# Composition and Basic Physical Properties of the Phobos Surface: A Comprehensive Review
## 7 Conclusions
Additionally, it is estimated that the average grain size of the regolith should be smaller than 2 mm in most regions of Phobos (thermal inertia of the surface 40-70 Jm\({}^{-2}\).K\({}^{-1}\).S\({}^{-1/2}\) and 20-40 Jm\({}^{-2}\).K\({}^{-1}\).S\({}^{-1/2}\)) [61; 62]. Assuming a surface thermal inertia of 55 Jm\({}^{-2}\).K\({}^{-1}\).S\({}^{-1/2}\), the most probable grain diameter should be below 1 mm [63]. The density values of Phobos, obtained based on the gravitational mass estimates (GM) from [PERSON] et al. [8] and the volume provided by [PERSON] et al. [7], fall within the range of 1845-1880 kg/m\({}^{3}\) (1857 kg/m\({}^{3}\)--average value) (Table 3, Figure 20). Comparing the bulk density of Phobos to the densities of other asteroids and meteorites, it can be observed that it is similar to the density of C-type asteroids. However, [PERSON] et al. [75] estimated the bulk density of the top several tens of centimeters of the surface layer to be around 1600 \(\pm\) 300 kg/m\({}^{3}\). This density is similar to the densities of Cl chondrite meteorites and the Tagish Lake meteorite, which is most likely a fragment of a D-type asteroid. Phobos is suspected to be highly porous, indicated by the presence of the Stickney crater on its surface [1; 25]. The formation of such large craters on a small body requires significant porosity, preventing the body from being destroyed during the impact process [81]. Utilizing grain densities (Table 4), the total porosity of Phobos was determined to be in the range of 31.73-50.21% (Table 4, Figure 23). These values closely mirror the porosity of C-type asteroids (40-60%). Although scientific research conducted through a spacecraft orbiting Mars (currently or in the past) has provided valuable information about Phobos, there are still gaps in our knowledge about it. Despite attempts to estimate Phobos' composition (spectroscopic data, high-resolution imaging), its composition remains unknown. It is known that two main materials (red and blue units) occur throughout its surface, but their properties, distribution, and spatial relationships are unknown. Additionally, the physical parameters of the regolith are only approximate, and its origin remains an open question. Acquiring detailed knowledge about Phobos would allow, among other things, resolving questions about its origin, understanding the geochemical processes occurring on it, comprehending the early geological history of Mars' environment, and preparing for future -crewed missions. However, determining the composition of Phobos, and, consequently, its origin, requires additional information about its mineralogical, elemental, and isotopic composition, which would need to be obtained through in situ studies or collected samples. Conceptualization, M.K., A.S. and M.C. ; methodology, M.K. and A.S.; formal analysis, M.K. ; investigation, M.K. and A.S.; resources, M.K. ; writing--original draft preparation, M.K. and M.C. ; writing--review and editing, M.C. ; visualization, M.K. and A.S.; supervision, M.C. ; project administration, A.S.; funding acquisition, M.C. All authors have read and agreed to the published version of the manuscript. This research received no external funding. Not applicable. The authors declare no conflicts of interest.