New Iapetian image series Options

[scalbers]

Or would the grainier (snowy) material be in Roncevaux Terra with more small shadowed areas between grains that help contribute to the opposition effect? I would surmise then that CR is less granular, perhaps more tarry or something. The actual size of the grains may be less important in deciding whether there's an opposition effect.

[ugordan]

It's quite possible, I was just trying to spark some discussion. Zero phase behavior difference seen here could be pretty interesting. I'm not acquainted with photometry and backscattering properties of different materials. My comment was based on the fact powdery surfaces (very beaten up regolith) seem to show the strongest opposition effect. It could just as well be dependant on the specific materials. The shadowing effect is only a part of the equation, coherent backscatter (if that's what it's called) is I think more important at zero phase. These shots seem to suggest CR shows very little of that effect.

[scalbers]

Gordan - these are interesting animations that you put together, and yes there are several factors that can contribute to the opposition effect. I think there was a post (by edstrick) mentioning the morphology of some features in Roncevaux Terra suggesting grainy snow, so the opposition effect may lend more support for this. He also suggested the dark terrain was granular though.

I wonder whether there is more of an opposition effect on the bright side of Iapetus compared with other icy satellites? There is a general discussion by Sanna Kaasalainen in this thesis, particularly in section 1.4.3 where Anne Verbischer is mentioned:

https://oa.doria.fi/bitstream/handle/10024/....pdf?sequence=1

And this paper might be interesting to read if I had a subscription:

http://pubs.nrc-cnrc.gc.ca/cgi-bin/rp/rp2_...19_81_ns_nf_cjp

Beyond that, this gives me something to look for when walking around during the upcoming winter days.

[ugordan]

Interesting PDF from a glance, thanks for the link. Regarding other icy moons, as far as I can see this is by far the lowest phase observation of any moon yet by Cassini (around 0.05 degrees minimum at disc center, comparable to the apparent angular diameter of the moon, 1.5'). I know there were some other low phase moon observations - Rhea comes to mind, but it was nowhere near this low a phase. Cassini did observe the rings at zero phase quite often, though. Earth-based observations such as the recent one are more favorable for catching several moons at the same time, but Cassini has the benefit of actually resolving the moons into discs.

Personally, I don't think Roncevaux Terra can come anywhere near Enceladus when it comes to the opposition surge. It's fairly bright albedo ice (comparable to Europa IIRC), but you can't beat the shiniest object in this stellar neighborhood. It might be interesting to compare it to Rhea on the other hand - similar albedos, but Rhea appears to be slightly dirtier.

[ngunn]

Interesting. I would expect ice deposited as frost in a near vacuum to be very pure (hence bright), and in very small particles (hence high opposition surge) wherever it happened to be located. If Roncevaux is both less bright and shows less opposition surge than Enceladus I'd be inclined to attribute the differences to radiation damage rather than 'dirt' as such. That would be consistent with a very much lower deposition rate on Iapetus than on Enceladus - not unreasonable.

[CAP-Team]

Below images from 17, 19 and 23 september together with simulated views (using Steve Albers' maps)

[scalbers]

Nice views that you put together CAP-Team. One thing I'm working on is to try and soften the image edge on the map near the terminator in the vicinity of the central meridian in these views. One minor suggestion on the presentation would be to consider rotating the Celestia views (with certain keyboard controls) to match the orientation more closely.

It's interesting to backtrack to what we were looking at with the July 2007 imagery (Harkeppler in post #138) from a similar vantage point. For example the dark escarpment on the eastern rim of the western Claude basin was visible in those images.