# Composition and Basic Physical Properties of the Phobos Surface: A Comprehensive Review
## 6 Characteristics and Physical Properties of Phobos Regolith  
### Characteristics of Phobos Regolith  
The surface of Phobos, similar to the surfaces of solid bodies in the Solar System, is covered with regolith--a fragmented and unconsolidated material composed of crushed rock and soil. Regolith is formed through various impact processes, erosion, and weathering, resulting in a mixture of particles with different shapes and sizes. The outer layer of regolith is continually modified by solar wind, cosmic radiation, and meteoroid impacts. However, the processes involved in regolith formation on small bodies due to these factors have not been sufficiently studied [16].  
The formation process of regolith on Phobos takes into account multiscale, multistage, and repetitive processes, including impact fragmentation, material melting, cratering with ejection, and mixing (accumulation of material both from the direct settling of ejecta and the return of high-velocity ejected fragments to the space near Mars), as well as cosmic weathering [12; 13]. As a result, Phobos' surface should be covered by an unsorted mixture of solid rock fragments and material melted under the influence of impacts, with sizes ranging from micrometers to meters [12].  
However, unlike the Moon, Phobos' orbital configuration allows almost all ejecta to re-impact it. Within approximately 1000 years after the collision that formed the Stickney crater, material was ejected at high speed from Stickney, then re-impacted Phobos' surface, strongly transforming it and creating a new regolith with a global thickness of 30-50 m (about 50% of the new regolith deposit produced by Stickney is the primary ejection from Stickney, and the remaining 50% is the ejecta from secondary impacts on Stickney) [51; 54].  
Additionally, an important characteristic of Phobos' regolith is the suggested shortage of particles with sizes <300 \(\upmu\)m. Particles <300 \(\upmu\)m are particularly susceptible to orbital eccentricity changes, and if they do not re-impact Phobos shortly after being ejected, they usually get lost in interplanetary space or drift away from the orbit into the Martian atmosphere [12; 13].  
The proximity of Mars is a significant factor influencing the properties of regolith on Phobos. Part of the ejected material from the surface of Mars most likely settles on the surface of Phobos, introducing Martian material into its regolith [16]. It is estimated that the concentration of Martian material is 250 ppm in the upper part (0.5 m) and one to two orders of magnitude lower in concentration at greater depths [12]. Analysis of the work of [PERSON] and [PERSON] [13] indicates that the most probable mass concentration of Mars-ejected fragments in the contemporary Phobos regolith is around 250 ppm within the range of 20-1250 ppm.  
Moreover, the thickness of Phobos' regolith, determined based on crater morphology, ejecta distribution, smooth sediment morphology, geological feature morphology such as boulders and lineaments, and spectro-photometric changes on the surface [55], is as follows:  
* 5 m minimum;
* 100 m maximum [12; 56].  
Moreover, the size of Phobos' craters with complex morphology indicates the presence of layered material, implying the existence of a regolith layer covering the bedrock. Layers within the regolith are likely not continuous but resemble lenses. The presence of layers in the regolith is also suggested by observations of material ejected from craters, showing varying apparent brightness [12].  
Additionally, the observed population of impact craters indicates that the upper several hundred meters to several kilometers of Phobos' surface are heavily fractured [12].