[X] My Assistant

[Phil Stooke]

Here's one of the latest Rhea images showing the prominent ray crater.

Phil

[alan]

Some subtle horizontal streaks at 4 o'clock in this image of Rhea
http://saturn.jpl.nasa.gov/multimedia/imag...2/N00036898.jpg

[Gsnorgathon]

I shamefully confess that I haven't given the matter much thought, but I still can't get over the fact that the ray crater doesn't look fresh to me. Am I missing something?

[DEChengst]

Two simple mosaics made with the images from yesterday's fly-by:

http://paranoid.dechengst.nl/saturn/Rhea4.jpg
http://paranoid.dechengst.nl/saturn/Rhea5.jpg

[Bob Shaw]

I shamefully confess that I haven't given the matter much thought, but I still can't get over the fact that the ray crater doesn't look fresh to me. Am I missing something?

Phil:

The surface certainly looks strange.

There's a lot of craters with central peaks, and a lot of distinctly non-circular craters, too - the old 'Lunar grid' afficionados would have a field day!

The population distribution seems a little odd. It looks to me like there's an underlying rolling and eroded landscape peppered with bigger craters, as if a well-gardened moon had suffered a series of periods of catastrophic impacts.

The 'bright' impact crater isn't as bright as I'd have expected - the ejecta blanket is still very visible, but the relationship between ejecta blanket visiblity and rays seems somehow skewed as compared to our own Moon.

Perhaps 'fresh' ejecta is subject to faster UV damage, sublimation, and/or micrometeoroid damage, and that's what we're seeing - a different sort of weathering...

Of course, it could all be down to the filters on the camera!

Just my 10 cents!

Bob Shaw

[Bob Shaw]

Two simple mosaics made with the images from yesterday's fly-by:

http://paranoid.dechengst.nl/saturn/Rhea4.jpg
http://paranoid.dechengst.nl/saturn/Rhea5.jpg

Nice mosaics - and, looking at the limb, I see relatively slight surface relief, which supports my perception of a moon with a well-stirred landscape...

[Decepticon]

Is this a second basin? Or is this a cluster of craters?

I must be seeing things!

[Decepticon]

Rhea map from Steves excellent Planetary Maps.


http://laps.fsl.noaa.gov/albers/sos/saturn...rgb_cyl_www.jpg

[Rob Pinnegar]

That second basin on Rhea is visible in some of the Cassini images.

It naturally makes one wonder if they are the result of the impact of a double asteroid. Could of course just be coincidence though.

[tedstryk]

Will Cassini ever get good imagery of the cliffs of Rhea? It seems they would be interesting, as it would make for comparison with Tethys, Enceladus, and especially Dione. They are definitely there in Voyager images, but Cassini doesn't seem to be getting these areas. Or have I missed something?

[malgar]

Rhea craters... Elevation map extracted thanks to photoclinometry.

[dvandorn]

...There's a lot of craters with central peaks, and a lot of distinctly non-circular craters, too - the old 'Lunar grid' afficionados would have a field day!

There are several sets of, for want of a better term, crater chains on Rhea, some of which appear to be hundreds of kilometers long. They are in varying states of degradation, so these chains have been forming over a fairly long time frame.

Something is controlling the orientation of those crater chains -- some seem very straight, others define gentle arcs, but they are definitely arrayed in chains.

The chains are either internally or externally controlled; those are the only two choices, right? I can see, on the conceptual level, arguments for either method.

On the internal control side, we can see that most of the icy moons of Saturn have surfaces that appear extensively cracked, twisted, or otherwise deformed. It wouldn't be a huge stretch to imagine Rhea being cracked, both internally and externally, prior to its final major resurfacing -- and that subsurface faults that still exist (and that occasionally exhibit movement under the proper gravitational triggers) could manifest on the surface as strings of huge sinkholes spaced evenly above the deep faults.

On the external side, we've also seen crater chains on some of the other moons, and of course we see the central bulge on Iapetus -- all of these attest to dynamics of ring-moon collisions that may be fairly unique to the Saturn system. With all of the ring debris in the environment, a stray ringlet arc that gets tossed out of its orbit via an unusual gravitational resonance might run across a moon, and the ringlet arc might well define a straight or an arced line as it intersects the moon's surface.

I tend to the latter theory, that in Rhea's case, the crater chains are probably where Rhea has collected up tossed-out ring arc fragments.

The craters in these chains don't look like linear ejecta from larger impacts to me, either -- they don't tend to vary a lot in size, and while there are a few somewhat larger craters to which some chains are arrayed radially, a single chain tends to be the *only* chain that's actually radial to that larger crater. If a crater chain was made up of ejecta, I would expect to see a considerable variation in the nature of the ejecta and the size/population of the secondaries as a function of distance from the primary, and that's just not what we see in these chains. So, I think we can safely eliminate the "easy answer" that the chains are simply rays of ejecta, and accept that one (or possibly both) of the more exotic options above *must* be true...

-the other Doug

[Bob Shaw]

other Doug:

The only fly in the ointment with regard to expelled ring-arcs hitting the surface must be the rate at which the bits disperse once they're not locked into place. Shoemaker-Levy certainly had time to become a l-o-o-o-n-g string of debris, too, which might suggest either (1) that broken snowball moons (AKA baby comets for all practical purposes) could form crater chains or (2) that debris would quickly fan out and become chaotic. On the other hand, perhaps the orientation of the observed crater arcs will tell us something - if, say, the leading edge of the moon gets whacked more than the poles, then that'd be a sure sign of something coming from outside.

Bob Shaw

[Gsnorgathon]

Is this a second basin? Or is this a cluster of craters?

I must be seeing things!

It sure looks like a big basin with scalloped edges to me. I'd guess the basin rim was eroded by landslides. Rhea in particular seems to have a lot of non-round craters, doesn't it?

[Decepticon]

New update... http://photojournal.jpl.nasa.gov/catalog/PIA07572