Highlights

• New option to make Mars proto-satellite disk: partial capture of a disrupted asteroid

• Combine SPH tidal-disruption simulations with orbital evolution of captured fragments

• Tens of percent of an unbound asteroid can be captured beyond collisional timescales

• A smaller asteroid could make the moons via this scenario than a giant impact

• Each scenario predicts different properties of the moons upcoming missions will test

The origin of Mars’s small moons, Phobos and Deimos, remains unknown. They are typically thought either to be captured asteroids or to have accreted from a debris disk produced by a giant impact. Here, we present an alternative scenario wherein fragments of a tidally disrupted asteroid are captured and evolve into a collisional proto-satellite disk.