[X] My Assistant

[Rob Pinnegar]

While looking over some pictures of Iapetus from the New Year's 2005 flyby, I came across this:



I'm not sure that we've been paying this image enough attention. For one thing, this big crater, on the edge of the big Saturn-facing basin, has a nice bright white ice band that extends most of the way around its edge. This has got to have some implications both for emplacement of Cassini Regio, and perhaps the thickness of the dark material as well. Also, this crater seems to have virtually no impacts inside it, which points to a recent origin -- and that in turn can place some age limits on Cassini Regio, which has to be even younger.

There's also that strange "flow feature" on the crater's floor. The hypothesis that was originally put forth to explain this was that part of the basin wall had caved in and caused a big landslide. The thing that bothers me about this idea is that it doesn't explain the shape of the crater's outline, which looks as if it really should extend a bit farther out beyond the basin wall. Shouldn't a landslide have caused the crater's outline to be deformed outwards on that side, not inwards?

Also, I've never been comfortable with the idea of a landslide extending over that much distance. Iapetus' low gravity has been cited as a reason for this, but that also would limit the amount of gravitational potential energy available to get the slide going. We probably need a bigger energy source.

So I wonder if this flow feature wasn't caused by a landslide at all, but instead was formed at the same time as the crater, as the shock wave caused by the impact reflected off of the basin wall? I don't know if this could explain the sharp boundary of the flow region, but it's probably worth considering, at least.

[dvandorn]

It looks *very* much to me that the large, circular basin feature is *not* an impact feature. It appears to be a collapse feature.

Consider -- the basin appears to be at a constant (though different) altitude both inside and outside the rim, there isn't much in the way of a raised rim or concave floor to this basin at all. The rim itself is a series of very steep cliffs, with evidence of landslides at its base. Some features seem to be cleanly cleaved apart by the rim.

It's possible that this is an impact feature, and the basin rim massifs have "relaxed" back into the ice crust -- but the terrain immediately outside the rim cliffs doesn't appear to be any more or less degraded than the terrain immediately inside of them, and some features seem transected by the rim but otherwise intact. For example, I cannot imagine that landslide crater could have predated an impact basin and still be recognizable as a crater. And yet, it appears likely that landslide crater *does* predate the basin.

If this is an impact feature, I'd have to think it's not a basin in the sense of basins on terrestrial planets. Perhaps a smaller impact triggered a set of concentric cracks around its primary crater, which allowed a general collapse of an area defined by the natural resonance of the impact? Because that basin rim looks a lot more like a radial graben than it looks like a basin wall.

-the other Doug