Saturn’s moon Iapetus is one of the more mysterious objects in the Solar System. It’s fairly large, the 11th largest moon in the Solar System, and made mostly of ice.

But Iapetus is a puzzle. Half of the moon is dark-coloured and the other half light, with no shades of grey. It is bulging around its equator, as if it is rapidly spinning. But it actually rotates rather leisurely, once every 79 Earth-days. Most puzzling of all is a ridge that stretches almost half way round its equator (see picture above).

Today Iapetus is slowly giving up its secrets. Planetary geologists recently solved the puzzle of its two-tone appearance. It turns out that the dark stuff is the chemical residue left when water ice sublimates.

The thinking is that about a billion years ago, one side of Iapetus began to sublimate a little more quickly than the other, leaving a dark residue. The ice then condensed on the other side of the moon making it lighter. The darker side then absorbed more sunlight, making it warmer and increasing the rate of sublimation in a positive feedback cycle.

It is this cycle that has left the moon in its current two tone state.

Today, Harold Levison and buddies at the Southwest Research Institute in Boulder, Colorado, have hit on an explanation for the other two puzzling features: Iapetus’s bulge and ridge.

Their theory is that early in its history, Iapetus spun very quickly, probably at a rate of once every 16 hours or so. This caused it to bulge around its equator.

At this time it was hit by another large moon, which catapulted a huge volume of ejecta into orbit (a little like the collision that formed our Moon). This orbiting mass of rubble then fell victim to two separate processes.

To understand these forces, we first need a little bit of background about orbital dynamics. Astronomers have long known that there is a particular distance from any gravitational object beyond which rubble can condense to form a solid body. This is called the Roche radius. However, anything closer than this gets torn apart by tidal forces and so never condenses.

Levison and co say the ejecta around Iapetus must have spanned the Roche radius. The stuff beyond this limit condensed to form a new moon, which gradually spiralled away from Iapetus. It was this loss of its own satellite that slowed Iapetus’s rotation to the sedate rate we see today. However, its frozen body preserved the shape of the original bulge.

But the ejecta inside the Roche radius could not have formed a solid body and so must have formed a ring around Iapetus instead. Levison and co think this ring was unstable and must have slowly closed in on the moon.

So the equatorial ridge we see today is the leftovers of this ring that settled onto the moon’s surface.

That looks like a neat idea. It explains at least two of the great mysteries of Iapetus. And that ain’t bad for a single theory.

Ref: arxiv.org/abs/1105.1685: Ridge Formation And De-Spinning of Iapetus Via An Impact-Generated Satellite