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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#1
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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, 06:19 PM
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Senior Member Group: Members Posts: 3419 Joined: 9-February 04 From: Minneapolis, MN, USA Member No.: 15 |
I can, however, imagine an impactor large enough to succumb to tidal break-up as it approaches Mars.
Bring a body the size of one of the larger asteroids across Mars just within the Roche limit. As the body breaks up, the release of the outer pieces would fling them into escape velocities, and breakup dynamics would allow some pieces to fall into stable orbits. The rest of the body would, of course, impact Mars. In such a break-up scenario, I can well imagine pieces the size of Deimos and Phobos coming out of the process as relatively intact chunks of rock. Collisions with other pieces of the original parent body during the break-up would knock off the roughest edges, and accretion of smaller chunks over the months and years after the break-up would provide craters and regolith. Perhaps Phobos looks like it has stretch marks because it was literally stretched in the process of the break-up on the original body? Subsequent mantling and alteration has not been enough to wipe the striations off the face of the little moon, since the cracking also controls impact-shock faulting and reinforces the surface expression of the cracks. On a gestalt level, it hangs together... at least, for me. -the other Doug -------------------- “The trouble ain't that there is too many fools, but that the lightning ain't distributed right.” -Mark Twain
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Jul 14 2008, 10:58 PM
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#3
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Senior Member Group: Members Posts: 2530 Joined: 20-April 05 Member No.: 321 |
I don't see a hard dividing line between accretion and impact. Mars was created by many impacts of bodies together. There wouldn't have been any special moment when God clicked a stopwatch and said "DONE!" and thereafter all impacts were impacts, whereas the day before they would have been accretion.
If material were being lofted skywards from Mars, we would see different possibilities as Mars evolved. But the new material being introduced by the impactor would be equally new to the Mars system whether it was this year or 4.5 GYA. And how much of the putative satellite was martian vs. foreign would also be on a continuum. |
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