Rev 49 - Aug 9-Sep 14, 2007 - Iapetus I1, The only close flyby of Iapetus Options

[elakdawalla]

[Moderator's note: This thread contains images from the Iapetus 2007-09-10 flyby and discussion of them. It was created by splitting this thread which contains pre-flyby discussion]


Another CL-UV3-GRN-IR1 set was posted today, though Iapetus seems to have been hiding from the green and IR filters...here's my best effort at making something from the CL and UV3 images. Lots of topography on the limb!

--Emily

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[dvandorn]

I don't know about anyone else, but in some of these images I'm seeing very definite evidence of material flow, in a direction that, if the shadows aren't playing tricks on me, seems parallel to Iapetus' orbital motion.

In other words, it looks a lot to me like most of the larger craters have debris on opposite sides, filling in both inner walls to some degree but piled up a bit more on the far sides (in relation to their orientation towards the direction of orbital motion). The debris patterns seem to consistently apply themselves 180 degrees apart on the larger crater rims (i.e., the thickest or most obvious sections of the debris accumulations lie approximately 180 degrees apart along the rims), and the vector through these points in the crater walls seem to align with direction of orbital motion.

I'm getting my concept of the direction of orbital motion from the shadow angles and from my (admittedly extremely rough) concept of where these regions lie in relation to the equatorial ridge. In any event, to make it a little simpler, it looks like the vector defined by the 180-degree-seperated rim distortions points back toward the center of Cassini Regio, i.e., towards the center of the region Iapetus that faces into its direction of orbital motion.

It will be more instructive, and prove more, when I can see exactly where these images are located and where those vectors actually point. After all, you *could* (and, on many Solar System bodies, do) see the same kind of phenomenah emanating from basins. But while some of this looks like ejecta, more of it looks like, well -- duning. To me.

This debris accumulation looks less obvious in the inter-crater "plains" (as rare as they seem to be) than on the rims and floors of the larger craters. But I really do think I see very specific patterns of debris accumulation here. It would seem to support the belief that material is, or has been at some point in the past, very slowly "flowing" from the midpoint of Cassini Regio out in vectors away from the leading face's central point. What's harder to tell is whether the "flow" is more radial to the center of the leading face, or more parallel to the equatorial ridge. It may well be a combination of the two...

I'd have to say it looks like the surface is very ancient, but that that the debris accumulations (which you don't see in such abundance on the other icy moons), while relatively thin in most places, seem to definitely overlay almost all of the craters to the same extent. In other words, this material is younger than the underlying ancient surface, but thin enough that it doesn't resurface the ancient terrain, it just piles dunes and sprays of debris on top of it. It's not enough material to resurface, but it does seem to be enough material to "paint" the cratered terrain a very dark brown.

-the other Doug