How Did the Moons of Mars Form?

Among the formation of moons in the Solar System, the history of the moons of Mars is the most mysterious. Most moons are all but confirmed to be formed together with the planet, captured as an asteroid, or broken away from the planet after a collision. But for the moons of Mars, research is still ongoing and a clear conclusion of this interesting case hasn’t been reached yet. What had happened to form the two moons of Mars?

Before discussing the theories, let’s discuss some background information first — the properties of the moons of Mars. Mars has two moons, Phobos and Deimos. They are both very small, at 21 km and 13 km wide, respectively. Because of their small sizes and low mass, they are irregularly shaped, causing them to look like asteroids. They orbit the red planet on nearly spherical orbits fairly close to Mars, only 6000 km and 19510 km from Mars’s surface, respectively.

To learn about the origins of the satellites, you first need to understand their chemical compositions. Specifically, you need to compare it with the parent planet’s composition. If the compositions are similar, it can be concluded that the satellite formed together with the planet, or that it came from the fragments of a major impact event. Otherwise, the difference in compositions indicate that the satellite is most likely a captured asteroid, formed from a different region in the Solar System.

The nearly circular, close-in orbits of the moons of Mars are indicative of co-formation, as with most of Jupiter’s and Saturn’s major moons. But research comparing the meteorites from Mars and its moons suggests that their compositions are not exactly the same (even after accounting for the atmosphere of Mars). This means that these moons are unlikely to be formed together with Mars, so this leaves two scenarios — a giant impact (which also requires some degree of similarity between the compositions) and asteroid capture (which is compatible with the case that both objects are completely different).

Out of the remaining conditions, let’s consider the captured asteroid scenario first. In an asteroid capture scenario, the relative velocity of the object with the planet is often close to the escape velocity of that planet. Therefore, the orbit of the object is often highly elliptical, coming close in and far out from the planet. Yet, what we see on the moons of Mars is that they orbit the planet in a nearly circular orbit. And so if those satellites are captured objects, how did they achieve this circular configuration?

One theory is that the asteroid being captured was tidally disrupted in the first place. That means, when the asteroid is initially captured by Mars, it ducks under its Roche limit, causing it to disintegrate into pieces. Those pieces then drift inside their orbits, dampening the eccentricity of the orbits through the collisions between the fragments. They eventually start coalescing once the orbits are dampened enough that their relative velocities are not too high when colliding, and this eventually forms a satellite orbiting in a near-circular orbit around the red planet.

Another possibility is that the moons of Mars formed from fragments of a giant impact. This is much like the mechanism that (most likely) formed Earth’s moon, but the scale of the collision would have to be smaller (due to the small size of the moons). Basically, it is an impact so powerful (often involving a large object colliding with a planet) that flings large amounts of material out of the planet. But some of them with the right velocity can enter an orbit around the planet, forming an accretion disk where the particles accumulate to form a satellite.

One might argue that, if the compositions are not similar enough, the satellite is probably not formed from a giant impact (since the materials of the accretion disk is a mix of materials from both the planet and the outside asteroid/object that collided with it). But it might not be the case. A study published to the MNRAS (Monthly Notices of the Royal Astronomical Society) pointed out that the impactor can contribute to up to 70% of that material, which means the difference in composition can be fairly significant. And another study using the data from the UAE’s Hope Mars orbiter indicated that the composition difference between Mars and Deimos could be smaller than expected. This puts the giant impact scenario on the table again.

In this article, we’ve briefly analyzed how the three possible satellite formation scenarios can potentially apply to the formation of the moons of Mars, and how there are two dominant theories, involving a somewhat chaotic satellite capture and a giant impact, respectively. The next step to hopefully resolving this problem might just be collecting more data from the moons of Mars. The Japanese MMX spacecraft, on a mission to collect samples from Phobos (one of Mars’s moons), is slated for launch in November 2026, and will bring back a sample back to Earth in 2031. By further analyzing the composition of the materials from the satellite, perhaps we might have a better comparison between the compositions of Mars and its moons so that we can identify the more likely scenario.

1. (n.d.). Mars Moons: Facts. Retrieved August 31, 2024, from https://science.nasa.gov/mars/moons/facts/
2. Kegerris, J. A. (2024, July 22). Origin of Mars’s moons by disruptive partial capture of an asteroid. Retrieved August 31, 2024, from https://arxiv.org/pdf/2407.15936
3. Mastrobuono-Battisti, A., Perets, H. B. (2017, May 3). The composition of Solar system asteroids and Earth/Mars moons, and the Earth–Moon composition similarity. Retrieved August 31, 2024, from https://academic.oup.com/mnras/article/469/3/3597/3795552
4. O’Callaghan, J. (2023, May 2). Where Did Mars’s Moons Come From? Retrieved August 31, 2024, from https://www.scientificamerican.com/article/where-did-marss-moons-come-from/