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?