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Mars's moons have the appearance of captured asteroids. How could this capture have taken place. An asteroid in solar orbit would need to loose some momentum to be captured. The gas Giants can capture new moons through 3 body interactions between the asteroid and their existing moons, but Mars doesn't have this issue. Airbreaking would lead to impact, not capture.

An answer here suggests that the moons could have formed recently (millions of years, not billions) after an impact ejected a substantial amount of material in Martian orbit. What is the evidence for this? Is there an impact crater of the right size, and age? Were both moons formed together?

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  • $\begingroup$ I think you are more knowledgeable about this than me, but what about the collision of asteroids with each other to reduce the momentum or is this just statistically improbable. $\endgroup$
    – Jack R. Woods
    Oct 31, 2015 at 14:25
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    $\begingroup$ They would need to collide inside Mars's gravity well. That means two decent sized asteroids colliding within a few hundred thousand km of Mars at the same time. I don't know the maths, but it feels unlikely. $\endgroup$
    – James K
    Nov 1, 2015 at 14:37

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You're right, Mars' moons likely did not form with from the same dust mass as it; their spectra, albedo, and density are suspiciously similar to those of C- or D-type asteroids. Thus, Mars presumably captured Phobos and Deimos from the asteroid belt. However, the origin of the Martian moons is quite controversial.

Burns (1992) notes that capture requires the loss of energy (and momentum, as you pointed out). So obviously, something would need to dissipate that energy. The immediate candidate is aerobraking, but we run into a problem: Mars' atmosphere is too thin to capture a Phobos-sized object through aerobraking. In fact, even if aerobraking could capture Phobos and Deimos, the densities of both moons indicate that they may not survive structurally.

Several hypotheses have been proposed to answer this. Landis (2001) suggests tidal forces are to credit:

The existence of asteroid moons provides a mechanism for the capture of the Martian moons (and the small moons of the outer planets). When a binary asteroid makes a close approach to a planet, tidal forces can strip the moon from the asteroid. Depending on the phasing, the asteroid can then be captured. Clearly, the same process can be used to explain the origin of any of the small moons in the solar system.

According to Geoffrey Landis, Deimos and Phobos could have each been moons of parent asteroids, and they were separated due to tidal forces:

enter image description here

This would explain how Phobos and Deimos could survive as moons. I could mention the other hypotheses, but Landis' would answer your question quite well.

Right now, there isn't any clear consensus as to how Phobos and Deimos became moons. More research is still necessary, and we may need to modify our models for the evolution of Mars and the asteroid belt.

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  • $\begingroup$ Very nice answer. Would it be correct to assume that a gravity assist wouldn't work? I'd assumed that would be a possible reason, but it would probably have been mentioned if it was possible, so I might not have my facts straight. (maybe that should be a separate question - your call). $\endgroup$
    – userLTK
    Oct 4, 2016 at 11:27
  • $\begingroup$ @userLTK Not quite sure what you're asking. How do you propose a gravity assist would work? $\endgroup$
    – Sir Cumference
    Oct 4, 2016 at 13:14
  • $\begingroup$ Maybe it belongs in a new question but an asteroid gets a gravity assist from Jupiter and it loses enough orbital velocity around the sun that it can fall into an orbit around Mars (perhaps eventually, wouldn't have to be right away). I'd assumed that's how it worked, but since your answer didn't mention that the experts see that as a possibility, I'm having 2nd thoughts on my assumption. $\endgroup$
    – userLTK
    Oct 4, 2016 at 15:20
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    $\begingroup$ @userLTK Don't have second thoughts just because it hasn't been mentioned before. Important theories develop because people think outside the box and propose new ideas. That said, I can't find any proposal involving a gravity assist, but I can't see anything inherently wrong with your idea either. Not too sure. $\endgroup$
    – Sir Cumference
    Oct 4, 2016 at 15:25

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