On a ring origin of the equatorial ridge of Iapetus Options

[Guest_AlexBlackwell_*]

Wing Ip just had an interesting Iapetus-related paper published in GRL.

[dvandorn]

Does Iapetus' ridge have to have been created by a ring orbiting Iapetus? Could Iapetus have traversed a ring strand (or series of ring strands) ejected from Saturn's early ring system while the moons settled into their various stable resonances?

-the other Doug

[tasp]

Maintaining focus (or collimation) of the strand through its travels through the Saturnian system would be a very difficult feat to achieve. Any plausible force acting to accelerate a strand or filament would disperse the materials.

Additionally, trifurcating the filament symetrically, and having it by chance emplace it self aligned to the Iapetan equator multiplies tiny probabilities.

The amazing collimation of a ring around Iapetus is assured by the tendency of chunky materials in randomly inclined orbits about Iapetus (lofted most likely by an oblique impactor) to collapse to the Laplacian plane in fairly short time spans. Deposition of the ring materials can result from the natural process of momentum transfer across the ring system once it has collapsed to the equatorial plane. (a better description of these two processes is in the Planetary Rings chapter in the excellent book The New Solar System.

Further enhancements to sharp, well defined rings around Iapetus is its' remoteness to other perturbing bodies. Distant Titan and Saturn would produce relatively small tidal affects across the diameter of an Iapetan ring system.

Orbital periods for materials just prior to contact with the Iapetan surface would be slightly short of 3 hours. I am not sure of the orbital period at the altitude of the Iapetan Roche limit.

I figured a while back that if the ring system depostied itself at 1 cubic meter per second, you could get a ridge system similar to what's seen (45 degree slopes, I forget the length and sloping heights I used) in around 350 years. I suspect the process was slower than that, but it gives some numbers to play with.


In the past, Iapetus would also have been the last significant moon of Saturn to achieve tidal lock with Saturn. It is possible the ring system emplaced onto an Iapetus rotating considerably faster than the once every ~80 days currently seen. Any increase in rotation rate for Iapetus slows the touchdown speed (<1500kph) for the emplacing materials.



We should be looking for a largish elongated crater on Iapetus, too. It being the possible oblique impactor crater that lofted the material that formed the rings.

There is a largish oval crater (axis about 45 degrees to the equator) with an interesting elongated central peak complex in the southern hemisphere on the eastern edge of Cassini Regio. Might be a good place to start calculating volumes . . .

[David]

I think I understand the idea to be one of a low-velocity deposition of debris onto Iapetus' surface. But even if the impact of a ring or ring fragment deposited a line of material, why wouldn't it slump to the level of the surrounding "plain", either from Iapetus' own gravity, or from the repeated impacts which have visibly struck the region? We have some pretty good side views of the ridge, and it seems to me that it's both hard and well bonded to the surface of Iapetus; impact craters that overlap it, instead of simply splatting a soft material down to the level of the original surface, appear to me to be tilted along the slope. Is this really possible for a feature created by a rain of debris measuring in metres rather than kilometres?