 Because of the similarity to the composition of C- or D-type asteroids, one hypothesis is that the moons may be objects captured into Martian orbit from the asteroid belt, with orbits that have been circularized either by atmospheric drag or tidal forces, as capture requires dissipation of energy.The current Martian atmosphere is too thin to capture a Phobos-sized object by atmospheric braking.Geoffrey Landis has pointed out that the capture could have occurred if the original body was a binary asteroid that separated due to tidal forces.