# Composition and Basic Physical Properties of the Phobos Surface: A Comprehensive Review
## 4 Surface Composition of Phobos  
### Spectral Properties  
Visible spectra obtained through spectrometer imaging in the near-infrared range are commonly employed for the identification or, at the very least, constraint of the surface mineralogy of Solar System bodies by identifying highly diagnostic absorption bands of minerals [35]. Available spectroscopic data indicate the presence of two distinct materials on Phobos' surface. Data from Mars Express and Mars Reconnaissance Orbiter have provided additional information regarding the properties of these materials and their spatial relationship. The surface of Phobos reveals areas with different spectral slopes, defined as red and blue units. The red unit covers the majority of the moon's surface and is characterized by a low albedo and a red spectral slope. The blue unit becomes apparent near the Stickney crater (Figure 4) and exhibits spectral properties similar to the prevalent red unit but with a smaller spectral slope [10]. Figure 3: Schematic representation of chemical modeling; the scenario of a giant impact [33] (the yellow region represents the part of the disk within the Roche limit [34]). The spectra of Phobos' surface reflectance in the near-infrared and visible wavelengths (approximately 0.4 to 4.0 \(\upmu\)m) show an increasing slope towards the infrared region, matching several low-albedo primitive asteroids [14]. Phobos' reflectance spectra have been confirmed by missions such as Mars Reconnaissance Orbiter, Mars Express, and Rosetta, as well as observations obtained using the Hubble Space Telescope and the ground-based 4 m Mayall Telescope [14]. So far, no satisfactory spectral match has been found between Phobos and meteoritic material considered representative of primitive asteroids. Additionally, the reflectance spectra do not clearly show absorption bands (Figure 5), which provide diagnostic information regarding the composition of carbonaceous, silicate, or hydrated material. This may be related to cosmic weathering, which can suppress such bands on airless bodies [14]. On the Moon, this process is responsible for darkening, reddening, and damping absorption bands in the reflectance spectra of silicate material [35]. The darkening, reddening, and damping of absorption bands have also been observed in the spectra of Phobos (Figure 5). Figure 4: (**a**) View of Stickney crater and its eastern rim; (**b**) the same view with key geological features highlighted. Craters containing red and blue materials are outlined in red and blue, respectively. Red lines indicate outcrops of reddish material along the edges of grooves. White lines show the boundaries of geological units. HiRISE_PSP_007769_9010_IRB image synthesized from photos taken in the infrared (800–1000 nm), red (550–850 nm), and blue-green (400–600 nm) channels [12] (numbers from 1 to 5 according to [12]). Numbers: 1 (and white arrows)—part of the massive landslide moving down from the W-NW sector of the crater inner walls; 2—the hummocky surface texture deposit; 3—the landslide body acquired sufficient momentum for the frontal part of the moving landslide mass to travel up the interior of the wall and onto the eastern rim of the crater; 4—the 700 m crater, whose interior, eastern rim, and the area further to the east are red, while the area around the crater is blue, except the part to the east; 5—crader about 100 m in diameter is surrounded by a relatively wide (200–500 m) red halo, interpreted as its ejecta [12]. Figure 5: Reflectance spectra obtained by instruments: (**a**) CRISM from Mars Express; (**b**) OMEGA Mars Reconnaissance Orbiter [36]. In the spectra of Phobos, an uncertain and very weak absorption band around 0.65 \(\upmu\)m has been detected (Figure 6), which can be interpreted as a potential signature of composition. However, the possibility of cosmic weathering effects cannot be ruled out [37].