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Did an ancient impact bowl Pluto over?
alan
post Oct 6 2007, 09:46 PM
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QUOTE
Pluto and its large moon Charon may have been bowled over when they were struck by wayward space rocks in the past, a new study suggests. If so, NASA's New Horizons spacecraft may find evidence of these rolls when it arrives at the distant worlds in 2015.

Jay Melosh of the University of Arizona in Tucson, US, first suggested about 30 years ago that the basins gouged out by impacts would redistribute the mass of planetary bodies, causing them to roll over to re-stabilise themselves.

Assuming Pluto and Charon have basins as big as those on Saturn's moons Tethys and Rhea and Uranus's moon Titania, the researchers calculate that Pluto probably tipped over by 10° and Charon by 20°.

"One [prediction] is that there will be a network of tectonic fractures caused by the satellites rolling over," Nimmo says, explaining that their 'equatorial bulges' – a widening at their equators due to their rotations – would create stresses when their equators shifted position.

http://space.newscientist.com/article/dn12...pluto-over.html

Also check out the illustration of 'Pluto'. It looks a lot like this image of Ganymede.
http://photojournal.jpl.nasa.gov/catalog/PIA00352
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tasp
post Oct 6 2007, 09:53 PM
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Interesting topic.

So many permutations of the scenario though.

Did either (or any) impact disrupt the mutual tide lock situation ??

Can we sort out any 'frozen' in bulging from pre and post impact effects and relaxation effects ??

If Charon is currently active (water/ammonia volcanoes) does that relax the crustal deformations, or reinstate them to the new orientation relative to Pluto ??


Why is Charon active and Pluto apparently not ?? Timing of recent impacts, and redistribution of Charonian bulging ??


Weirdly complex topic.
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Rob Pinnegar
post Oct 6 2007, 11:09 PM
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I'm not impressed with this one.

Iapetus has got huge basins all over it. If *any* moon should have tipped due to impacts, it's Iapetus. However, the ancient bellyband (which most likely has something to do with the moon's original non-synchronous rotation) is still on the equator, suggesting that its rotational axis is stable.

In fairness, I guess one could make the argument that Iapetus' walnut-shaped profile would make it almost impossible to tip over, due to the enormous amount of mass distribution that would be necessary. But it's still just speculation -- and the fact that it's in New Scientist doesn't breed much confidence. Those guys seem willing to print anything.
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elakdawalla
post Oct 7 2007, 01:58 AM
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It may be in New Scientist now but it refers to an article published in Geophysical Research Letters, which is a peer-reviewed journal. Francis Nimmo (first author) is a well-respected geophysicist working on the tectonics of icy bodies -- in fact, he won this year's Urey prize from the DPS (outstanding young scientist). I don't have online access to the full version of the publication, but I can assure you that these guys have done their homework. From the abstract, I think that what they are saying is not that they have proven Pluto and Charon have reoriented (not enough data for that), they're just suggesting that the interpretation of the geologic history of Pluto and Charon may very likely be complicated by a past episode of reorientation.

The question of Iapetus is an interesting one though, it's worth asking Francis about.

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tasp
post Oct 7 2007, 02:02 AM
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Re. Iapetus, perhaps equatorial ridge dates back only to 3.7 GYA, big 'moon rockin' impacts probably much older.

Joan for instance, if the 'tiger scratches' record to some degree a continuation of the flight path of it's impactor, studying crustal rigidity, and Joan's diameter, we might come up with a good estimate for the mass of that impactor. Looking at Iapetan radius in the bottom of Joan (not easy, I concede, due to subsequent impact damage) and comparing it to 'average' radius for the entire Iapetus (irregular though it is) might allow us to estimate increase in mass of Iapetus since that specific impact. If we can start working up some (relative, perhaps) dates for these features, maybe we wind up helping Alan when NH gets to Pluto. Additionally, more examples of this axial tilt idea might help pin it down (if it not spurious).

I am thinking Iapetus is taking on increased relevence to interpreting the NH Pluto/Charon/Nix/Hydra data in 2015. We are seeing several features unique to Iapetus, perhaps a subtle preview to some Plutonian strangeness.
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brellis
post Oct 7 2007, 02:16 AM
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Question: if objects are moving more slowly the farther they are from the sun, is it less likely for Pluto and Charon to get tipped over?

If they are tipped, and so is Uranus, isn't it that much weirder that planets really far out are tipped and those closer in to the sun are not?

My attempt to answer: many KBO's orbit at wildly inclined angles to the ecliptic, making it more likely for an object in that region to get tipped, even if objects aren't traveling as fast as they do when closer to the sun.

2nd attempt: before things settled into the plane of the ecliptic, orbital inclinations varied greatly. But if that's the case, wouldn't it be unlikely to find a nice neat ring of pre-planetary dust around other stars, as we have observed with COROT and other advanced instruments?

