Study finally explains Mars got its moons

Long believed to have been asteroids captured by Mars, the planet’s two moons were likely created by a collision between the planet and a second, smaller object, the French National Centre for Scientific Research (CRNS) has discovered.

In two new, independent and complementary studies, one of which was published in Monday’s edition of Nature Geoscience and another scheduled for future publication in The Astrophysical Journal, CRNS scientists and their colleagues reported that the shape and orbits of Mars’ moons eliminates the asteroid hypothesis, and that surface features are indicative of a collision involving an object approximately one-third the size of Mars.

Based on numerical simulations, they believe that Phobos and Deimos accreted from the outer part of a debris disc that formed following a giant impact on Mars. These models show that the moon formed from material in the denser inner disk and migrated outwards due to gravitational interactions with the disk. The ensuing orbital resonances resulted in the gathering of outer disk debris, ultimately forming two satellites approximately the same size as Phobos and Deimos.

Writing in Nature Geoscience, the authors explained that “the larger inner moons fall back to Mars after about 5 million years due to the tidal pull of the planet, after which the two outer satellites evolve into Phobos- and Deimos-like orbits. The proposed scenario can explain why Mars has two small satellites instead of one large moon. Our model predicts that Phobos and Deimos are composed of a mixture of material from Mars and the impactor.”

Findings rule out the possibility of asteroid capture

The asteroid hypothesis originated in part due to the small sizes and irregular shapes of Phobos and Deimos. However, even though they resembled asteroids, scientists were unable to explain how the planet captured them and turned them into satellites with nearly circular and equatorial orbits, the CRNS-led research team explained in a statement.

Similarly, a competing theory that Mars had been hit by a protoplanet in the late stages of its formation raised questions about why two moons would have formed rather than one. A third possibility suggested that Phobos and Deimos formed at about the same time as Mars. Again, however, this notion had issues: specifically, that the moons should have had close to the same composition as Mars, something that their low densities indicated was not the case.

The authors of the two new studies believed that they have found the answer: the moons were indeed the product of a giant collision that took place between Mars and a primordial body one-third its size. This collision would have occurred between 100 and 800 million years after the beginning of the planet’s formation, and the debris from the collision formed an extremely wide disk that included both a dense inner part of primarily matter in fusion, and a thin outer part that was comprised primarily of gas.

In the inner region of the disk, a moon about 1,000-times the size of Phobos formed. That moon has since disappeared, but its gravitational interactions acted as the catalyst for the accretion of debris that formed smaller, more distant moons, including Phobos and Deimos. Once the debris disk dissipated a few million years later, the tidal effects of the planets brought nearly all of the moons back down onto the planet. Only the two most distant ones survived.

Composition of the satellites supports early collision hypothesis

In The Astrophysical Journal study, another team of researchers further eliminated the asteroid capture hypothesis by showing that the light signatures given off by Phobos and Deimos are not compatible with that of the primordial matter that formed Mars, which included meteorites made up of ordinary chondrite, enstatite chondrite and/or angrite.

Based on this, and statistical arguments based on the compositional diversity of the asteroid belt, the researchers indicate that the moons had to have been formed as the result of a collisions, and their light signatures indicate that they are comprised of fine-grained, less-than-micrometer-sized dust particles.

While erosion from interplanetary dust can partially explain the small size of these grains, there also has to be another explanation: the moons had to have been made up of very fine dust grains to begin with. This could only have been the result of gas condensation in the outer area of the debris disk, not magma from the inner part. Furthermore, moons forming from fine grains would be extremely porous at their depths, which would explain their low densities.

The collision model proposed by these two studies could explain why the southern hemisphere of Mars has a higher altitude than the northern one, as the impact that caused Phobos and Deimos to form likely would have taken place in the Borealis basin. Observations and samples collected by upcoming missions to the Red Planet could confirm or invalidate this scenario, the CNRS said in a statement. Such missions are scheduled to begin within the next decade.

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Image credit: Labex UnivEarths / Université Paris Diderot

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