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.

[hendric]

I still don't see how the two divergent ridges get created by a decaying ring. The orbital speed of the ring at low altitude would not be the same as the rotational velocity of a proto-Iapetus. So, how do the angled ridges get created?? If the ring was at an angle to Iapetus, as it descended it would not stay above a single location.

I favor a tectonic explaination. We have symmetrical ridges here on Earth due to seafloor spreading. Seems like a reasonable explaination to me. The question would be why would there be a single crack along a great circle? Maybe Iapetus had a Europa-like episode with a shallow ocean, and as it slowly froze it expanded/contracted enough to crack open. Without nearby moons and tides, the crack went around the planet symmetrically.

[tasp]

I still don't see how the two divergent ridges get created by a decaying ring. The orbital speed of the ring at low altitude would not be the same as the rotational velocity of a proto-Iapetus. So, how do the angled ridges get created?? If the ring was at an angle to Iapetus, as it descended it would not stay above a single location.

The single location is the highest spot along the Iapetan equator. That is what synchronizes the emplacement of the diverging features.

For every rotation of Iapetus, the highest spot along the Iapetan equator penetrates the ring plane twice. Once as it crosses from north to south, and again 180 degrees later when the high spot penetrates the ring plane crossing from south to north.

The lowest edge of the ring descends (via the 'bump' or dynamical ring spreading process) during the interval the high spot is not in the ring plane. As the highest spot passes through the ring plane, the chunks comprising the low edge of the ring impact the high spot, making it higher. The 'spray' from the impact interacts with materials still in orbit immediately above the high spot, decelerating them, and causing them to contact the surface downrange of the highspot, along the ground track of the ring at that time. This also explains why all three ridges taper downward from the high end.

That's why the 2 diverging attendent ridges are identical (less subsequent cratering damage), and are 'keyed' to the highest spot along the equator.


Note: inclined features in the ring or a small shift in the spin axis of Iapetus during emplacement will produce the same result, 2 diverging attendent ridges. I am not sure how to distinguish between shifting the axis of Iapetus a few degrees and inclined features in the ring. I think the 2 causes will produce identical features.

I cannot imagine an internal process that can create perfectly matched attendant ridges to the main ridge structure as elegantly as the emplacing ring with either an inclined element, or a small shift of the Iapetan spin axis during the emplacement.



I also note, it appears to my inexperience eye that the high end of the ridge structure is antipodal to the sub-Saturn point of Iapetus. Is the ridge structure massive enough to be the deciding factor in aligning Iapetus to Saturn? (as mass distribution in earth's moon does to earth?)



Does anyone with more computer graphics savy than me (that would be everyone else) want to take a stab at modeling this process for all to see?