Understanding the formation of Mars’ satellites

The formation process of the Solar System’s satellites is not well understood. In the case of Mars, there are several hypotheses. The first is that Phobos and Deimos may be asteroids captured by an unknown mechanism. But they orbit exactly above the Martian equator, making this hypothesis unlikely. The second is to imagine a formation scenario similar to that of our own Moon. Around 4.5 billion years ago, the young Earth and a protoplanet would have collided in a giant impact, producing a lot of debris that formed a disk around the proto-Earth exactly in its equatorial plane. The latter cooled and condensed, creating a protolune. It’s a pleasing hypothesis, as it’s a simple way of creating a satellite in the equatorial plane. This idea applied to Mars is reinforced by the presence of a gigantic impact basin in its northern hemisphere. This has served as a guideline for our study. But it remains to be explained why Phobos and Deimos are so different from our Moon, why Phobos orbits so close to Mars (at 3 Martian radii) and Deimos is so far away (at 7 Martian radii) with a large gap in between.

Originally a large moon

We ran a series of calculations on the S-CAPAD platform, including a simulation of a giant collision on Mars. We noticed that the debris disk created a large, compact and massive ring between 1 and 3 Martian radii, leaving a very tenuous but extensive disk of debris on the outside. As Phobos and Deimos must have formed around 6 Martian radii, we studied the dynamics of this thin disk to see if the debris accreted spontaneously. This first attempt was unsuccessful. We had to find the ingredient that would disrupt this system and trigger accretion. We then studied the role of the inner disk and found that very large satellites form at its outer edge. This was the spark: perhaps one of these large moons from the inner disk is stirring up the outer disk as it tides away with it?
Coupled simulations of the two disks show that the large inner moons formed at 3 Martian radii agitate the outer disk sufficiently to pack accretion into exactly 2 moons in particular regions called “resonances”. Martian tides then move the satellites. All the moons formed below the synchronous orbit, including Phobos, fall towards Mars, but Deimos, having formed just above the synchronous orbit, is pushed outwards. After 4 billion years of evolution, only Phobos and Deimos remain in their current positions.