For a long time, the two potato-shaped moons of Mars were thought to be just captured asteroids, but scientists are not that sure anymore. In fact, it seems that the Martian satellites are actually the leftovers of a planetary-wide impact.

Taken at face value neither scenario has been able to completely take into account the dynamics of the current system, but a new series of numerical simulations, published in Nature Geoscience, suggest that the impact hypothesis can actually explain it.

According to a Belgian-French-Japanese collaboration, Phobos and Deimos, the moons of Mars, actually accreted from the outer portion of the debris disk, which formed after Mars was hit by a planetoid about one-third the size of the Red Planet. This scenario is similar to the way our own Moon formed.

“A major difficulty has been to explain why a giant impact on Mars would have left two moons so different from our own Moon, a huge single mass, that also formed from Earth undergoing such an impact,” explained co-author Sébastien Charnoz in a statement.

According to the simulations, over 4 billion years ago a Pluto-sized object hit the northern hemisphere of Mars, creating in about 30 hours a wide debris disk around the planet. At the edge of the inner disk, located within 3,620 kilometers (2,250 miles), a large moon, a thousand times the mass of Phobos, took shape

This large moon was repelled by the inner disk due to resonance forces, which then propagated to the rest of the outer disk forming more moons. Unfortunately for Mars, as the inner disk slowly fell towards the planet so did these moons, including the large one. After 5 million years, only Phobos and Deimos were left orbiting the Red Planet.

Evidence is mounting that the impact scenario is the correct hypothesis, but more is needed to confirm their origin once and for all. With this in mind, the Japan Aerospace Exploration Agency (JAXA) has announced a space mission for 2022 to the Martian moons, which plans to collect samples from Phobos and bring them back to Earth in 2026.

Analyzing the composition of Phobos might tell us once and for all where these moons come from.

^{Mars is violently hit by a proto-planet three times as small (1) and a debris disk is formed in a few hours (2). A large moon rapidly emerges (3) from the disk close to the planet. As it migrates away from Mars, its two zones of "resonance" influence propagate like ripples (4), facilitating accretion of debris further away into two small satellites, Phobos and Deimos. The large moon falls back to Mars in a few million (5) years, while the smaller Phobos and Deimos reach their present position around Mars within the next billion years (6). Antony Trinh / Royal Observatory of Belgium}