Origin of Mars’s moons by disruptive partial capture of an asteroid

The origin of Phobos and Deimos remains uncertain. Most active hypotheses for the formation of Mars’s small moons fall into two categories: direct capture and a giant impact. The moons’ spectral oddities suggest that they might be asteroids caught by the planet. However, their near-circular and near-equatorial orbits more naturally align with accretion from a disk around Mars, typically assumed to have arisen from a large impact. Distinguishing between these two scenarios is the primary goal of the imminent JAXA Martian Moons eXploration (MMX) mission.

We present a new alternative scenario wherein fragments of a tidally disrupted asteroid are captured and evolve into a collisional proto-satellite disk. We model both the initial disruption and the fragments’ subsequent evolution, using a combination of high-resolution smoothed particle hydrodynamics (SPH) simulations and orbit integrations.

We find that tens of percent of an unbound asteroid’s mass can be captured and survive beyond collisional timescales, across a broad range of periapsis distances, speeds, masses, spins, and orientations in the Sun–Mars frame. Furthermore, more than one percent of the asteroid’s mass could evolve to circularise in the moons’ accretion region in the outer disk, compared with a far lower fraction for a post-impact disk. The resulting lower mass requirement for the parent body than that for a giant impact could open up a greater population of potential parents in the early Solar System, contributing to a higher likelihood route to forming a proto-satellite disk that, unlike direct capture, could also naturally explain the moons’ orbits.

The satellites that would arise from this new scenario of disruptive partial capture or from each of the two established origin hypotheses would have different bulk compositions, volatile contents, and other properties that will soon be tested by MMX to constrain the origin of Mars’s moons.