# Composition and Basic Physical Properties of the Phobos Surface: A Comprehensive Review
## 6 Characteristics and Physical Properties of Phobos Regolith
### Physical Properties of Regolith
#### 6.2.2 Density  
Measurements conducted as part of the radio-science experiment (MaRS) on Mars Express significantly improved the estimation of Phobos' mass [25; 64]. This improvement was attributed to both the elliptical orbit of MEX (Mars Express--ESA's mission to the Red Planet) and refined ephemerides for Phobos, derived from the Super Resolution Camera (SRC) on board MEX [1; 65]. The size of Phobos was more precisely determined thanks to images from the HRSC/SRC cameras [7; 66]. Utilizing gravitational parameter (GM) estimates derived from [PERSON] et al. 's work [8] (GM obtained from spacecraft tracking during flybys or long-term planetary motion integration) and the volume estimated by [PERSON] et al. [7] (5742 \(\pm\) 35 \(\mathrm{km^{3}}\)), the average volumetric densities of Phobos were determined (Table 3). Densities were calculated only for masses where the mean GM value fell within the 3\(\sigma\) range, with GM obtained from the close flybys of MEX 2008 ROB (7.11 \(\pm\) 0.09 \(\times\) 10\({}^{-4}\) \(\mathrm{km^{3}\cdot s^{-2}}\)). The resulting density values fall within the range of 1845--1880 \(\mathrm{kg/m^{3}}\) (1857 \(\mathrm{kg/m^{3}}\)--mean value) (Table 3, Figure 20). \begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline \multirow{2}{*}{**Database**} & \multicolumn{2}{c}{**GM**} & \multicolumn{2}{c}{**Error**} & \multicolumn{2}{c}{**Source of**} & \multicolumn{1}{c}{**Mass 1**} & \multicolumn{1}{c}{**Error**} & \multicolumn{1}{c}{**Volume**} & \multirow{2}{*}{**Error**} \\  & \(\times\)**10\({}^{-4}\) \(\mathrm{km^{3}\ s^{-2}}\)** & \(\times\)**10\({}^{-4}\) \(\mathrm{km^{3}\ s^{-2}}\)** & & **Adopted GM** & \(\times\)**10\({}^{-4}\) \(\mathrm{kg}\)** & \(\times\)**10\({}^{-4}\) \(\mathrm{kg}\)** & \(\times\)**10\({}^{-4}\) \(\mathrm{kg}\)** & \(\times\)**11\({}^{-4}\) \\ \hline \multicolumn{8}{c}{Close spacecraft flybys} \\ \multicolumn{8}{c}{Phobos-2} & 7.163 & \(\pm\)0.0080 & [8;67] & 1.0732 & \(\pm\)0.0012 & 1869 & \(\pm\)12 \\ \multicolumn{8}{c}{Viking} \\ \multicolumn{8}{c}{Viking} \\ \multicolumn{8}{c}{(w/o nongrav)} & 7.0770 & \(\pm\)0.0030 & [15] & 1.0603 & \(\pm\)0.0004 & 1847 & \(\pm\)11 \\ \multicolumn{8}{c}{Phobos-2} & 7.0910 & \(\pm\)0.0050 & [15] & 1.0624 & \(\pm\)0.0007 & 1850 & \(\pm\)11 \\ \multicolumn{8}{c}{Phobos-2} \\ \multicolumn{8}{c}{(w/o nongrav)} & 7.0910 & \(\pm\)0.0050 & [15] & 1.0624 & \(\pm\)0.0007 & 1850 & \(\pm\)11 \\ \multicolumn{8}{c}{Viking and Phobos-2} & 7.0920 & \(\pm\)0.0040 & [15] & 1.0626 & \(\pm\)0.0006 & 1851 & \(\pm\)11 \\ \multicolumn{8}{c}{Mars Express (MEX) 2006} & 7.1200 & \(\pm\)0.1200 & [25] & 1.0668 & \(\pm\)0.0180 & 1858 & \(\pm\)33 \\ \multicolumn{8}{c}{(UniBw)} \\ \multicolumn{8}{c}{Mars Express (MEX) 2008} & 7.1270 & \(\pm\)0.0210 & [25] & 1.0678 & \(\pm\)0.0031 & 1860 & \(\pm\)13 \\ \multicolumn{8}{c}{(ROB)} \\ \multicolumn{8}{c}{(Mexpress (MEX) 2010} \\ \multicolumn{8}{c}{(with C20)} \\ \multicolumn{8}{c}{(Mex)}  
Figure 20: The volumetric density of Phobos was determined using a volume of 5742 \(\pm\) 35 km\({}^{3}\)[7] and mass obtained from (**A**) GM derived from close spacecraft flybys, (**B**) GM derived from the gravitational field of Mars or distant perturbations in the spacecraft’s orbit. The volumetric density was calculated only using masses for which the average value of GM fell within the range of 3\(\sigma\) of the GM obtained from the close flybys of MEX 2008 ROB [8]. The horizontal dashed lines represent the upper and lower bounds of the 3\(\sigma\) error associated with the volumetric density derived from the GM of MEX 2008 ROB [8]. The volumetric density of Phobos was calculated based on the mass values from MEX 2008 UniBw (German Armed Forces in Munich) [25; 68] and the mass from the MEX 2010 flyby (with C20--second-degree gravity coefficients) [68], resulting in densities of 1860 kg/m\({}^{3}\) (\(\pm\)13 kg/m\({}^{3}\)) and 1845 kg/m\({}^{3}\) (\(\pm\)12 kg/m\({}^{3}\)) (Table 3, Figure 20).