# Composition and Basic Physical Properties of the Phobos Surface: A Comprehensive Review
## 5 Surface Morphology of Phobos
### Grooves  
Another type of morphological feature characterizing the surface of Phobos is the numerous grooves (Figures 14 and 15) [12; 48; 49; 50; 1, 51]. The origin of Phobos' grooves has been a subject of discussion since their discovery. Most likely, the grooves are fractures appearing on the surface of Phobos during strong impacts. Three different mechanisms for the formation of grooves are considered [12; 46; 49; 50]:  
* Cracks/faults formed by the shockwave, due to tidal forces and resistance forces during capture;
* Chains of secondary impact craters or ejecta from Stickney (traces of rolling and bouncing boulders);
* Chains of secondary impact craters formed due to large primary impacts on Mars (chains of craters formed by ejecta from large impact craters on Mars). Presently, it is believed that the grooves on Phobos result from a combination of processes such as impact-induced fracturing with low-velocity ejections, the re-accumulation of ejecta, and tidal stresses [52]. Phobos' grooves are elongated, linear, or nearly linear depressions, typically with a width of 100-200 m and a depth of 10-30 m. Their length usually ranges from a few kilometers, but some may extend up to about 20 km [12; 46; 53]. At least one groove can be traced continuously for 168\({}^{\circ}\), approximately 30 km [49]. Grooves exhibit variations in appearance. Chain-like groove formations have been identified, formed by connecting depressions/craters of roughly the same diameter. Grooves with serrated edges and almost linear groove formations have also been observed. Figure 16 illustrates the morphological variations in grooves and the relationships between the grooves and the craters Drunlo (D = 4.2 km) and Clustril (D = 3.4 km) [12]. Figure 15: Phobos (**a**) view of the northern hemisphere (Mars Express HRSC Image 401-20080729-5851-6-na-1b, orbit 5851), (**b**) sub-Martian hemisphere, (**c**) leading orbital hemisphere, (**d**) trailing orbital hemisphere, and (**e**) anti-Martian hemisphere [51]. There are several \"families\" of grooves, with grooves within a family being approximately parallel to each other with an accuracy of a few degrees. Twelve such families have been identified by [PERSON] and [PERSON] [49], labeled from A to L. The surfaces of grooves from different families are oriented at various angles to each other, causing the grooves to form nearly rectangular grids in some areas (Figure 17) [49]. At the intersections of grooves with different orientations, there are no clear lateral displacements (Figure 18), unlike what is often observed at intersections of different tectonic features. Some families are spatially extensive, covering areas with a meridional or longitudinal extent greater than 90-120\({}^{\circ}\)[12]. Figure 16: The HRSC images depict the craters Drunlo (bottom left) and Clustril (top right), showing fragments captured during orbits 0756 (**a**), 2780 (**b**,**f**), 2813 (**c**), 3310 (**d**), and 5851 (**e**) [12]. Figure 17: The image of Phobos illustrates three “families” of grooves. The orange color represents the C groove family, the yellow color represents the D groove family, and the purple color represents the F groove family. Groove families C and F intersect nearly perpendicularly near the center of the leading hemisphere, while family D (yellow arrows) intersects the other two at an angle of approximately 45\({}^{\circ}\). The right half of the image, where groove families overlap, constitutes the “overlap zone”. Image credit: ESA/DLR/FUB, h4847 nd [49]. Figure 18: Intersections of grooves southwest of the Drunlo crater. Fragments of images taken during orbits 6906 (**a**,**b**), 4847 (**c**,**d**), and 2813 (**e**,**f**); on the left—original images; on the right—stretched images to enhance contrast [12]. Grooves similar to those on Phobos are also observed on other small bodies, such as Eros, Lutetia, and Vesta. In terms of morphology and size, the grooves on Eros most closely resemble those on Phobos.