Will MMX find evidence that Mars once had rings?

A key question for the Martian Moons eXploration mission is how the two moons of Mars were formed. Competing theories suggest that the satellites are either captured asteroids or that they were formed during a giant impact with the red planet (video explainer).

Evidence for the impact scenario is that both the Martian moons have nearly circular orbits in the Martian equatorial plane. This would be a natural consequence of the moons coalescing from a disc of debris ejected during a violent collision with the Martian surface. It would be more difficult to replicate if the pair were once asteroids, as the process of capturing the moons should result in different elliptical and inclined paths around the planet.

But despite the moon orbits more closely agreeing with formation via impact, there are a few anomalies. The moons sit either side of the distance for a synchronous orbit around Mars, meaning that Phobos moves around Mars faster than the planet rotates while the outer Deimos orbits Mars more slowly. The resultant tidal tugs on the moons from Mars’s gravity mean that Phobos is gradually being pulled closer to Mars while Deimos is pushed outwards. Ultimately, Deimos is expected to escape Mars’s gravity entirely and cease to be a moon, while Mars’s gravity will eventually rip Phobos to pieces.

Both the outward movement of the orbit of Deimos and the timescale for the demise of Phobos present challenges to the impact scenario. The Martian crust shows evidence of a large impact, with a difference in elevation between the two hemispheres. This could be caused by a moon-forming impact over 4.3 Gyrs (4,300,000,000 years) ago. If that impact created Phobos, then the inward-moving moon must have formed about twice as far out as its present location to be seen in its current orbit today. But such a migration would have meant the moon passed a 2:1 resonance with Deimos as it moved inwards, and that should have left visible evidence.

Resonances occur when the ratio of the orbital periods of planets or moons in the same system is a small integer. In a 2:1 resonance, Phobos would orbit Mars twice for every orbit by Deimos. This creates a regular gravitational pull between the moons that is similar to pushing a child on a swing. Like a swing, the regular nature of the force creates a strong response. In the case of the 2:1 resonance between the Martian moons, the expected results is the smaller Deimos would be pushed onto a strongly elliptical orbit. Yet, the moon shows no sign of this having occurred. However, if Phobos formed inside the 2:1 resonance position, then it should have moved far past its current location.

An additional issue is that the disc formed during a giant impact with Mars should not extend out as far as the distance to the synchronous orbit. This is a challenge for the orbits of both moons, but especially that of Deimos which must have formed beyond the synchronous orbit position to be moving outwards.

An intriguing solution to this conundrum is that over the last 4 Gyrs, Mars has had a series of rings. The idea (Nature Geoscience, Hesselbrock and Minton, 2017) proposed that a giant impact created a massive debris disc around Mars. This disc was heavy enough to form both Deimos and an inner moon far more massive than Phobos. Once the remaining debris dispersed, the inner moon moves inwards until Mars’s gravity disrupted its structure. The moon was shredded to form a ring of debris around the planet, which spread outwards to form a new disc that coalesced into a new inner moon. This second generation Martian moon was in turn pulled towards Mars, and disrupted to create yet another ring and a third generation moon. This would make Deimos the direct product of the giant impact with Mars, but Phobos would be a later generation of moons born in the series of ring-forming episodes.

One instant advantage with this scenario is it explains the small size of the Martian moons. An impact large enough to create a hemisphere dichotomy on Mars should have ejected enough material to form far more massive moons than Phobos and Deimos. But each successive cycles of moon shredding would lose material to Mars, leaving only a low mass ring with which to form Phobos.

Now a research paper this month led by Matija Cuk at the SETI Institute suggests evidence for this ring-forming scenario may be seen in the orbit of Deimos.

The paper presents computer simulations of moon generation prior to Phobos but after the initial impact that created Deimos. The pre-Phobos moon sits in a massive disc created from the ring of the shredded remains of its predecessor. Interactions between the disc and the embedded moon push the moon initially outwards until pre-Phobos reaches a 3:1 resonance with Deimos, orbiting Mars three times for every orbit of Deimos.

The regular tugs between the moons in a resonant orbit create a very stable configuration. As pre-Phobos continues to move outwards, Deimos therefore also moves outwards to maintain the resonant position. This connection between the pre-Phobos moon and the generation of moons before it, takes Deimos past the synchronous orbit location and onto its current trajectory.

Unlike the 2:1 resonance, the 3:1 resonance has been found to increase the orbit of the smaller body’s inclination but not cause a large change in the orbit’s ellipticity. As a result, the orbit of Deimos tilts by a couple of degrees but does not become strong elliptical. This is exactly what is seen in the current orbit of the moon. Once the inclination has developed, the resonance between the moons breaks. The debris disc disperses and removes the outward pull on pre-Phobos. The inner moon then feels only the tidal tug from Mars and begins its fatal descent towards the planet.

Phobos forms from the shredded remains of the pre-Phobos moon. The new disc initially also pushes Phobos outwards, but the moon’s mass is too low for a resonance with Deimos to have any impact on either moon. The disc once again disperses and Phobos begins its march inwards to where it is today.

While the orbits of the two moons support this multi-ring hypothesis for Mars, far more compelling evidence would come from a close-up analysis of the moons. Notably, if this scenario is correct, Phobos should be significantly younger than Deimos, having formed after multiple generations of moons were born and disrupted.

A sample collected by MMX from Phobos will provide information on the strength of the moon’s surface, indicating how easy it is to form a deep crater on the moon. This will link the visible craters on Phobos to the size fo the impactor that made them. By knowing the expected rate of impacts (found through models of the Solar System), the age of the moon can then be accurately determined. This was done recently for asteroid Ryugu that was visited by JAXA’s Hayabusa2.

If the age of Phobos turns out to be significantly younger than the probable date for a giant impact with Mars, it will be compelling evidence that Mars is a planet with a history of repeated rings.

Further reading:

Journal paper by Cuk et al., 2020: Evidence for a Past Martian Ring from the Orbital Inclination of Deimos

How were the moons of Mars formed? Video explainer