Uranus Moon Theory Debated

FortKnox writes "Y! has an interesting article on, what is said to be the most puzzling moon in the solar system, The Uranus moon Miranda. Due to the extreme differences in the textures of the surface, a theory of how it happened came about. The old theory was that the moon was shattered in the early ages of the solar system, but fell back together. But now the theory is in question. Instead of something smashing the surface breaking it, they believe it may have to do with seismic activity. Quite an interesting read."

Comments

Re:Hey, two articles in a row! ;-)

[tolan's my name]

If you suppose that the 'breaking-up' wasn't too violent then the relative velocities of the fragments post break would be relatively small. They would also be traveling at something close to their original velocities relative to the planet. Now the pieces in isolation would continue to orbit fairly normally in this scenario, and of course would have an attraction towards the centre of mass of the pieces. If things were aproximately right they'd all clunk back into each other over time. As to why they might bond rather than stay as seperate pieces I could only guess heat caused by the impacts.

I admit that I dont personally believe in this scenario, but it is generally plausible.

Re:Hey, two articles in a row! ;-)

[hubie]

Remember that you can't "lose" gravity. Even when the pieces are separated, if there is nothing else to continually draw them apart they will still want to reassemble through mutual attraction. It is like watching an exploding firework; after the explosion all the pieces are moving with the same center of mass. In the case of the firework the aerodynamic drag on the little pieces as well as wanting to fall back to Earth keeps the little pieces from reassembling. If the firework was exploded in space, if the initial explosion was not large enough to give the pieces enough escape velocity (that is, enough velocity to escape the gravitational attraction from all the other pieces), the firework would eventually fall back together.

It all depends on what the local environment is like. For instance, the rings around the planets (most notably Saturn) are composed of a bunch of material that doesn't seem to want to reassemble (at least on the several hundred year timescales that we have observed it), and it is believed that the shepherd moons provide enough disturbance to keep the ring material in a ring (actually they keep the material from spreading out uniformly; it is the Roche limit that keeps them from clumping).

Re:Hey, two articles in a row! ;-)

[JabberWokky]

It is like watching an exploding firework

Which begs the question of *what* causes moons to explode in such a fashion. The term "explode" implies a fairly uniform outward expansion together with fragmentation. A collision would (presumably) cause a very directional push that would not only scatter the fragments into positions where they would gather to more localized centers of gravity, but would also (I'm assuming quite a bit about the energy necessary to shatter a moon) likely kick the moon into an unstable orbit, or at least a highly eccentric one.

I assume the research and simulations have been done - a fairly in depth google search didn't come up with any. Anybody got any explinations/pointers to more info?

--
Evan

Re:Hey, two articles in a row! ;-)

[hubie]

I believe current thinking on these collisions is a planet being struck by a very sizeable object. I presume these kind of collisions are highly inelastic, so I think you have to be careful about thinking of the struck planet being kicked into significantly different orbits. I believe that the modelling of these things show that most of the kinetic energy goes into friction that liquefies the planet. There is more and more evidence that the Moon was formed when a Mars-sized object hit the Earth. A nice link that shows the results of a model as well as discussing some of the stuff mentioned above can be found here.

Incidentally, some believe that Charon was formed when something struck Pluto in a similar fashion.

Link for images and ??

[CXI]

Images are here:

http://www.nasm.edu/ceps/etp/uranus/uran_moons.htm l

Now, the thing that puzzles me is that this guys theory isn't any different from the previous theory.

The "explosion" (aka impact by comet or something) theory says that the act of the moon heating up as it reformed caused volcanic like activity, which stopped when the moon finally settled.

The new theory says that some random and undetermined force (maybe tidal forces, maybe something else) caused the moon to heat up and cause volcanic like activity. Um, that's the same thing! All this theory does is say "no, it wasn't an impact that caused this! It was... Something Else!" What else? "Well, we're not sure, but man this is getting me good publicity for my PhD!"

*sigh*

Photographs

[gwyrdd benyw]

photo 1 - this one nicely shows the chevron discussed in the article, as well as a broad expanse of Luna-like craters

photo 2

photo 3, from the APOD.

photo 4

photo 5, allowing zoom controls

photo 6 - a closeup of the ridges

Also, here is NASA's writeup on Miranda.

Re:Hey, two articles in a row! ;-)

[krlynch]

Do you know what stops the shepherd moons from breaking up? (they are within the Roche limit)

Well, it is not the case that ALL objects within the Roche Limit will break up. The Roche Limit is defined as the orbital radius at which an object with _no tensile strength_ (that is, with no internal forces holding it together) will be torn apart by the tidal forces of the parent body. Objects with non-vanishing tensile strength (rocky or icy moons, or space probes, for instance) can survive at an orbital radius well within the Roche Limit; exactly how far inside that radius is dependent on many factors (the material, the orbital eccentricity, the rotation speed of the object, etc.), and isn't generally easy to calculate.