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

TL;DR

This paper proposes a novel scenario where Mars's moons originated from fragments of a tidally disrupted asteroid that was captured, offering an alternative to previous theories like direct capture or formation from a debris disk.

Contribution

It introduces a new model of moon formation through disruptive partial capture of an asteroid, supported by simulations showing feasible mass capture and orbital evolution.

Findings

A significant portion of an asteroid's mass can be captured and survive long enough to form moons.

More than 1% of the asteroid's mass can circularize in the moons' region.

This mechanism requires less mass than giant impact scenarios, increasing its plausibility.

Abstract

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. We simulate the initial disruption and the fragments' subsequent orbital evolution. 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. This implies a lower mass requirement for the parent body than that for a giant impact, which could increase the likelihood of this route to forming a proto-satellite disk that, unlike direct capture, could also naturally explain the moons' orbits. These three formation scenarios each imply different properties of Mars's moons to be tested by upcoming spacecraft missions.