Moon with a Screw Top

Iapetus is Saturn's third-largest moon. It was discovered over 300 years ago, back in 1671 by Giovanni Domenico Cassini. Cassini didn't call them moons back then, he called them the "stars of Louis" in deference to the current king. It's a good thing we stopped this tradition, else with all of these new moons we've been finding lately we would quickly run out of names. Heck, we're running out of mythological entites already. And I really don't want to be studying the "liver spots of Carter", "Bush's Points Of Light", or "Clinton's Follies."

One of the first things that puzzled Cassini about Iapetus was that every 79 days or so it appeared to "wink out" for a short period of time. This happened like clockwork, so Cassini figured (correctly) that part of the moon must be much darker than the other part of the moon. When we finally got good images of Iapetus, he was right! It was very strange looking!

When Voyager passed Saturn in 1980 we got our first realy good images of Iapetus.
It has a most unusual dark spot covering a large part of the moon's surface. What is it?

When this image first came out, Iapetus was called the "Yin-Yang moon", since this view looks like the Chinese symbol of the same name. As it turns out, this dark spot (known as the Cassini Regio) is actually an oval, as you can see below.

The dark area looks like it is in the shape of an oval.

This view makes Iapetus look like a huge Pac-Man! Traveling around the Saturnian system, gobbling up little bits of dust. Seriously, though, more is unknown about this feature than is known. The composition of this oval is unknown. The depth of the material is unknown. Maybe it's ammonia. Maybe not. Maybe it came out of a cryro-volcano. Maybe not. Nobody knows. And it gets weirder.

Taking a look at the full-color image of Iapetus released by JPL, you can see something that has never been seen anywhere before in the solar system. Take a look.

This is the best full view of Iapetus we have. Notice something strange?

Here we have a major moon in our solar system, and all around the equator is a huge wall of -- something. It looks like it stands about 12 miles tall. It's in a perfect straight line.

Objects in nature do not form huge straight lines. Or if they do, we haven't seen that anywhere else so far. Natural objects are curvy. There are some exceptions, like the fake "underwater cities" that turn out just to be rock formations weathered by the tides. But those formations are on a small scale, meters wide. This wall is over 750 miles long and sticks up 12 miles from the surface around it.

What could form such a wall? Nobody knows. The leading theories are:

Rapid rotation change and cooling from radioactive elements found in the proto-Saturn system. I'll be nice and say I find this explanation lacking. But then again, what do I know?

Icy material welling up from inside the moon. Ok fine. Makes some sense. Lots of icy moons have stuff welling up from inside of them. But why a straight line? Why just along the equator?

Collision with the rings. This theory supposes that Iapetus used to orbit much closer to Saturn, grazed the rings (and got a big scratch!) and then somehow moved into it's current outside orbit. This also has a lot of "what ifs" and I find it lacking. And unlikely. But I'm just an amateur.

There are also some ludicrous crazy ideas, which I present.

Extra-terrestrial intelligence made the wall. The idea that Iapetus was a sign-post put there by advanced alien civilizations was first suggested back in 1980 by Goldsmith and Owen. Just a couple of years ago, Richard C. Hoagland speculated that it was an alien artifact. Hoagland also thinks that there is a Martian City at Cydonia, NASA is covering up the true color of mars, NASA killed the Apollo 1 astronauts on purpose, and the Apollo astronauts discovered crystal cities of glass on the moon.

Let's be kind to Hoagland and call him "eccentric." Crazed wacko also works for me, as well as "conspiracy theorist capitalist" -- there's a lot of money to be made with wild speculation. You can sell tapes, books, go on talk shows, etc. It's quite the cottage industry. Let's also just say kindly that I find Hoagland severely lacking in the credibility department.

Finally there's my theory.

Giant hollow moon. Iapetus is a giant twist-off ball, with some kind of prize inside. We're going to need a really big pipe wrench to open that sucker! I hope we get a good prize, and not one of those cheap plastic toys.

That's the problem with lack of data. People can make up anything they want. While it's not a world crises or anything, Iapetus certainly has the public's interest. And scientists are perplexed too. Wouldn't this be a good time for an Agency who needs better PR to find out a little more information? After all, when's the last time you saw something like this on a moon?

Maybe Iapetus has an immigration problem too

Iapetus is just a strange place. If you take another look at the "Pac Man" picture above, you'll note that there are a lot of straight lines. I've never seen anything like it, but then again, what do I know? In fact, it almost looks like the moon is a constructed body, which is why Hoagland got so much mileage out of his wild theorizing.

Just to have fun, let's take a couple more looks at weird stuff from Iapetus. Here's one showing something along the horizon.

"Tower" on Iapetus stands over a mile from the surface. Wouldn't it be cool to see a better picture of this?
Notice all of the straight lines on the rest of the moon. Kind of looks like there is some sort of structure underneath, huh?

Looks like a bunch of buildings who have lost their roofs.
Then it kind of looks like "snowfall" on the ground all around the buildings.
Why so many straight lines?

As far as I know, these are all just strange artifacts of some process we have yet to understand. I'll bet a hundred bucks right now there is some natural cause for all of this. But that still doesn't stop it from being interesting. How come all of the craters on Iapetus look like geometric shapes? How come there are these huge structures that rise from the ground? How could you come up with an incredible wall like that? Why does this moon have so many straight lines? Why is the moon on such an odd orbit compared to the rest of the moons of Saturn? What's in that dark stuff anyway? Is that snow on the ground (or the cryro equivalent)? How could Iapetus be so different from the other moons? From all of the other moons we've observed?

Nobody seems to know any of these answers. We've spent billions to get the Cassini probe out to the Saturnian system. I understand that it may be too much to task the probe to radically change orbits to study Iapetus extensively, but couldn't we get some more radar data? Long range imaging might be very useful as well. And towards the end of the mission, would there be anything wrong with doing some up-close study of Iapetus? I'll vote to put that burn in the fuel budget. After all, it's a big puzzle. And aren't we supposed to be paying all of this money to solve some of the big puzzles out there?

UPDATE: More recent information on the Iapetus mystery here: