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Posted by: Rob Pinnegar Apr 27 2006, 03:36 PM

On quite a few occasions I've seen Mimas' big crater Herschel described as being the result of an impact that came close to breaking Mimas apart. Today I was looking at an image of Mimas and what really struck me is the fact that Herschel sits pretty close to the middle of the leading hemisphere (hope I've got that right) and it's also just about right on the equator.

This is a long shot, but could Herschel be the result of a collision between Mimas, and a Mimas co-orbital that wasn't able to stay in the Lagrange point because of perturbations from Enceladus and the other large moons? The fact that Herschel is a bit off-centre in the leading hemisphere isn't a _huge_ problem, since Lagrangian orbits can be a bit elliptical.

What I'm thinking is that, if the co-orbital was big enough, this could provide a larger crater than what would normally be the limit for shattering the moon, due to the relatively low impact velocity.

Herschel is pretty deep, and has a pronounced central peak, which speaks to a violent origin. And Mimas is so small that it would have had difficulty grabbing a Lagrangian moon in the first place. So this hypothesis probably doesn't have a whole lot of merit. Still, I was curious to find out what people think -- and whether this mechanism could explain features on any other Solar System moons. Would a very-low-velocity impact give an identifiable crater configuration?

I'm tempted to speculate about Mare Orientale and solar perturbations, but have probably gone far enough out on a limb for one day.

Posted by: tasp Apr 27 2006, 04:48 PM

Tethys has a largish crater, equatorial and leading hemisphere too.

IIRC, the Valhalla and Asgard craters on Callisto are on the leading hemisphere too. Perhaps a binary co-orbital got zorched?

Additionally, the mass ratio of Titan to Hyperion, and Dione to (help me out here, Calypso or Telesto or whatever one it is?) is quite similar.

Perhaps the crater size of Valhalla could be extrapolated back to figure a range of possible masses that may have created it. Curious if the mass ratio is in the ball park for that one too . . . .

Might be some subtlety going on here.

Posted by: Richard Trigaux Apr 27 2006, 05:54 PM

I think there is an anomaly into the statistical repartition of crater size, and this anomaly is present througout the solar system. This anomaly is that there are many large craters, into the 200kms-300kms, but there are too less larger sizes. For instance if we have a Pareto law (power law) crater size repartition, we would see more 300-500kms craters than we see 500-3000kms craters. There was already a discution here about this, one of the hypothesis being that there would have be some co-orbitals for the Moon, to explain the series of large bassins, which all formed into a relatively short time. But others said that such Moon co-orbitals were unlikely.


The origin of this anomaly would be probably that the bulk of impacts would be comets, giving craters in the 200kms size, with a sharp upper limit. But larger craters would have a different source, for instance co-orbitals, or other smaller bodies which formed into unstable places and were later ejected.

There was also a discution about Japet's equatorial ridge being the result of the destruction of a satellite.


About the relation between the mass/velocity of impactor and crater size, it seems that the crater size depends only on the total energy of the impactor, so that we cannot distinguist a small fast impactor from a large slow one. But I think that if the speed is VERY slow, say subsonic (the sound velocity into rocks, of course) there would not be an explosion crater, but rather a spreading of rocks around a central heap, or a shallow crater without sharp limits. I don't know examples of this, but perhaps we could find this on small far Kuyper belt objects, which have very slow relative speeds.

Posted by: BruceMoomaw Apr 27 2006, 07:25 PM

This is an interesting idea, and it just might be worth your while to mention it to a scientist -- I don't think any of us has the expertise to appraise it.

Remember that after Voyager, one popular view was that Saturn's moons showed cratering patterns suggesting that they had been hit by two different populations of impactors -- one being the usual Sun-orbiters, and the other being a large separate population of slower Saturn-orbiting objects. However, I gather that Cassini's more accurate census of crater sizes suggests that the apparent bipolarity in size and shallowness of craters indicated by Voyager doesn't really exist. (There's a recent LPSC abstract on this, once I find it.)

Posted by: Richard Trigaux Apr 27 2006, 07:50 PM

Thanks, Bruce. There may be many other ideas worth of a scientific recognition into this forum. But who to tell it?

I know that if, for instance I discover a comet, I must tell it to some organism I don't remember the exact name, and they give my name to it. But this is a special case. Usually a science idea must be presented into the form of a paper, with calculations, checks and all. And into the instance I don't have figures or statistics of crater sizes. (I could use raw Cassini images, but they are not a complete sample). So I (we us) can only make guesses like this, that more exact calculations may prove false.

