# Composition and Basic Physical Properties of the Phobos Surface: A Comprehensive Review
## 6 Characteristics and Physical Properties of Phobos Regolith
### Physical Properties of Regolith
#### 6.2.3 Porosity  
The low bulk density of Phobos (average value of 1857 kg/m\({}^{3}\), Table 3) can be explained by its relatively high porosity and/or the presence of light materials, such as water ice, within its interior [1; 68]. [PERSON] [1], to determine the porosity of Phobos, assumed that its rocky interior corresponds to a single material with a bulk density \(\rho_{a}\) (Table 4) and that there is no light material inside the pores. With this assumption, the porosity (\(n\)) needed to achieve the determined bulk density \(\rho_{b}\) (1857 kg/m\({}^{3}\), Table 3) can be expressed by the formula  
\[n=1-\frac{\rho_{b}}{\rho_{a}}, \tag{1}\]  
The calculated porosity does not account for the inherent porosity of the rocks constituting the moon but only corresponds to the void spaces between potentially constituting rock blocks (macroporosity). Using a broad range of density values for analogs that could potentially compose Phobos (Table 4), a macroporosity ranging from 17.47% to 46.64% was determined (Figure 3). Assuming an analog of hydrated carbonaceous material, a porosity of approximately 17.5% is obtained. This is less than the range of 40% to 60% reported for C- type asteroids constructed from hydrated carbonaceous material [76]. For analogs of silicate materials, the estimated porosity value is higher than that for hydrated carbonaceous material, reaching around 47% (SNC achondrites, upper limit, Table 4, Figure 22), similar to the obtained porosity value for black chondrite (around 45%). These values are significantly higher than the porosity range estimated for S-type asteroids (20-30% or less, except for near-Earth asteroids, which have a macroporosity estimated at around 40% [76]). Moreover, the density of Tagish Lake is lower than the volumetric density of Phobos, suggesting that achieving the average volumetric density determined for Phobos (1857 kg/m\({}^{3}\), Table 3) would require compressing the Tagish Lake material. Figure 22: The macroporosity of Phobos (dark green curve) estimated based on the bulk densities of analog rock materials, which it may hypothetically be composed of (Table 4), with an error range (dark-green dashed curve) (partially based on [1]). \begin{table}
\begin{tabular}{l l l l l l l l l l l} \hline \hline  & \multicolumn{2}{c}{**Bulk**} & \multicolumn{2}{c}{**Bulk**} & \multicolumn{2}{c}{**Range +/-**} & \multicolumn{2}{c}{**Source**} & \multicolumn{2}{c}{**Thobos**} & \multicolumn{2}{c}{**Range +/-**} & \multicolumn{2}{c}{**Crain**} & \multicolumn{2}{c}{**Range**} & \multicolumn{2}{c}{**Total Porosity of**} \\
**Types of Analog Materials 1** & **Density** & **[g/cm\({}^{3}\)]** & **Range +/-** & **Macporosity** & **Range +/-** & **Density** & **+/-** & **Source** & **Phobos** & **Range +/-** \\  & **[g/cm\({}^{3}\)]** & **[g/cm\({}^{3}\)]** & **[g/cm\({}^{3}\)]** & **[g/cm\({}^{3}\)]** & **[g/cm\({}^{3}\)]** & **[g/cm\({}^{3}\)]** & **[g/cm\({}^{3}\)]** & **[g/cm\({}^{3}\)]** & **[g/cm\({}^{3}\)]** \\ \hline \multicolumn{11}{l}{Hydrated Carbonaceous Chondrite (CM)} & 2.25 & 0.08 & [76] & 17.47 & 3.03 & 2.9 & 0.08 & [76] & 35.97 & 1.86 \\ \multicolumn{11}{l}{Tingels Lake} & 1.64 & 0.02 & [78] & – & – & 2.72 & – & [78] & 31.73 & – \\ \multicolumn{11}{l}{Black Chondrite} & 3.39 & 0.045 & [79] & 45.22 & 0.88 & 3.5 & 0.025 & [79] & 46.94 & 0.62 \\ \multicolumn{11}{l}{HED Achondrites (Asteroidal)} & 2.62 & – & [80] & 29.12 & – & 2.99 & – & [80] & 37.89 & – \\ \multicolumn{11}{l}{HED Achondrites (Asteroidal)} & 3.37 & – & [80] & 44.90 & – & 3.51 & – & [80] & 47.09 & – \\ \multicolumn{11}{l}{(Upper Limit)} & 2.83 & – & [80] & 34.38 & – & 3.08 & – & [80] & 39.71 & – \\ \multicolumn{11}{l}{Meteoreorites (Lower Limit)} & 3.48 & – & [80] & 46.64 & – & 3.73 & – & [80] & 50.21 & – \\ \multicolumn{11}{l}{Meteoreorites (Upper Limit)} & \multicolumn{1}{l}{} \\ \hline \hline \end{tabular} \({}^{1}\) Types of analog materials adopted from Roseblatt [1], except for SNC achondrites.  
\end{table} 
Table 4: Densities of materials that could form Phobos and their corresponding porosities.