The strangest moon in the Solar System

Saturn’s Iapetus looks like nothing else we’ve ever seen. What made it so?

“The dance between darkness and light will always remain — the stars and the moon will always need the darkness to be seen, the darkness will just not be worth having without the moon and the stars.” -C. JoyBell C.

Yet the dance between light-and-dark is sometimes incredibly puzzling. Such was the case for the 17th century astronomer, Giovanni Cassini, when he gazed upon the wondrous ringed planet of our Solar System: Saturn.

Image credit: NASA, ESA and the Hubble Heritage Team (STScI/AURA). Acknowledgment: M. Wong (STScI/UC Berkeley) and C. Go (Philippines).

Cassini made many great discoveries about this planet, including the divisions between the rings and a number of Saturn’s moons. In fact, the second of Saturn’s moons ever discovered, Iapetus, presented a tremendous mystery to Cassini. While it clearly orbited Saturn like the other moons, it was only visible during half its orbit, remaining completely invisible during the other half.

As telescope technology improved, we discovered why: one half of Iapetus is only about one-fifth as bright as the other!

Image credit: NASA / JPL-Caltech / Space Science Institute / Cassini.

What a strange occurrence. Thanks to Cassini — the NASA mission, not the Italian scientist — we’ve actually gone to Iapetus itself to photograph it, finding that in fact one side looks like it’s been plowing into a dirt storm. Your car’s windshield might look like this after driving through a particularly dense patch of insects, but what could be causing this on Iapetus?

For a long time, this remained a mystery.

Image credit: © 2012–2014 Guide to the Universe

You see, all of Saturn’s major moons orbit in the same plane as its rings: all but Iapetus, which is significantly tilted. And no one knows why; no other large moon in the Solar System that formed along with its parent planet has such a tilt, and yet Iapetus does.

Image credit: NASA / JPL-Caltech / Space Science Institute / Cassini.

Iapetus also has a giant ridge along its equator: some 10 kilometers higher than the rest of the rocky, icy world. It isn’t rotating quickly enough to explain this, and the surface of Iapetus appears to be many billions of years old, so it likely isn’t recently coalesced debris, either. While many ideas abound concerning what causes this ridge, no one theory is the clear front-runner.

There are many ways in which Iapetus is unusual for our Solar System, and a few mysteries that haven’t been answered.

Image credit: NASA / JPL, via

But one of them has! What makes Iapetus two-toned? Not the inclination of its orbit, but the fact that it’s the farthest large moon from Saturn. That, and the presence of another, also remarkable moon even farther out.

Image credit: NASA/JPL/Space Science Institute.

Well beyond the orbit of Iapetus lies a moon that looks like none of the others orbiting Saturn: Phoebe. Phoebe isn’t made up of the same materials that Saturn’s other Moons are, and moreover, it orbits in the opposite direction from all the others. Rather than (looking down from the north pole) orbiting counterclockwise around its parent planet, which all the other moons do, Phoebe revolves clockwise around Saturn.

How is this possible? Because Phoebe didn’t originate with Saturn, but is rather a captured Kuiper belt object! In addition, Phoebe is responsible for the largest, most elusive ring in the Solar System.

Image credit: NASA/JPL-Caltech/Keck.

Only detected in 2004 with the (infrared) Spitzer Space Telescope, the Phoebe ring is a diffuse ring of debris that originates from Phoebe, and is extremely dark. It’s also incredibly sparse: the material comes at a density of about seven dust-sized grains per cubic kilometer! Of course, since this debris revolves around Saturn in the opposite direction to all the other moons, the outermost moons of Saturn might plow into it, exposing the leading side of that moon to that darkened debris.

That’s exactly the configuration we’ve got with Iapetus, slamming into Phoebe’s debris ring!

Image credit: Smithsonian Air & Space, derived from NASA / Cassini images, via

Since Iapetus is tidally locked to Saturn — meaning that the same “side” always faces forward as it moves through its orbit — the front of it accumulates this dark material, while the rear side doesn’t.

Now you might think that this, alone, could account for Iapetus’ appearance. But if there were only this darker material, it would heat up while in direct sunlight, causing the ices to sublimate and resolidify atop the debris, leaving both sides the same color. Amazingly, this does happen, at least partially. The darker material does get hotter than the lighter material, and this causes the surface ice to sublimate. But in that gaseous phase, it has a significant amount of kinetic energy. Not enough to have it escape Iapetus’ gravity, but enough so that it can migrate to the light side, where it can stably remain!

Image credit: NASA / JPL, via

So the debris from Phoebe is important for landing on one side and causing more severe heating of that side, which prevents ice from stably forming atop it. If there were no Phoebe, Iapetus would be as snowy-white as Saturn’s other brilliant moon: Enceladus.

Image credit: NASA/JPL/Space Science Institute, via

Instead, the presence of the Phoebe ring darkens one side of Saturn’s outermost giant moon, giving it an appearance unlike anything else in the Solar System.

While Iapetus might still be the most unusual moon around, we can be proud of ourselves for one huge reason: we’ve solved at least one of its major mysteries, and finally know why it has a yin-yang coloration unlike anything else!