# Composition and Basic Physical Properties of the Phobos Surface: A Comprehensive Review
## 7 Conclusions  
In the process of evolution, all planets and most of their satellites undergo various changes due to external factors and endogenic processes. These processes have radically transformed the building blocks of planets and almost entirely altered the original material. However, smaller bodies in the Solar System, such as Phobos, due to their small size and normal levels of radioactive isotopes, exclude internal heat and endogenous tectonic activity. Therefore, these bodies serve as a source of raw materials similar to the primordial cloud substance from which the planets of the Solar System originated ([PERSON] et al., 2016). Additionally, each planetary system exhibits unique features. For instance, the Martian system lacks a single large moon, like Earth, or multiple moons of varying sizes, as found around giant planets. Instead, it possesses two small moons: Phobos and Deimos. The formation mechanism of such a system is still under investigation ([PERSON] et al., 2016). There are two main hypotheses explaining the origin of Phobos, the first being the capture hypothesis, e.g., ([PERSON] et al., 2016; [PERSON] et al., 2016), and the second, the in situ formation hypothesis, e.g., ([PERSON] et al., 2016; [PERSON] et al., 2016). Currently, the capture hypothesis is considered less likely. This is associated, among other factors, with the fact that dynamic capture models require specific conditions, including aerodynamic resistance in the early proto-atmosphere of Mars ([PERSON] et al., 2016; [PERSON] et al., 2016; [PERSON] et al., 2016). On the other  
Figure 24: The density of the rocky material as a function of porosity and water ice content according to equation (2) (modified graph from [PERSON] et al. (2018)). hand, numerical simulations conducted to replicate the complete scenario of the in situ formation hypothesis (impact, accretion, and long-term evolution of two moons) show better agreement with the current orbits than the capture hypothesis, and they are consistent with observations regarding their presumed composition [14]. Despite the lack of precise data on Phobos's composition, scientists, based on hypotheses about its formation, have proposed a hypothetical composition. If Phobos is a captured asteroid, then most of its mass may consist of primitive carbonaceous material (possibly a cousin of D-type bodies from the outer asteroid belt). However, if Phobos formed due to a giant impact (high-energy collision), it should be composed of roughly half Martian material and half impactor material [34]. The available spectroscopic data indicate the presence of two distinct materials on the surface of Phobos, namely areas with different spectral slopes--red and blue units. The red unit covers the majority of the moon's surface, while the blue unit is exposed near the Stickney crater [10]. Spectral observations highlight the ambiguity of Phobos's composition, where reflectance spectra suggest the presence of carbonaceous material [37], while emissivity spectra indicate the presence of silicates [40]. The composition of Phobos remains uncertain. The current preferred model for Phobos, in line with available observational data, suggests that it is primarily a heterogeneous object consisting of a mixture of red and blue material blocks of varying sizes [12]. The surface of Phobos is characterized by numerous craters (1300 craters with a diameter greater than 200 m [12]), grooves (12 families of grooves [49]), as well as blocks and rocky boulders. Its surface is most likely covered with an unsorted mixture of solid rock fragments and impact-melted material, ranging in size from micrometers to meters, with a suggested deficiency in fractions smaller than \(<\)0.3 mm (e.g., [12]). However, it is not excluded that a thin layer of dust (grain size \(\sim\)0.1 mm) may cover its surface layers [59; 60].