I submitted only once a paper (in economy) and it took me nearby one year and many tries until finding the good gateway toward a peer referee review.

Posted by: BruceMoomaw Apr 28 2006, 09:49 AM

Here's that abstract (by Neukum and Wagner) concluding from Cassini's crater counts that the icy moons, contrary to post-Voyager belief, have NOT been extensively bombarded by a large number of small Saturn-orbiting impactors in addition to their bombardment by solar-orbiting ones:
http://www.lpi.usra.edu/meetings/lpsc2005/pdf/2034.pdf

Posted by: Richard Trigaux Apr 28 2006, 11:12 AM

... hmmmm....

At first glance, it seems that these statistics contradict my theory above, as what there would be two impactor families: "small" but numerous comets, and larger less numerous co-orbital.


But as you see on the curves, there are real changes in slope. They interpret these changes as effects of resurfacing of the icy moons. I would suggest that these change in slope look exactly as what we could expect from a double population.

Posted by: Phil Stooke Apr 28 2006, 11:12 PM

Reply to Rob's idea about Mimas:

The idea that the Herschel impact was nearly large enough to disrupt Mimas is a Voyager-era idea (akin to the Phobos-Stickney story too) which is not much believed now. Studies by people like Peter Thomas and his colleagues on asteroid craters have shown that it's much harder to disrupt a body than people used to assume. I'm not sure about the effect of a lower-speed impact... worth doing some modelling!

Phil

Posted by: Rob Pinnegar Apr 28 2006, 11:46 PM

Oh, okay. Thanks, Phil. (I guess the hemisphere-sized crater on Vesta should have clued me in to that one, eh?) Anyways that still leaves the main issue open here.

Posted by: ljk4-1 May 31 2006, 08:42 PM

General Relativity and Quantum Cosmology, abstract
gr-qc/0605111

From: Lorenzo Iorio [view email]

Date: Fri, 19 May 2006 15:49:22 GMT (24kb)

Does Cassini allow to measure relativistic orbital effects in the Saturnian system of natural satellites?

Authors: Lorenzo Iorio

Comments: Latex2e, 5 pages, no figures, 4 tables

In this paper we address the following question: do the recent advances in the orbit determination of the major natural satellites of Saturn obtained with the analysis of the first data sets from the Cassini mission allow to detect the general relativistic gravitoelectric orbital precessions of such moons? The answer is still negative. The present-day down-track accuracy would be adequate for Mimas, Enceladus, Thetys, Dione, Rhea and Titan and inadequate for Hyperion, Iapetus and Phoebe. Instead, the size of the systematic errors induced by the mismodelling in the key parameters of the Saturnian gravitational field like the even zonal harmonics Jl are larger than the relativistic down-track shifts by about one order of magnitude, mainly for the inner satellites like Mimas, Enceladus, Thetys, Dione, Rhea, Titan and Hyperion. Iapetus and Phoebe are not sensibly affected by such kind of perturbations. Moreover, the bias due to the uncertainty in Saturn's GM is larger than the relativistic down-track effects for all such moons. Proposed linear combinations of the satellites' orbital elements would allow to cancel out the impact of the mismodelling in the low-degree even zonal harmonics and GM, but the combined down-track errors would be larger than the combined relativistic signature.

http://arxiv.org/abs/gr-qc/0605111

Posted by: ljk4-1 Jun 6 2006, 03:38 PM

Date (revised v2): Mon, 5 Jun 2006 15:14:03 GMT (10kb)

Does Cassini allow to measure relativistic orbital effects in the Saturnian system of natural satellites?

Authors: Lorenzo Iorio

Comments: Latex2e, 9 pages, no figures, 3 tables, 12 references. Removed discussion on pericentres, focus on the mean longitudes only in view of the small eccentricities of the Saturnian satellites'orbits

In this paper we address the following question: do the recent advances in the orbit determination of the major natural satellites of Saturn obtained with the analysis of the first data sets from the Cassini mission allow to detect the general relativistic gravitoelectric orbital precessions of such moons?

http://arxiv.org/abs/gr-qc/0605111

Posted by: ljk4-1 Sep 12 2006, 01:59 PM

Unravelling Temporal Variability in Saturn's Spiral Density Waves: Results and Predictions

http://arxiv.org/abs/astro-ph/0609242