# Composition and Basic Physical Properties of the Phobos Surface: A Comprehensive Review
## 3 Hypotheses of Formation
### In Situ Formation Hypothesis  
The challenges in reconciling the capture hypothesis with the current orbital properties of Mars' moons motivate the exploration of alternative hypotheses. One such hypothesis proposes that the moons of Mars originated from an equatorial disk of remnants containing material beyond the Martian realm. This alternative hypothesis suggests that the Martian moons formed from an equatorial disk of debris, which also included matter beyond the Martian vicinity [23]. In this hypothesis, the parameters most challenging to explain in capture scenarios (orbital eccentricity and inclination) are considered natural outcomes of the accretion of the circumplanetary disk [1,14,24]. Additionally, if Phobos originated from loosely aggregated material from the accretion disk, its small mass (low bulk density) would be easily explainable, implying likely high porosity [1,14,25]. These characteristics would account for the formation of major morphological features on Phobos' surface, such as the Stickney crater [14,26] and some of its grooves [14,27]. There are two main scenarios for in situ formation. One of them posits that a body significantly more massive than Phobos was captured by Mars; subsequently, its orbit rapidly decayed due to tidal forces, and the object was destroyed upon crossing the Roche limit [28]. From the resulting debris below the Roche limit, a ring formed around Mars, giving rise to small moons. However, the Roche limit is approximately 3R\({}_{\rm M}\), well below the synchronous limit, which is around 6R\({}_{\rm M}\). It is challenging to identify a mechanism by which moons formed near the Roche limit could migrate far from Mars and then remain in orbit for an extended period [14,29]. Another possibility is the formation of Phobos resulting from a giant impact [1,24]. The scenario of the Martian moons forming due to a giant impact is one of the more recent hypotheses. [PERSON]'s research [30] suggests that a large body (from one-quarter to one-third the size of Mars) collided with proto-Mars at least 4 billion years ago, ejecting fragmented material into space. Simulations by [PERSON] et al. [31] indicate that impacts forming basins the size of Borealis could eject a sufficient amount of material, creating an accretion disk from which the Martian satellites formed (accretion in the strong tidal regime hypothesis). The key factor in this formation scenario is the rotation imparted to the planet by the impactor because it determines the location of the synchronous boundary (whether it is within or beyond the Roche limit). A slow rotator, like Mars post-impact, corresponds to a relatively distant synchronous boundary. Moons that form near the Roche limit, due to the viscous spreading of the disk, migrate outward through interactions with the remaining disk but eventually settle back towards Mars under the influence of tidal forces. It is expected that only small moons formed near or beyond the synchronous boundary will remain in orbit for an extended period [14]. Phobos and Deimos may be the last surviving moons that emerged from the formed accretion disk (Figure 2). The current rotation of Mars suggests a relatively massive impactor, which could also be responsible for the formation of the Borealis basin [32]. The giant impact scenario is also consistent with telescopic observations of the surfaces of both moons, suggesting a composition different from that of Mars. In this case, the composition of the disk materials is treated as a mixture of Martian and impactor materials (Figure 3) [14]. Numerical simulations used to replicate the entire scenario of Phobos' formation, from the initial impact to accretion and the long-term evolution of the two moons, provide better agreement with their current orbits than the previous capture hypothesis and remain consistent with observations regarding their composition [14]. Figure 2: Schematic of the formation of Phobos and Deimos—giant impact scenario.