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

[Ian R]

Here's my contribution to the Iapetus party:



*Yawn* I'm off to bed then!

[TritonAntares]

Only six new images have been transmitted so far - probably prior to a large RADAR data downlink.
I think these images are connected with those CIRS observation mentioned above - but actually I'm not sure and I can't appreciate the numbers of NACs and WACs we see there now...

EDIT: Info from Tilmann - 2 are the only ISS-shots of ISS_049IA_FP1NITMAP001_CIRS prior to a large RADAR SAR data set from the dark side.

[volcanopele]

ICYEXO003_UVIS
=========
http://saturn.jpl.nasa.gov/multimedia/imag...iImageID=126232
(I presume the bright spot near the center of the image is Nunki or Sigma Sagittari)

ICYMAP003_UVIS
=========
http://saturn.jpl.nasa.gov/multimedia/imag...3/N00091840.jpg
http://saturn.jpl.nasa.gov/multimedia/imag...3/N00091841.jpg
http://saturn.jpl.nasa.gov/multimedia/imag...3/N00091842.jpg

FP1NITMAP001_CIRS
=========
http://saturn.jpl.nasa.gov/multimedia/imag...3/N00091839.jpg
http://saturn.jpl.nasa.gov/multimedia/imag...iImageID=126231

So we got a little bit more than we expected.

[antipode]

This surface makes most of Earth's moon look young....



P

[nprev]

Yeah... ...doesn't look like it ever had anything in the way of a major resurfacing event like the lunar maria, ever, at least the terrain we've seen. Think that exogenic origin of the splotch is looking more likely now...

Anyone else find these close-ups oddly evocative of Phoebe?

[CosmicRocker]

Good catch, antipode. I was so caught up in the wonder of the encounter that I somehow managed to miss that.

[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

[volcanopele]

dvandorn, I'll have to look at these images again to see what you are talking about. Any chance you can draw up something in Paint or Photoshop to illustrate what you are talking about?

To me, these images tell the tale of a very ancient world. The surface is very heavily cratered. We do do see some relatively young craters (don't ask me for an age estimate) with flat floors, central peaks, and mild to major slumping along crater walls. We also see some very ancient craters that have been eroded by later impacts, which have ruggidized the crater floors and space weathering, which has smoothed out many of the earlier hummocks produced from slumping.

From the images that have been returned so far, expect the highest resolution images to show a relatively smooth surface with impact craters down to the limit of resolution, though it maybe interesting to see if the dark material deposition has buried some of the smallest impact craters. This would be evidenced of course by a lower limit to impact crater size. Could be interesting to see. I don't expect the kind of bouldery surface we saw at Enceladus.

[mchan]

The distribution (size and number) of smaller craters (if that's what they are) in the inter-crater "plains" appear odd. There appears to be some number of craters of one narrow size range, with very few or none that are, say, twice as large and larger, or half as large and smaller. At first glance, they appeared to be a smattering of huge boulders.

[edstrick]

<beats his head on the keyboard in frustration>

When, did somebody say, is the next batch of pictures likely to be posted?

[TritonAntares]

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.
...

You're talking about a crater like this:

There seems to be some kind of debris inside, probably a landslide like in the major bassin found during the newyear encounter 2004/5.
Btw. there should have been a saturnshine image of it - did anyone see it?
Maybe it was among the completly dark pics Emily wrote about...

[volcanopele]

There are Saturn-shine images. Some of them look great (over bright terrain), some of them, not so much (over dark terrain). They seems to show up as quite dark on the JPL raw images page.

Not sure when the images from the current downlink will be on the raw images page. They are coming down, and so far look good.

[edstrick]

Ooooh. <tries not to drool on his keyboard>

[ugordan]

Is it me or does Iapetus' surface look different than other icy moons at this scale? The surface is saturated with big craters and then peppered by very small ones at certain locations. It almost looks like it's missing "intermediate" crater sizes. Another thing in that picture is the bright terrain near the horizon, is that the transition region?

Fascinating and very crisp imagery, considering the difficulties with exposure for dark material at high phase angle. So far only UV3 frames were underwhelming, but that might change as well as phase angle rapidly decreases after C/A.

Can't wait to see the Voyager mountains pop up.

EDIT: Regarding bright terrain, nevermind. Just looked up Tilmann's page and yes the limb shot catches a bit of northern transition zone.

[brellis]

It's definitely had some "ring events". In many places the surface looks like it slammed into a ring or stream of substantial objects at substantial velocity.