Whoa

I would like to put down my wild vote right now that the explanation of the ridge is that it is the edge of some colossal impact feature.

The gory details on the flyby.

makes sense; OTOH, if two such large bodies collided, wouldn't the resultant plasticity due to heat, combined with gravity, end up smoothing out such a feature? Or is this a small enough body that maybe there was insufficient gravity to smooth it out before re-solidification?

I'm thinking of the current popular earth-moon formation theories, for example.

the impact essentially plasticized the earth, and then gravity smoothed out the resulting impact effects.

I keep thinking what a big nutcracker it would take to crack that walnut.

It looks like the large impact crater in the lower right caused some deformation of the ridge.

look that way to you?

oh, btw, I'm referring to this pic:

http://www.planetary.org/image/iapetus_cassini_20051231_mosaic_gray.jpg

hmm.

what if the ridge was formed when the moon was spinning rapidly on its axis (which would explain its equatorial position and fairly symmetric height), and then explain the current rotational rate by a large impact that slowed the rotation (but not large enough to cause plasticity)?

kinda parallel to the current explanation for the rotations of Venus and Uranus?

...would also explain why the moon appears "squashed" longitudinally.

I think the most persuasive hypothesis put forth so far is that it's due to some sort of settling, much as ice buckles. But the settling would have to happen at exactly the right time, while the material was still plastic enough to settle, but not so plastic as to keep relaxing the feature away altogether.

In favor of settling is the detailed, multi-ridge structure typical of compression ridges, and that this thing is *exactly* on the equator.

and that this thing is *exactly* on the equator.

The net gravitational force of a rotating object is less at the equator than at the poles. Settling would therefore start at the poles and continue to the equator, where a compression ridge would develop.

Hey, that looks like a half-buried face!

No wonder Seldona disappeared from Mars. It went straight to Iapetus

Okay, but was intelligent falling involved, or not? :D

It's actually a giant coat-hanger. God tried a pinata moon on a cheap wire frame before falling back on plastic rocks.

The net gravitational force of a rotating object is less at the equator than at the poles. Settling would therefore start at the poles and continue to the equator, where a compression ridge would develop.

(Usual disclaimer about not being a physicist)

The equatorial position seems to me too convenient for an impact formation. Besides the settling hypothesis, is there any chance it could be some kind of tectonic effect, like the oceanic ridge?

Helloooo, Spaceworms are the only real explanation and you know it.

As others have noted, the fact that the ridge occurs on the equator would seem to indicate that the formation of the ridge is at least partially due to centripedal effects.

Personally, I like the idea that the material from whech Iapetus is made behaves a bit like water/ice, actually expanding as it solidifies. The natural release point would be the equator, where centripedal forces make expanding outwards easy.

The nice thing about this explanation is that it obliges the ridge to form just at the moment where Iapetus would be losing it's plasticity - ie the passage from liquid to solid. The downside is that it would require Iapetus to be mostly made from a material that has a property that is rather rare - most materials don't expand on solidifying...

God tried a pinata moon on a cheap wire frame before falling back on plastic rocks.

Now I know whom to blame for Dr. Who.
.

Looks designed to me!!!

All Science So Far!

Clearly, that's the seam where Allah stitched the two halves of the moon together.

Someone call PZ, the moon is an egg developing a neural fold.

Centripedal effects? What do feet have to do with it? Pedals can make things spin quickly. Is that how goddidit?

centripedal effects

The equatorial position seems to me too convenient for an impact formation.

The nice thing about this explanation is that it obliges the ridge to form just at the moment where Iapetus would be losing it’s plasticity - ie the passage from liquid to solid. The downside is that it would require Iapetus to be mostly made from a material that has a property that is rather rare - most materials don’t expand on solidifying…

The ridge seems to be made from heavier plate like pieces, forced into an equatorial ring by centrifugal forces.

The net gravitational force of a rotating object is less at the equator than at the poles. Settling would therefore start at the poles and continue to the equator, where a compression ridge would develop.

Scientists calculate Iapetus originally rotated much faster -- at least five hours, but less than 16 hours per revolution. The fast spin gave the moon an oblate shape that increased the surface area (in the same way the surface area of a round balloon stretches when the balloon is pressed into an oblate shape). By the time the rotation slowed down to a period of 16 hours, the outer shell of the moon had frozen. Furthermore, the surface area of the cold moon was now smaller. The excess surface material was too rigid to go back smoothly into the moon. Instead, it piled up in a chain of mountains at the equator.

Paul Burnett suggests The game’s afoot!

Quite right. Not content with the finished product I hypothesize that the grand old designer just stepped on it resulting in the equatorial ridge. Evidence for this would be in the form of a large foot print at one of the poles. This would have the added benefit of orienting the solar system with respect to the designer. While the Muslims orient themselves toward Mecca for prayers, Christians would now be able to orient themselves in the relevant direction toward the designer.

Delta Pi Gamma (Scientia et Fermentum)