Iapetus is weird. It's a moon of Saturn, and it's always been known to be weird. One of Iapetus's hemispheres is much brighter than the other, for one thing (probably due to collecting material as it orbits the planet). For another, it's got a pretty big equatorial bulge; it's not even close to being a sphere. And third, right around its equator, there is this vast ridge of material that's something like 20 kilometers high!
Yup. It is the walnut of the solar system.
Those two features -- the bulge and the ridge -- are just crying out that they are related somehow. And now it looks like it may be understood why.
New results just released state that when it was very young, Iapetus rotated very rapidly - something between 5 and 16 hours per rotation. This is what formed the equatorial bulge. But its spin rate now is much longer, about 80 days. Obviously, something in its past slowed the spin.
That something is the immense tidal force of Saturn. This force (really, a product of the gravitational force) can slow the rotation rates of objects. But for Saturn to slow Iapetus, it turns out that there must have been something warming the interior of the little moon when it was very young, and that was found to be radioactive heat. Aluminum-26 and iron-60 are radioactive, and their decay can heat up the surrounding material. Furthermore, they have such short half-lives -- meaning they decay away rapidly -- that in geological terms it's as if the heat source switches off.
Now follow this logic: Iapetus spun quickly when it was young, and got the bulge. Its interior was heated by radioactivity. But then that heat source shut down. The moon started to cool, and simultaneously its rotation slowed due to tides from Saturn. When the rotation slowed, the centrifugal force at its equator dropped, and it tried to shrink and resume a spherical shape. But by then the outer crust had frozen. Instead of flowing smoothly into a sphere, the equatorial crust piled up as the Moon shrank, forming the ridge.
Voila. Walnut moon.
Incidentally, because radioactive materials decay at a known rate, and the amount of heating needed to make the theory work implied how much radioactive material Iapetus had, the scientists were able to calculate the age of the moon. The answer? 4.564 billion years, pretty much the known age of the solar system.
Swallow that nut, young Earth creationists!
Speaking of garbage science, I have to mention -- regular readers know I can't help myself sometimes -- that Iapetus has long been the target of some, um, nutty ideas. The king of these is of course one Richard Hoagland, who claims that the ridge around Iapetus is artificial. Yes, built by intelligent beings (though as usual he never says who he thinks they are).
You can't make this stuff up.. oh wait, duh, of course you can. Here's what he has to say:
[...] it could really be a "wall"... a vast, planet spanning, artificial construct!!
Man, you know this is serious if he uses two exclamation points. I mean, "exclamation points!!" To drive the artificiality point home, he compares the moon to the Death Star from Star Wars in a side-by-side picture -- not just once, but twice!
I mean, "twice!!"
He goes on to say:
There is no viable geological model to explain a sixty thousand-foot-high, sixty thousand-foot-wide, four million-foot-long "wall" spanning an entire planetary hemisphere... let alone, located in the precise plane of its equator!
It's unclear when Hoagland wrote that page, though it's dated 2005 and there are clues it was in February or March of that year, but at the same time he was feverishly banging away at his keyboard producing that goofiness, a real scientist by the name of Paulo CC Freiere was finishing up an actual paper on the ridge around Iapetus (and you can read a popular-level synopsis of his findings over at Universe Today). In a nutshell (ha! a double pun!) the idea was that Iapetus could have formed that ridge when it slammed into one of Saturn's rings. The material piled up on the equator, forming that vast range of mountains. That also could explain the difference in brightness of the two hemispheres.
This new idea about Iapetus getting its bulge and ridge by the freezing out and piling up of matter seems more plausible than having the Moon plow into a ring and gathering up matter, but still, we now have two theories on how that structure could have formed. Either or both may turn out to be wrong, but I think the extraterrestrial alien pyramid builders can be dismissed.
Of course, Hoagland continues on his pages to bark on about artificial constructs, doing his usual sleight of hand with over-magnifying images and claiming JPG artifacts are buildings or some such nonsense. And I'll admit, it's rather fun to read his stuff, in a schadenfreude kind of way. But in the end, I prefer actual, y'know, science. Speculation is fun, but real science will be more interesting, more exciting, and more satisfying every time.