[X] My Assistant

[Rob Pinnegar]

There's been a fair amount of speculation that Iapetus' despinning, from its initial rapidly-rotating configuration to its current tidally-locked one, might have taken a long time, and that Iapetus might be the only major Saturnian moon that froze *before* it was despun. This has been presented as a possible explanation for Iapetus' strange shape.

Well, there are at least two huge basins on Iapetus that, while rugged, have relatively flat floors: the big one in Cassini Regio that faces Saturn, and the one in the southern part of Roncevaux Terra. Has anyone checked to see whether the "best fits" to the shapes of these basin floors match the curvatures of surrounding parts of the moon? If either of those basins formed before Iapetus was completely despun, their shapes might match some ancient Iapetean geoid, not the current one (some allowance for "slump" would be necessary here).

Given the bashed-up state of the basins, Iapetus' crazy shape, and our very incomplete knowledge of its gravitational field, I don't know whether this would even be possible. It might be worth a look, though.

[tasp]

Need to add a clarification.

Debris blasted from the Iapetan surface by an impact that is destined to recontact the surface in ~ an hour will be traveling at less than 1000 mph.

I am not sure the size range the debris chunks will exhibit, and I am not sure of their mechanical strength.

When the debris contacts the surface, will it crater? Or will it just skid to a stop?

For cryogenically frozen blocks of mostly water ice, these pieces could be pretty sturdy. Seems I remember seeing Mr. Wizard hammering a nail with a banana dipped in LN2. These pieces, contacting a disconsolidated surface of icy grains might simply skid to a stop. Would 'skid marks' be evident after so many eons? Would the 'skids' point back at the primary crater (with skewing for rotation) or would local slope conditions govern skid direction? Is the surface of Iapetus going to be covered with ~meter sized chunks like Mars? How do we pick out the ones we are interested in?

Might mess up what I am trying to discern here . . .