[X] My Assistant

[Decepticon]

Here's what extrapolating the current SPICE kernels yields:

IAPETUS 2005 NOV 12 14:34:49 185078153 415,451 km arcmin=12.1
IAPETUS 2006 JAN 25 21:13:08 191495653 873,345 km arcmin=5.7

IAPETUS 2006 APR 11 17:16:28 198047853 602,419 km arcmin=8.3
IAPETUS 2007 SEP 10 12:31:28 242699553 1,259 km arcmin=2186.8

Posted by jmknapp

This time a little further out.

Jan. 2006 - 879,000 km - 5.3 km/pixel - eastern Cassini Region and transition zone, part ofthe "moat" posted by VP


If anyone has made a animation of the encounter please post it!

[Decepticon]

Very Nice!

[Michael Capobian...]

Yes, this map shows the dotted line north of the moat much better than the official one. I'd be interested in seeing an exogenous explanation for it.

Michael

Very Nice!

[JRehling]

Yes, this map shows the dotted line north of the moat much better than the official one. I'd be interested in seeing an exogenous explanation for it.

Michael

I agree. In fact, I'd be interested in seeing a purely endogenous explanation, too!

(I'll interject here, high in my message: continued great work by Steve Albers!)

A purely endogenous explanation (eg, the ridge is a Europa/Enceladus style rift that sprayed the dark stuff out over hundreds of km, painting the surface) is going to have a very hard time explaining the patchy irregularities in eastern CR. One possibility (but a reach) is that the terrain is so extremely rugged that at the furthest reaches of the spray, when the material is coming in almost tangential to the geoid, that pronounced and bizarre topography could allow almost arbitrary complexity in the boundary. Really, any "geyser"ish explanation demands that. And sure enough, we know that Iapetus has, in general, appreciably nonspheroidal topography. But we won't be able to characterize the topography of eastern CR within the primary mission -- that would have to remain an ad hoc explanation. Even that seems to strain credulity at the Snowman craters, where the topography can only be so irregular (is a crater still vaguely bowl-shaped or not)? We can similarly (almost!) reject the exogenous explanations that have Iapetus having passed (in one event or many) through a cloud of material. The bizarre boundary of eastern CR is implausible in that case.

An endogenous explanation holding that the dark stuff flooded upwards can already be rejected. The northern boundary clearly shows that the dark stuff was emplaced by ballistic delivery from the south.

I think we're going to be left to conclude that the material sprayed outward from the surface of Iapetus, within what is now CR, in a fashion much more energetic and irregular than geysers could plausibly explain, and that some sort of impact event led to the staining of Iapetus.

The remaining flybys are going to show us western CR at very high resolution, and the southern boundary at decent resolution. We will not see the eastern/Snowman region in very high resolution (and sunlight rather than saturnshine) at all during the primary mission -- excepting decent coverage as Cassini departs the Sept. 2007 flyby and eventually the Snowman will rotate in.

For the sake of discussing whether or not Iaptus merits more flybys in the extended mission, I think not. The remaining flybys are going to show so much of the hardly-seen boundaries of CR at higher resolution that I can't see dedicating another flyby to finish the almost-complete map. ALTHOUGH, the one portion of the border that will not be mapped may be the single most intriguing! The big Snowman crater will be seen at about 1.3 million km distance in Sept 2007. A flyby to see that same region at higher resolution seems like a big cost for a small gain. In the absence of other reasons to have apoapsis be so far out, I would expect the extended mission to pull the apoapsis way in, so Titan encounters happen much more frequently, and the RADAR mapping can add a swath or two per month instead of a swath every few months.

[jmknapp]

The remaining flybys are going to show so much of the hardly-seen boundaries of CR at higher resolution...

The interior of CR also contains much information as to the direction of the emplacement, where the crater walls facing away from the equator have been shielded from any deposits. This color image is from Tilmann Denk's 1/25/05 CHARM presentation:

Viewgraph 38:


Viewgraph 39:

Pete: Tillman, this is Pete Goldie from San Francisco.  I have several questions.  The bright edge of the crater walls relative to the distance from the albedo change, is that oriented from the equator?  Did you have enough images to see whether there was any relation between the bright walls and the equator or just the bright walls and the edge of the albedo change?

Tillman: These are facing away from the equator, that is right.  So, if you are referring to viewgraph number 38, I think--this is the first one with color image--then the equator is somewhere to the left of the image.  Just outside the lower left edge of the image and it's running approximately parallel to the boundary between the dark and the bright terrain.  So this means that the bright crater walls are indeed facing towards the northern hemisphere – the North Pole – and away from the equator.  Is this what you wanted to know?

Pete: Yes.  Is that true south of the equator?  Were you able to tell that?

Tillman: The New Year’s Eve flyby did not observe the area south of the equator, but from low resolution images that are a little bit further to the south where you can really see the craters that are in the dark terrain, also it's possible near the edge.  There it's also very obvious in the same way.  Let's say the bright faces, yes, these are obvious somewhat.  What's even more obvious is that the dark crater walls facing within the dark terrain; this was also very obvious.

How can the material NOT have come from the equatorial ridge, given this pattern within the CR craters?