A giant impact scenario to form the Martian moons?

Abstract:

The formation of Martian moons is one of the most enigmatic in the solar system. Long considered to be asteroids captured by Mars, a giant impact origin has recently been proposed, motivated by the exploration of Phobos by Mars Express (Paetzold et al., 2025). The objective of this study is to review recent formation scenarios proposed within the framework of this giant impact hypothesis.

 

1- The formation of two small moons from a giant impact

The asteroid capture scenario fails to explain the current near-circular and near-equatorial orbits of both moons (e.g., Rosenblatt, 2011). Alternatively, the giant impact scenario naturally explains the near-equatorial orbits by forming the two moons in an impact-induced debris disk (Craddock, 2011). Rosenblatt et al. (2016) and Canup and Salmon (2018) have shown that it is possible to accrete two small moons of the mass of Phobos and Deimos into this debris disk, on either side of the synchronous boundary. However, these small moons have elliptical orbits, and to match the near-circular orbit of Deimos requires a tidal dissipation rate of its interior that is not consistent with a rocky composition (Rosenblatt et al., 2016). Moreover, the composition of these small moons should be a mixture of materials from Mars and the impactor, which is not consistent with a purely primitive composition as suggested by remote sensing data from the surface of Phobos and Deimos.

 

2- The question of primitive composition

The giant impact is expected to eject fully molten rocky material into Martian orbit, composed of similar amounts of Martian and impact materials (Hyodo et al., 2017). This mixed material crystallizes primarily into olivine and pyroxene phases (Pignatale et al., 2018), which have not yet been identified in the remote sensing data (e.g., Wargnier et al., 2025). However, the ejected material is also expected to contain a small amount of volatile compounds (Pignatale et al., 2018), depending on the impactor composition, which could mimic the remote sensing signature of a primitive composition.

Kegerreis et al. (2024) also proposed the disruptive capture of an asteroid by the planet's tidal forces, leaving a cloud of primitive material in orbit. However, the authors did not study the accretion of two small moons from this cloud.

 

3- The question of the long-term orbital evolution of the Martian moons

Bagheri et al. (2021) proposed a large tidal dissipation rate in the Martian system, leading to a faster orbital evolution of Phobos and Deimos. They found that both moons could be at the same distance from Mars 1–3 Gyr ago, suggesting that they could originate from the breakup of a single progenitor. Nevertheless, the origin of such a progenitor is not explained in this model and the breakup process yields to the formation of several smaller fragments instead of just two (Hyodo et al., 2022). However, such a large tidal dissipation rate merits revisiting the orbital evolution of two small bodies formed around Mars or even captured by Mars.

 

4- Could Phobos have formed after Deimos?

Under the giant impact scenario, Hesselbrock & Minton (2017) propose that Phobos could have formed after Deimos. In this model, Phobos would result from multiple cycles of a ring-moon system. This model is consistent with the overall shape of Phobos (Hu et al., 2020) and the slightly inclined orbit of Deimos above the Martian equator, which could reflect gravitational interaction with a larger, former inner moon (Cuk et al., 2020). Nevertheless, a faint dust ring is expected to remain around Mars today (Madeira et al., 2023), which has not been observed so far. Furthermore, the retention age of Phobos surface craters is as old as 4.3 Gyr (Schmedemann et al., 2014), which is inconsistent with a younger Phobos according to this model.

 

5- How could the Martian Moon eXplorer data help discriminate between these formation scenarios?

The sample collected at Phobos’ surface will provide determined its composition (Usui et al., 2020). However, this surface could be contaminated by ejecta from the Martian surface, making it difficult to distinguish between the impact debris disk model and the asteroid disruptive capture model. One solution to resolve this ambiguity is to measure the magnetic properties of the sample. Indeed, if the material forming Phobos is condensed in Martian orbit, it should record the strong magnetic dipole field of the early Mars (Rosenblatt et al., 2024). If the Circum-Martian Dust Monitor measured a low-dense ring of material around Mars (Kobayashi et al., 2019), this would support the ring-moon cycle model. The geodesy experiment will allow for better constraint on the internal structure of Phobos (Yamamoto et al., 2024), thus the modeling of its tidal dissipation properties. The MMx data will open the way to exciting research aimed at answering the following question: Is Mars capable of maintaining small accreted moons in orbit throughout its history, or even capturing asteroids, to ultimately form the current Martian lunar system?

 

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