Of all the moons in the solar system, Iapetus has to be among the weirdest. Named after a spear-wielding Titan, the strange Saturnian satellite is less than half the size of Earth’s moon. But it’s a cluster of enigmas: Squished at its poles, the moon is walnut-shaped, has a face as black as coal and a bright white backside, and wears a big, spiky mountain range as a belt.
Even its orbit is weird: Iapetus is roughly three times farther from Saturn than its closest neighbor, Titan. And the path it takes around the planet is tilted, meaning it swings up and down as it orbits, rather than staying in the plane of Saturn’s rings like the rest of the “normal” satellites.
In other words, it’s kind of like the rebel of the Saturnian system, a moon who’d prefer to hang out behind the dumpster and cut class rather than play ball with the other kids.
Among the strangest of Iapetus’ unsolved mysteries is its super-chic, spiky mountain range. Running straight as an arrow along three-quarters of the moon’s equator, the thing is huge: Roughly 20 kilometers tall and up to 200 kilometers wide. (The peak of Mt. Everest, in comparison, rises only 8.85 kilometers above sea level.)
There’s nothing else like it in the solar system.
Scientists first spotted the ridge in 2004, and since then, they’ve been trying to figure out how such a thing formed. Early theories suggested geologic activity within the moon itself – maybe something akin to Earth’s plate tectonics or volcanism had forced the ridge to rise up along the equator. But that didn’t make a lot of sense. The moon’s crust wasn’t spongy when the ridge formed, the evidence for active geology tepid.
Then, scientists thought maybe the ridge had formed as a result of the moon’s rotation period abruptly slowing down. Some early simulations suggest a day on the moon used to last for a mere 16 hours. Now, though, a day on Iapetus lasts 79 Earth-days – the same amount of time it takes the little guy to shuffle once around Saturn (the moon is tidally locked, meaning it’s plowing through space with the same face forward, always).
Maybe, teams said, a giant impact had knocked Iapetus into its current rotation state, and the resulting braking action caused the crust to buckle.
But most of these theories also predict other strange geologic features (which aren’t observed), or hinge upon the crust being a certain thickness (which it may not be).
In 2010, a new theory emerged. Perhaps the ridge is the remains of a former moon – a moon-moon, suggested Andrew Dombard of the University of Illinois at a meeting of the American Geophysical Union. Sometime during the evolution of the Saturnian system, he said, Iapetus may have had a little friend, roughly 100 kilometers in diameter. Whether the moon captured the moonlet, or the moonlet formed from the debris ejected by a giant impact (this is how Earth’s moon formed) isn’t known.
But eventually, that little friend wandered too close to cranky Iapetus and ended up being shredded by the planet’s gravity.
As the moonlet broke up, Dombard said, its pieces formed an ephemeral ring around Iapetus’ equator. The ring eventually rained down upon the satellite and deposited the giant ridge.
In 2011, another team suggested something similar, this time with a giant impact forming both a ring and a moonlet. The ring would go on to form the mountain range, while the moonlet would smash into Iapetus and create one of its many large impact basins.
Recent evidence, gleaned from the shape of the mountain ridge itself (steep and triangular), suggests that pieces falling from on high could make total sense. It’s kind of the same shape you get when you take a handful of sand and slowly sprinkle it into a pile.
Why the ridge only runs along three-quarters of the equator isn’t explained by this scenario, though.
In short, we still don’t know how Iapetus grew its monstrous mountains. But the idea of a moon with a moon, or a moon with a ring, is strangely compelling. Too bad Iapetus had to go and tear its little friend to bits.