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.

[tasp]

An object skimming the surface of Iapetus will have a velocity of ~1500 km/hr. (under 1000 mph)

For cryogenic water ice, (~-300 F) I suspect vaporization upon impact would be nil. There may be a 'blast of particles though.



Consider a ring around Iapetus (glossing over how it got there, for the moment).

What happens to the lowest orbiting particle?

It strikes the highest point along the Iapetan equator.

What happens to the second lowest orbiting particle?

It hits the first one.


You originally start to form a pile. At some point in time, you literally run out of room in the pile area (insufficient kinetic energy of the oncoming particles) and the pile starts to accrete only into the direction of the oncoming particles. (analogy-you plow snow from all over your driveway into one pile, eventually, your pickup is not powerful enough to cram the pile any higher, and if you keep shoving snow into the pile, always from the same direction, you will start to form an elongated pile, and the pile will lengthen into the path of the oncoming snowplow)

The orbitally decaying ring system will form a ridge. Ridge 'grows' into the oncoming stream of material. Process concludes when ring material is depleted.

Iapetus had enough ring material to make a ridge 90 to 120 degrees long.


An atmosphere is not necessary for the process to occur. Differential ring spreading (transference of angular momentum radially across a ring system is well understood. Check out the Planetary Rings chapter in The New Solar System. That chapter also outlines clearly how a cloud of debris in virtually any orbital inclination will eventually settle into an equatorial ring system.


That the ridge is observed to be non-continuous may be due to the ridge suffering subsequent cratering damage, or perhaps settlement due to crustal overloading.


The best Cassini images also show perfectly (less subsequent crating damage) symetrical diverging 'attendent' ridges (the off ramps). I cannot imagine any internal geological process that could make such elegant matched attendents. They are the 'smoking gun' for declaring an external process made the ridge.

(the attendants can be explained by a percentage of the ring system being inclined to the equator. Twice each orbit for the inclined material, it will cross the equator. Once, from north to south, and once from south to north. The high spot on the ridge 'synchronizes' the simultaneous equal deposition of material into 2 matched divering stuctures.)


(I suspect at the contact point with the high spot on the surface for the inclined material, a small debris cloud forms, and material orbiting above that spot at the instant of the lower materials contact will be subjected to a drag force and accumulate down range.)

-or-

As the material deposits into the ridge structure, a point is reached where the spin axis of Iapetus shifts a few degrees (like unbalancing a gyro with a small weight) and Iapetus starts preceesing underneath the still descending ring system, the high spot still penetrates the ring plane twice per rotation, and the symmetrical attendant ramp form as before.

-or-

A large (unrelated) impact occurs somewhere on Iapetus and it knocks the spin access off a few degrees. Effect is still the same as above.



Why don't we see ridges any where else?

Iapetus is the most remote from it's primary, satellite we have seen so far. Tidal effects (causing the ring system to not form in the first place on other moons) are uniquely low at Iapetus (also one of the reasons we will not see a ridge system on either Pluto or Charon).

Also, Iapetus plods along at 5000 kph (or is it mph?, doesn't matter) in it's orbit around Saturn. Iapetus probably accreted very slowly (compared to the other satellites) and perhaps had a solid crust far earlier in its' history than the other moons, the impact heat being reduced by the lower incoming velocities and longer periods to radiate the heat away. Also, Al 27 heating in the accretable materials of Iapetus had longer to radiate away.

Iapetus is the closest we get to a 'cold formed' moon considering its' size.


We may see some equatorial ridge structures on the larger KBO's, if there are enough of them that are spherical (think about it), remote enough, and had an originating oblique impact to orbit a ring forming debris cloud.

This post has been edited by tasp: Aug 30 2006, 05:15 AM