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."

Hey, two articles in a row! ;-)

[FortKnox]

Question: the article states that moons breaking apart and coming back together isn't such an akward theory. I know it mentions a moon on Saturn that almost blew apart, but that kinda seems far-fetched to me. How does it "fall back together"? Once its in little pieces, it looses its gravity, so how do the "pieces" stay together? I'm just a touch puzzled by this.

Sorry about the bad grammar in the article, its still morning for me...

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 [nasa.gov] 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 [nasa.gov] 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 [spacedaily.com].

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

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

[JabberWokky]

There is more and more evidence that the Moon was formed when a Mars-sized object hit the Earth.

Yes, and I should have tossed that in - it seems to refute the concept of shattered pieces recoalesing. A body being hammered into a binary system (Earth - Moon, wherein the center of gravity is outside and between the two bodies) makes sense, but when the local center of gravity of the planet-moon system lies within the planet (as most do)...

We have to face the fact that we don't know *what* is common in orbital evolution. We only have (roughly) eleven datapoints in the same system (planets + the scattered bodies between Mars and Jupiter, plus the Oort cloud), and we have no idea if the entire system was exposed to some unknown force in the past. While I don't mean to denegrate the quite excellent work done by astronomers, much of it is guess work that is heavy on the guess. It's a testement to perserverence that they keep hammering at theories until things start matching the observed behaviour and mathematical simulations.

--
Evan

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

[hexx]

it is believed that the shepherd moons [nasa.gov] 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 [nasa.gov] that keeps them from clumping).

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

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.

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

[krlynch]

Oops... I hate responding to my own posts, but I do need to correct a slight error. I said:

that is, with no internal forces holding it together

I should, of course, have said:

that is, with no internal forces other than gravity holding it together

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

[Stephen Samuel]

Of course, the escape velocity of your average firework is going to be in the range of one furlong/fortnight (`units furlong/fortnight mph`) -- unless it's a 3 megaton (conventional explosive) firework...

Excuse me, dear boy, just what are you trying to do with this island?
...Oh, it's a firework! Well, that explains the 'no smoking' signs.

Pictures

[Lord_Of_The_Beer]

Does any one know where I can see these pictures of Miranda that the article is talking about. I am having a hard time "getting" the images from the article.

Link for images and ??

[CXI]

Images are here:

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

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*

Re:Link for images and ??

[CheshireCatCO]

I should defend Bob Pappalardo since I was on the committee that recommended his hiring here at CU and because otherwise my officemate (his grad student) will throw something heavy at me.

Bob's thesis was done years ago, the mid-90's as I recall, so I don't know why the press is just picking this up now. I have a hard time imaging that it's Bob's fault as much as the media's odd way of getting interested in things. Still, it is important work even if it did only show that Miranda probably wasn't broken apart, simply because removing one theory makes the research field that much smaller and easier to handle.

That said, the article is misleading. Bob's contention is seriously different from the beaking up hypothesis, because he believes Miranda suffered slow geological processes over long time periods, similar to processes seen on Earth. The smashing theory would have been a much more sudden event, not akin to anything in terrestrial geology. Bob's contention avoids the stochastic nature of the old theory, as well as allowing us to use what we know from Earth to study Miranda.

It's also worth noting that it had been known that Miranda could have been tidally heated since the late 80's, when Murry, Dermott and Malhorta published a paper on this topic. I got the sense for the article that Bob's work had come first, and the tidal study later, but this isn't the case.

Photographs

[gwyrdd benyw]

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

photo 2 [nasa.gov]

photo 3 [nasa.gov], from the APOD.

photo 4 [nasa.gov]

photo 5 [nasa.gov], allowing zoom controls

photo 6 [nasa.gov] - a closeup of the ridges

Also, here is NASA's writeup on Miranda [nasa.gov].

Another aricle with same subject

[maddogsparky]

Check out the article [space.com] at space.com [space.com].