# Composition and Basic Physical Properties of the Phobos Surface: A Comprehensive Review
## 5 Surface Morphology of Phobos  
### Craters  
On the surface of Phobos, there are 1300 craters with a diameter greater than 200 m. Among them, 70 craters have a diameter greater than 1 km, and 26 have a diameter greater than 2 km [12]. Seventeen craters have been officially named [44]. The largest crater is Stickney, with a diameter of approximately 8 km (Figure 10) [12] (other researchers provide slightly different dimensions, approximately \(\sim\)11 km--[45]; \(\sim\)10 km--[1]; \(\sim\)9.5 km--[46]). Comparisons between the surface of Phobos and the Moon suggest that craters with diameters in the decimeter and centimeter range should also be present, and impact microcraters should be found on the surfaces of rock fragments. Despite Phobos being a very small celestial body, it is covered with craters that resemble those found on larger planets. Craters with diameters of 100-200 m or smaller are very similar in size and shape to craters on the Moon [47]. Most craters are of impact origin, although some small craters may be features formed by the penetration of regolith into subsurface fractures [12; 46]. Most of the craters on Phobos have a bowl-shaped morphology (Figure 11), while some display complex internal features (approximately 60 craters). Three types of craters with complex morphology can be distinguished: flat-floored craters, craters with a central mound, and concentric craters (Figure 12). Among craters with complex internal morphology, concentric craters are the most common, while craters with a flat floor and a central mound are relatively rare [12]. Figure 11: A fragment of the HRSC (High-Resolution Stereo Camera) image h2780 shows the crater Drunlo (bottom left corner, diameter 4.2 km) and the crater Clustril (top right corner, 3.4 km) [12]. Figure 10: The HRSC (High-Resolution Stereo Camera) image h0756 depicts craters and grooves [12]. Craters with a diameter greater than 200 m also exhibit external morphological variability (Figure 13). A relatively small number of craters have an elliptical shape (Figure 13a). Some craters feature polygonal planimetric outlines, such as the square-shaped Hall crater (Figure 13b). This crater has two opposite linear edges coinciding with the undulating surface features of the substrate and two opposite linear edges approximately perpendicular to the previous ones, parallel to the system of grooves. The planimetric outlines of the 4.6 km long Drunlo crater (Figure 13c) show a certain triangular shape, with two directions coinciding with the groove systems [12]. Utilizing the digital terrain models (DTMs) of [PERSON], [PERSON] et al. [12] classified craters with a diameter greater than 2 km into three morphological classes based on the ratio of d/D (depth to diameter) and the maximum slope/steepness of the inner walls (Figure 14). The following classes were distinguished [12]:  
* Class 1--d/D > 0.1, maximum slope of inner walls > 20\({}^{\circ}\) (may rich 40-50\({}^{\circ}\));
* Class 2--0.05 < d/D < 0.1, maximum steepness of inner walls 10-20\({}^{\circ}\);
* Class 3--d/D < 0.05, maximum steepness of inner walls 10\({}^{\circ}\). According to this classification, 38% of Phobos' craters belong to class 1, 29% to class 2, and 33% to class 3. This subpopulation of Phobos' craters (craters with a diameter greater than 2 km) has a significantly higher number of craters with gentle slopes compared to craters on the Moon. This may be due, in part, to Phobos' very low gravity [12]. Figure 14: Geomorphic map of Phobos [12]. Figure 13: Portions of HRSC images h8974, h9574, and h2780: (**a**) elliptical crater with lengths of 750 m and 250 m, (**b**) Hall crater with a diameter of 6.2 km, and (**c**) Drunlo crater with a length of 4.6 km [12]. It is believed that the crater population on Phobos is close to equilibrium, a state in which a further increase in crater density is impossible because each time a new crater forms, a previously formed one is destroyed [12]. The density of craters with a diameter of 41 km is close to the limit of their occurrence.