out in 1705: Iapetus must be very bright on one hemisphere (the side facing forward in its orbit) and very dark on the other.
It took another two centuries to figure out how this two-faced personality came about. Close-up views taken in 2007 by NASA's Cassini orbiter revealed that the dark stuff has neither erupted from within nor drifted in from its neighbors — which had been the two leading suspicions. Instead, the moon's topmost layer of ice has apparently sublimated (vaporized) away, leaving behind a thick coating of carbon-rich dregs.
However, in solving one mystery about Iapetus, Cassini scientists were confronted with another: a towering, mountainous ridge up to 10 miles (15 km) high that stretches along the moon's equator for more than 800 miles (1,300 km) — a third of the way around its circumference. This remarkable and apparently ancient ridge can't be explained by geologic processes. It's had planetary scientists baffled since its discovery.
There's yet another vexing problem. Iapetus rotates in lockstep with its 79-day orbit around Saturn, long ago forced into synchrony by Saturn's gravity (just as the Moon synchronously spins and revolves around Earth). But Cassini's close-ups prove that Iapetus is distinctly out of round. Its overall shape implies that it must have become frozen while spinning quite fast, once every 16 hours. Saturn might be massive, but Iapetus is so far away that the planet's tidal tugs would take an estimated 10 billion years to slow down such a rapid spin.
Now dynamicist Hal Levison and three colleagues at the Southwest Research Institute have taken a stab at making sense of both that big "fossil bulge" and the despinning conundrum. At the just-ended Lunar & Planetary Science Conference, team member Kevin Walsh described how something must have whacked Iapetus hard enough to create a close-in debris disk and a small moonlet orbiting farther out.
The SwRI scientists find that, by tidally interacting with Iapetus, the moonlet dramatically decreases the despinning time and — bonus! — forces the disk to collapse to form the equatorial ridge in no more than a few thousand years. "The infalling debris will have velocities nearly tangential to the surface at only 300 m per second," Walsh explains, and there'd be more than enough of it to form the ridge.
Meanwhile, all this gravitational give and take would slowly push the moonlet farther away until it escaped Iapetus's grasp and started orbiting Saturn instead. But that freedom would have been short-lived: there's a 90% chance that it eventually collided with Iapetus and gouged out one of the big basins now scarring its icy crust.
As the team details in an article submitted to Icarus (and summarized here), this scenario succeeds or fails depending on assumptions about the stiffness of Iapetus's interior (a function of its composition and temperature) and how that changed over time.
Is this rock-'em, sock-'em story even remotely plausible? Well, why not! Catastrophic impacts have been invoked to explain everything from Earth's Moon to Mercury's oversize iron core to the sideways spin of Uranus. What's one more big splat in the grander scheme of things?