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Matt McIrvin's Steam-Operated World of Yesteryear

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Mold marks
Take a look at this crummy-looking picture of Iapetus:

The mostly-dark side of Iapetus, showing large craters and a long equatorial ridge in the dark area

This is a picture from the Cassini raw image archive, taken just yesterday, that I downloaded in JPEG form, then brightened excessively and unsharp-masked to hell and gone. The curious thing about it is that, besides the gigantic craters that Cassini found earlier, Iapetus seems to have a tremendous, absolutely straight mountain range on it that runs right down the axis of the dark area like a seam on a badly made plastic toy. The thing's so tall it's poking right out along the horizon.

I'm not sure about the orientation of this image, but I wonder if it connects to the line of white mountains pictured here. Yeah, I think it does! In that color picture you can see the faint streak continuing off to the right... that straight ridge goes at least halfway around the moon!

Let me just say, what the hell?

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It's a giant superball!

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The whole moon has a weird bright/dark pattern covering it, with the boundary shaped sort of like the seam on a baseball, so that many pictures of it look like a yin-yang symbol. In the dark hemisphere you're looking at here, only the poles are bright; the other hemisphere is completely bright except for the eastern and western extremes, which are the tips of the dark region. And now it looks as if most of the dark region has this enormous ridge running right along the equator.

Nobody knows why this is. No other object in the solar system looks anything like this. The Moon has most of its maria on the side facing Earth, but that's a much less extreme brightness difference.

The dark hemisphere is the one that leads in Iapetus's orbit around Saturn. Other moons of Saturn like Dione and Rhea have different-looking leading and trailing hemispheres, but none this extreme. One theory popular for a while was that the dark area is made of material blasted off of Phoebe. But it turns out to have a slightly different spectrum from Phoebe. It could have come from some of the other small moons.

..My theory is that Frank Gorshin lives there and has eternal fistfights among stock footage of burning buildings.

...I still tend to think that it has something to do with orbital dynamics and collisions. The whole Saturn system shows definite signs of being a giant moon-grinder; there's just so much crap orbiting it near the equatorial plane, so many dynamical oddities like ring shepherds, orbit-swapping moons, moons at the L4 and L5 points of other moons, stuff like that. It stands to reason that lots of moons would have unique phenomena that only happen on the leading or trailing hemisphere, because that's what you'd expect if the geology of these objects were governed by the process of things smashing into other things in front or rear collisions.

(Jupiter has more known moons big enough to be called moons, but most of them are dinky outer moons in random-looking orbits, probably captured bodies.)

Then there's the fact that Iapetus has a more inclined orbit than most of them (it's one of the few satellites that would have an impressive view of the rings, since it's not orbiting exactly in the ring plane). The first thing I thought when I saw that equatorial ridge is that it's actually the rim of a crater covering one entire hemisphere (kind of like the Moon's South Pole-Aitken basin), and that at some point Iapetus got knocked by a polar collision so big that it left that ring and changed Iapetus's orbital inclination. I'm sure the bright/dark dichotomy can be worked into this just-so story somehow. But it's the kind of appealing, pat catastrophic explanation that is probably wrong. It's also hard to resist the idea that the positioning of that ridge precisely on the equator, at the widest point of the dark oval, is more than a coincidence.

(it's one of the few satellites that would have an impressive view of the rings, since it's not orbiting exactly in the ring plane)

...The little outer moons are further from the ring plane, but are also further from Saturn. From Iapetus, Saturn would be about twice as wide as the full Moon seen from Earth, not counting the rings.

This old Astounding cover is extremely well-researched and stands up pretty well even today, though he exaggerated the banding on Saturn a little, and it'd have to be taken through a long lens for Saturn to look that big relative to the little spacemen.

I like the allusion in the foreground landscape to the bright/dark motif, which was well-known ever since Iapetus was discovered by Cassini (the guy, not the spacecraft). Those men in the red spacesuits must be somewhere in the boundary region. In 1939 the shape of the boundary was not well-known, but I just checked, and there are places on it where you could contrive to get pretty much that view of Saturn near the horizon.

The other moon with a good view is Mimas, since it's just slightly out of the ring plane and it's so close. Saturn and the rings would dominate half the sky.

The most stable configuration is to have the thickest great circle of a spinning object be at the equator, isn't it?

Yes, but the energetically preferred shape is a smooth ellipsoid, not a ball with a ridge around it. On the other hand, maybe you could get an equatorial rubble pile from a ring around the body that then falls to the surface.

Right, I was just thinking that if you started with an object with a ridge around it and set it spinning then after millions of years it might settle down in that configuration.

Oh, I see. Well, it probably would, but only if there were some mechanism for dissipating energy, such as a semi-molten interior, or some other orbiting bodies nearby for the ridge to pull on.

...But obviously Iapetus was plastic enough to become tidally locked to Saturn-synchronous rotation. Actually, how Saturn's tides figure into this is an interesting question: I'd say that, starting out in a random orientation with a great-circle ridge, the moon was likely to end up in a configuration where the ridge ran under the sub-Saturn point, but whether there would be a correspondingly large restoring force to make the rest of it settle into the orbital/equatorial plane of Iapetus is another matter. I think maybe not, because the centrifugal effect and the Saturn tide are working in opposite directions.

(The sub-Saturn point is pretty near the position of the weird dark ring that inspired Carl Sagan to mail Arthur C. Clarke, at one end of the dark oval. It's nighttime there now, but I think Cassini is going to try to get pictures of it by Saturn-light.)

Hmm, I think some things I said in the previous paragraph might be wrong. Well, it hardly matters, I'm talking to myself and don't feel like doing the math.

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