[X] My Assistant

[Bjorn Jonsson]

I'm now getting very close to finishing a near-global DEM of Rhea using stereo images. This has been a true 'monster project' involving 71 image pairs, a significant amount of software improvements and some impatience .

This has been an extremely interesting project but the results are fairly interesting. I have 'discovered' a highly degraded impact basin similar in size to Tirawa - it's not obvious in the source images. There are some interesting features near Tirawa's antipode that I think are real but I'm not completely convinced - if real they may or may not be associated with Tirawa's formation. I will post more details plus some crater depth measurements once I finish the DEM sometime in April.

There are a few frustrating gaps in the DEM, the biggest one in the north polar region for obvious reasons and another one near longitude 0. The latter may get filled following a nontargeted flyby in June 2010 (assuming Rhea gets imaged the way I want ).

I will probably be posting a flyover movie before mid-April and possibly a rotation movie as well. Meanwhile I'm posting two test renders:





The first one is from 3100 km from Rhea's center and has a FOV of 30°. The other one is from an altitude of 340 km and has a 45° FOV. I didn't drape a texture map over the DEM. The DEM is global so this wouldn't look realistic. The first one looks fairly realistic even though it is very obvious that it is computer generated. The second one reveals that the resolution of the DEM is several times lower than the resolution of the source images - they typically have a resolution of ~1 km/pixel.

And as previously noted: There's more to come in the not too distant future. And an interesting decision: Which satellite to do next.

[Bjorn Jonsson]

Thanks - I had forgotten the second basin was seen by Voyager but needless to say it is conspicuous in many of the Cassini images.

However, the basin I mentioned in the first message of the thread I had 'discovered' is a different one. It is the big one at the center of this image:



I'm sure this feature is real and not a processing artifact. The maximum depth of the basin is ~2 km, typically ~1.5 km and parts of it are less than 1 km deep. In contrast, Tirawa's maximum depth is ~5 km (this is a somewhat preliminary number), typically ~3 - 4.5 km and in addition it has a ~10 km high central peak relative to the crater floor. I don't know exactly how accurate these numbers are but they should give a fairly accurate view of the altitude of specific features relative to neighboring features.

This big basin isn't exactly conspicuous in the DEM's source images. This is its approximate boundary (the exact boundary is somewhat indistinct):



Finally I should mention that the big gap in the DEM I mentioned previously is visible in the first image. It should be possible to fill this gap following a June 18, 2010 nontargeted flyby. Actually this terrain has been fairly well imaged - the problem is simply lack of stereo coverage (or at least I couldn't find anything useful there for stereo).