Origin of Phobos and Deimos, Where did these guys come from? |
Origin of Phobos and Deimos, Where did these guys come from? |
Mar 25 2006, 02:49 PM
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Member Group: Members Posts: 154 Joined: 17-March 05 Member No.: 206 |
So what is everyone's thoughts on the origin of Mars' moons Phobos and Deimos? They are a bit of a mystery.
Here are the different theories: 1. They formed along with Mars when it accreted out of the plantary nebula. Pros: explains how both are in the same circular, equatorial orbit around Mars. Cons: Seems a strange coincidence that we are around to witness Phobos in such a low orbit that it is about (in a couple million years) to crash out of orbit. Also this would be the only case in the solar system where such small "asteroid-like" moons formed around such a large body. 2. They were captured into orbit around Mars. Pros: This would explain their similarity to asteroids out in the Belt. Cons: The probability that they would be both be captured into circular and equatorial orbits is virtually zero. Also, there is no know mechanism for asteroids to be captured by such a small body like Mars (after all the moons didn’t do perigee burns to brake them into orbit) 3. They were once part of a larger moon that that broke up into several pieces. Phobos and Deimos are the last remnants of it. Pros: This would explain how both moons have circular and equaltorial orbits (since they started from the same body). Theoretically, there would have been many more moons at one time, but they have crashed into Mars one by one, as Phobos is on course to do. Cons: Phobos and Deimos do not appear to be very similar compositionally, which is strange if they came from the same moon. Of course it was large enough, the large proto-moon may have been differentiated. 4. The moons were formed from a large impact early in Mars history, perhaps from the impact that created the Hellas basin or the northern lowlands. This impact formed a small debris field around Mars which accreted into the moons. Pros: Explains the circular orbits of the moons and Moons created from early gigantic impacts seems to be a re-occurring theme we see in the rest of the solar system (i.e. Earth's Moon and likely Pluto's moons) Cons: While it explains the circular orbits, it does not explain how they are equatorial. I believe the favored theory this decade is number 3, where a large body was present, but was broken up. What is everyone's thoughts? |
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Jul 13 2008, 07:48 AM
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Member Group: Members Posts: 214 Joined: 30-December 05 Member No.: 628 |
One thing has bothered me for some time about the impact origin theory of moon formation. If a bunch of pieces get thrown into space by the impact, and then later coalesce to form a moon, the orbital momentum of the new moon should be equal to the net angular momentum, with respect to the parent body, of its component pieces of ejecta. It follows that moons can only result from glancing blows, because a symmetric 360 degree spray of debris resulting from a head-on vertical hit can never come together to form an orbiting body, even if some fraction crashes back into the parent body and some other fraction escapes entirely. Tiny close-in moons like the ones around Mars - that I can believe, because maybe they represent only a small fraction of the debris created by a large glancing impact. But big moons like our Luna?
Given a handful of parameters: size of the parent, size of the impactor, their densities, rotational speeds, relative velocity and angle of impact, it should be possible to project the portion of debris that will be too slow to escape and too fast to crash back, and then calculate their net angular momentum. I certainly couldn't do the calculation myself, but it must have been done by someone during the general discussion of moon formation. Whether by calculation or simulation, I would like to know what constraints the basic physics of collisions places on the size of the residual net momentum, and under what circumstances it is really possible to form moons that way. If so, it should then be possible to do the calculation backwards for any known moon, and establish the boundaries of the possible parameter values that could have brought it about. Some people here must know the literature well enough to point out where this has been done. In the case of the Martian moons, of course, even if you can prove that such small close-in moons could come together from collision debris, they would likely be rubble piles, so the "Stickney objection" would still cause problems. |
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