I appreciate any answers from the UMSF Masters! smile.gif
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As old as Voyage...
post Oct 7 2007, 04:56 PM
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QUOTE (Rob Pinnegar @ Oct 7 2007, 12:09 AM) *
I'm not impressed with this one.

Iapetus has got huge basins all over it. If *any* moon should have tipped due to impacts, it's Iapetus. However, the ancient bellyband (which most likely has something to do with the moon's original non-synchronous rotation) is still on the equator, suggesting that its rotational axis is stable.


I mentioned the huge basins on Iapetus to Dr Nimmo while writing the following feature on the Moon's South-Pole Aitken basin:

http://space.newscientist.com/article/dn11625

He seemed intrigued but I'm not sure if he looked into it further. I also pointed out that Dione has a large south polar basin and crater evidence suggests it may have reversed its leading hemisphere in the past; both possible evidence of reorientation.

I wonder why Herschel and Tethys have not reoriented their large basins in a similar manner to the Moon.


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djellison
post Oct 7 2007, 05:12 PM
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QUOTE (Rob Pinnegar @ Oct 7 2007, 12:09 AM) *
If *any* moon should have tipped due to impacts, it's ....


Mimas, Phobos ?


Doug
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elakdawalla
post Oct 8 2007, 12:35 AM
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I got a reply from Francis:
QUOTE
I didn't do any calculations on Iapetus, but I figured that an uncompensated global mountain belt 20km high and 200km across was likely to outweigh the effect of any basin. (Roughly speaking, a 5km deep basin would have to have a radius of 700km to have a similar effect).

Questions for the geologists: 1) are there superposition relationships which give the relative ages of the band and the basins? 2) how equatorial is the band? If it is slightly off-equator, then we could calculate whether the observed basins are big enough to have caused it to move.

Here's a page listing his recent papers with links to PDF versions; this one should show up there soon.
http://es.ucsc.edu/~fnimmo/website/papers.html

--Emily


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Jyril
post Oct 8 2007, 12:51 PM
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QUOTE (As old as Voyager @ Oct 7 2007, 07:56 PM) *
I wonder why Herschel and Tethys have not reoriented their large basins in a similar manner to the Moon.


Mimas is surprisingly oblate. Even a large impact can't prevent its major axis from pointing Saturn.


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dvandorn
post Oct 9 2007, 07:40 AM
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QUOTE (As old as Voyager @ Oct 7 2007, 11:56 AM) *
I wonder why Herschel and Tethys have not reoriented their large basins in a similar manner to the Moon.

The Moon actually has its basins evenly distributed around the globe. Within statistically significant limits, there are just as many basins on the far side as on the near side, and there as just as many in the southern hemisphere as the northern hemisphere.

However, the basins on the near side are almost all filled with mare lavas, while the basins on the far side are almost exclusively not mare-filled. There is also a tendency for the southern hemisphere basins to be less mare-filled than those in the northern hemisphere, even on the near side.

The question is more why the Moon's lava flows occurred preferentially on its Earth-facing side? The best theories I have heard involve tidal interactions, the "freezing" of a tidal bulge on the near side, and these factors interacting with a non-homogenous melting of the upper mantle around a non-melted chondritic lower mantle. But there is not yet a satisfactory theory that addresses all of the observed facts.

It actually occurs to me that the Moon is similar to Iapetus, in that it is quite a bit darker on one side than it is on the other. The albedo range isn't as great, but it is significant.

-the other Doug


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JRehling
post Oct 9 2007, 08:39 AM
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QUOTE (brellis @ Oct 6 2007, 07:16 PM) *
Question: if objects are moving more slowly the farther they are from the sun, is it less likely for Pluto and Charon to get tipped over?

If they are tipped, and so is Uranus, isn't it that much weirder that planets really far out are tipped and those closer in to the sun are not?

My attempt to answer: many KBO's orbit at wildly inclined angles to the ecliptic, making it more likely for an object in that region to get tipped, even if objects aren't traveling as fast as they do when closer to the sun.

2nd attempt: before things settled into the plane of the ecliptic, orbital inclinations varied greatly. But if that's the case, wouldn't it be unlikely to find a nice neat ring of pre-planetary dust around other stars, as we have observed with COROT and other advanced instruments?

I appreciate any answers from the UMSF Masters! smile.gif


Their orbital velocity shouldn't directly have much to do with the ability to knock them over. That is to say, for them to experience a change in the vector component of their angular momentum.

Mercury and probably Venus have axes that are governed by solar tides, so they are two special cases.

The other planets tend to have an axial tilt of 20-something degrees. They are not fully independent of one another, and Jupiter is yanking just about everything else around. I once observed that the planets' axial tilts suggest (as well as such a small number of data points can) a Gaussian distribution. That is, if you have a component of angular momentum at right angles to the solar plane and you add a certain magnitude of randomness to that, you might get a peak of 20-something degrees.

I always have found theories involving an object being knocked around to be somewhat vacuous. Objects accreted. Obviously, they got knocked around thousands of times. A "collision" is just a knock that came later than most of the thousands of knocks.
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