[X] My Assistant

[alan]

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

[Rob Pinnegar]

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.

[As old as Voyage...]

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.

[dvandorn]

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