“The larger inner moons fall back to Mars after about 5 million years due to the tidal pull of the planet, after which the two outer satellites evolve into Phobos- and Deimos-like orbits.” -Pascal Rosenblatt, et al.
Compared to the other moons we know of in the Solar System, Mars’s two, Phobos and Deimos, are incredibly difficult to explain. They look like captured asteroids, being small, irregular, and exhibiting the right surface features. But captured asteroids form inclined or even retrograde orbits quite distant from their planet, while Phobos and Deimos live in circular, equatorial, close-in orbits to Mars.
The orbits of Phobos and Deimos, Mars’ moons, are in the same, equatorial plane as the other planets. Various satellites in orbit around Mars are also shown. Image credit: NASA / JPL-Caltech.
An alternative theory to the captured asteroid scenario is that the moons of Mars formed from a giant impact that kicked up a circumplanetary debris disk, similar to how Earth’s moon formed. But those scenarios never lead to merely two small moons; there’s always at least one large one. Thanks to a new simulation, all the pieces might finally be coming together.
With a circumplanetary disk, the inner, dense region forms a large moon quickly, which then creates instabilities in the outer reaches of the disk, leading to multiple, smaller moons. Image credit: Figure 1 from Perez et al. (2015), via https://arxiv.org/abs/1505.06808.