[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.

[tasp]

IIRC, that crater (which in other posts both here and at other sites I have referred to as 'landslide crater') is just about on the equator.

I would note that the protion of the crater wall on the equator itself, seems to show no 'weirdness' or anything else suggestive of an uncovered geological structure related to the equatorial ridge structure further west.

Additionally, it would seem that what ever object produced that crater was much smaller than the resulting excavated area. During the crater forming explosion, the outward bowl of the crater would have been deformed where it encounted the existing basin wall. Apparently, the energy of the impact was insufficient to 'erode' into the basin wall much.

But it did weaken it. Subsequently, the basin wall failed and a debris field extends across the crater floor now.

The basin originally would have been light colored, and the CR darkening effect changed it to match the albedo of the rest of CR.

Then the smooth bottom crater formed, light colored upon formation and then it too darkened. Then the landslide swept across the crater floor, and the darkening effect again occured.

I think the darkening is an ongoing process, and it is also an ancient process.

Would the radar instrument or any of the narrow spectral filters on Cassini help discern even minute differences in the CR coating in these adjacent areas?

[Rob Pinnegar]

But it did weaken it.  Subsequently, the basin wall failed and a debris field extends across the crater floor now.

Good points here, Tasp. Here's another idea to toss into the ring.

Suppose that I'm wrong about the debris field being due to reflection of the crater-formation shock wave off the basin wall, and that it really was primarily due to a landslide, as you (and others) have suggested.

If that landslide occurred immediately after the formation of the "landslide crater" -- I mean within the first minute or two after the impact -- wouldn't the areal extent of the landslide have been affected by relaxation of the crater floor? Immediately after the impact, the crater floor would probably have had more of a classic "bowl" shape than it currently has, after which it would have relaxed into its modern flat-bottomed shape.

If the landslide occurred during the early bowl-shaped phase, the landslide material might have been able to slide quite a long ways downslope, because the crater floor would've been steeper then than it is now. Once the crater floor relaxed into its current "flat" shape, the landslide material would thus appear to have moved further horizontally than would normally seem possible.

Just an idea. [Edit: I have no idea whether it is dynamically possible.]

[Bob Shaw]

In Terrestrial volcanoes, pyroclastic flows - the infamous 'nue ardentes' which destroyed St Pierre - can travel large distances and, because they are so massive, will readily climb hills and sweep over all sorts of obstacles. We see rampart craters on Mars which may be the result of a mud-like fluid, and I see no reason why, post-impact on an ice world, fluidised icy debris could not travel a very long way, especially in a low-gravity environment. So there's likely to be no need for any special dynamic case.

You might get them on Titan, too - turbidity flows under the Earth's oceans carve channels, so why not on Titan?

And here's the test: is the landslide just a heap of rock, or are there flow structures, lobate fronts etc visible?

Nice 'eye-witness' (after the event, the actual eye-witnesses were toast!) article on St Pierre here:

http://dominicapsn.freeyellow.com/MtPeleDominica.html

Do we call ice world pyroclastic flows cryoclastic flows?

Bob Shaw