  This study aims to analyze Phobos' photometric properties using Mars Express
mission observations to support the Martian Moons eXploration mission. We
analyzed resolved images of Phobos acquired between 2004 and 2022 by the HRSC
and the SRC cameras on board the Mars Express spacecraft. We performed
photometric analysis using the Hapke model for both integrated and
disk-resolved data. The Phobos phase function has a strong opposition effect
due to shadow hiding, with an amplitude and a half-width of the opposition
surge of 2.28$\pm$0.03 and 0.0573$\pm$0.0001, respectively. Overall, the
surface of Phobos is dark, with a geometric albedo of 6.8 % in the green filter
and backscattering. We also found a surface porosity of 87\%, indicating the
presence of a thick dust mantle or of fractal aggregates on the top surface.
The SSA maps revealed high reflectance variability, with the blue unit area in
the northeast Stickney rim being up to 65\% brighter than average, while the
Stickney floor is among the darkest regions, with reflectance 10 to 20% lower
than average. Photometric modeling of the regions of interest selected in the
red and blue units indicates that red unit terrains have a stronger opposition
effect and a smaller SSA value than the blue ones, but they have similar
porosity and backscattering properties. The HRSC data provide a unique
investigation of the Phobos phase function and opposition surge, which is
valuable information for the MMX observational planning. The Phobos opposition
surge, surface porosity, phase integral, and spectral slope are very similar to
the values observed for the comet 67P and for Jupiter family comets in general.
Based on these similarities, we formulate a hypothesis that the Mars satellites
might be the results of a binary or bilobated comet captured by Mars.
