what we're seeing on Iapetus, which may in turn be helpful in understanding some of the equatorial anomalies of Mars...

Evil's surmise:

My best uneducated guess: A massive ring of debris containing many micro-sized and pebble sized particals that collapsed into a flat ring about the world's equator where they eventually rained down on the surface, solidifying into a single global encirling geologic feature. It would have had to have happened over a fairly long period of time, but still have been fairly recently in geologic times (like the last 50 million years or so)?

Now: Why don't we see similiar features on other bodies in the Solar system?

Anybody else have an explanation?

Scott Evil

That there might have been a ring around such a dainty satellite of Saturn is interesting in itself. That there might not have been a ring around Mars is as interesting.

Recall the standard model of the Moon's formation: ancient impact of the Earth with a Mars sized body, formation of a ring around the Earth from the debris, coalescence of Moon, collapse of any remaining ring material onto Earth and Moon. Little/no evidence left behind.

But on Mars, we see indications of numerous low-angle impacts that trace great circles, suggesting locations of previous equators. Now we see a proposal that the giant basins of Mars are arranged along a great circle as well, indicating yet another fossil equatorial region. These impacts, if that's what they are, indicate the presence of numerous asteroidal bodies (or a singular body that broke up) in equatorial orbit over much of the history of Mars. Two of them are still -- and Phobos will impact the surface eventually. It might break apart before it does so. Would a ring briefly form from the dust of break up... or...

Back to Iapetus: the mysterious dark hemisphere may be directly related to the equatorial ridge, although it appears that the dark material overlays the ridge. If there were a small temporary satellite of Iapetus, something like 253 Mathilde which suddenly disintegrated in the path of Iapetus, it's possible there would have been a cloud of dusty debris -- which would hit the leading hemisphere forming the dark splotch -- and a ring could have formed at the same time. That ring then would spiral to the surface eventually disappearing except for the ridge.

Yet there are signs the ridge was not deposited on top of the surface but was generated from the interior. And then there are all the "landslides" and basins on the surface of Iapetus.

Hmmm....