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Posted by: BruceMoomaw Jul 11 2005, 11:15 PM

We now have the first density estimate of it from Cassini -- and it appears
that we have yet ANOTHER icy rubble pile. The estimate: 0.6 grams/cc.
Also, the surface is now officially recognized as very weird, in a way that
suggests that it is in no way a solid world:
http://ciclops.org/view.php?id=1192

Posted by: alan Jul 11 2005, 11:33 PM

Obviously Hoagland is calling the wrong moon an alien artifact

Posted by: Decepticon Jul 12 2005, 12:09 AM

LOL

Posted by: BruceMoomaw Jul 12 2005, 02:03 AM

Actually, those strange, small, relatively uniformly-sized and dark-centered craters on Hyperion bear a disturbing resemblance to the similar surface pits observed on Helene in Charles Sheffield's novel "The Ganymede Club" -- in which Helene turns out to be a Sinister Alien Artifact that does Horrible Things to the investigating astronauts.

The real problem with landing on Hyperion, however -- as pointed out by Poul Anderson in his story "Scarecrow" -- is that, given its chaotic rotation, its surface will be sliding around horizontally undeneath a descending lander at entirely unpredictable speeds and directions. (The main subject of the story, believe it or not, is the development of religion among intelligent machines.)

Posted by: BruceMoomaw Jul 12 2005, 02:07 AM

At any rate, now we know: Hyperion's non-spherical shape comes not from (as so many of us assumed) the fact that it's a solid moon that has had big chunks knocked off it by impacts, but from the fact that the whole moon is a
loosely bound and tidally distorted clump of small debris like Saturn's
innermost moons. Obvious question: where did the debris that accreted to
make Hyperion come from? Is it the finely shattered remnants of a single
solid moon that was originally near that orbit? Or have complex gravitational
interactions (mostly from Titan) caused debris from elsewhere in the Saturn
system to accumulate in a 3:4 resonance orbit with Titan and re-accrete
there into a rubble-pile moon?

Posted by: abalone Jul 12 2005, 12:27 PM

It maybe the portion outside the Roche Limit of a much larger satelite the rest of which now forms the rings. The life span of the rings is relatively short on the scale of the age of the Solar system 100-200 my? They have formed recently and must have formed from something and "the left over material accumulate in a 3:4 resonance orbit with Titan and re-accrete there into a rubble-pile moon" as you say

Posted by: Decepticon Jul 12 2005, 01:45 PM

Ok here's one that might be hard to swallow.


I think Hyperion is a captured comet.

Posted by: Myran Jul 12 2005, 02:40 PM

Yes, I cant but agree with you, I find that idea hard to swallow.

I would say its ar more likely Hyperion used to be a more 'normal' icy moon, that once in the past took a tremedous blow by one impacting body. The moon shattered and after time recollected but the weak gravity couldnt compress the pieces leaving the spaces that gives it the low density.
If thats not the case it could contain frozen gases that are lighter than water, and with a dark material on the surface I do understand how you come to think of a comet, but I seriosuly dont think it is.

Posted by: Bob Shaw Jul 12 2005, 08:30 PM

If we're accepting *any* captures as the origin of outer planetary moons (as opposed to a mini solar accretion disk) then surely comet/cometoid bodies are quite likely to be captured - there's lots of 'em, and they have orbits which mess with the gas giants on a regular basis (bet the Jovian Trojans are cometoids!). We saw evidence in the Jupiter system of crater chains from disrupted comets, as well as an actual disrupted comet impact, and as for the first port of call when Cassini hit Saturn's outer suburbs...

...but - and it's a b-i-g BUT: wouldn't we preferentially see these guys on the outer fringes of the systems which they've come to inhabit, not slam-bang up against the planets? The orbital mechanics surrounding a d-i-s-t-a-n-t capture are going to be much more benign than something just above the clouds (well, almost!).

Maybe there's a size thing going on, with some ancient KBO population being captured and preserved in almost the inner Solar System while their brethren were gradually pumped out - or in - and only standard-sized comets were left...

Posted by: abalone Jul 12 2005, 11:18 PM

Capture favours retrograde motion and at the outer edge of the family of moons, this is not what we have here.

Posted by: Bob Shaw Jul 13 2005, 02:09 PM

Of course; capture and 'un-capture' on the fringes is relatively easy ('Hello, Apollo 12 S-IVB! Cheerio, Apollo 12 S-IVB!') but perhaps we're seeng a relict cometoid population which for some reason or other has survived in very low orbits rather than hitting the planet or being expelled. If there isn't a common mechanism, then how do we explain the pattern of small, low-density moons near the giant planets, then?

Posted by: Rob Pinnegar Jul 14 2005, 12:10 AM

What about disintegrative capture? If, several billion years ago, an unassuming icy planetesimal on its way through the Saturn system happened to pass within Titan's Roche limit, would Titan have been able to break it up?

From what I remember of disintegrative capture, the fragments of the disrupted body end up leaving the vicinity of the primary body (here Titan) with a pretty large velocity distribution. There is also a lot of dissipation due to internal friction and gravitational interaction of the fragments during the breakup, which means that any debris trapped in Saturnian orbit wouldn't necessarily end up crossing Titan again. One would expect that at least _some_ of the debris would end up in the 4:3 resonance. (Any stuff that didn't end up in a stable resonance would probably end up beating the heck out of Titan and Saturn's other moons.)

The debris that would be "slowed down" the most during the disintegration would be the stuff on the Titan facing side. If Titan's upper atmosphere got involved this effect would of course be even more pronounced. If the planetesimal was differentiated this could in part explain the low density of the material captured by Saturn... it would be mostly ice.

I have absolutely no idea whether this hypothesis is even remotely feasible from a dynamical standpoint. (Hope I'm not getting any of the basic facts about disintegrative capture wrong. It's been about seven or eight years since I last read up on it.)

Cheers
Rob

Posted by: Bob Shaw Jul 14 2005, 09:55 AM

Rob:

Seems reasonable - and presumably would also work in other gas-giant systems with large moons. I'd still like it if some o-o-o-o-ld material could have survived!

I still wonder about the Jovian Trojan Asteroids...

Bob Shaw