Phobos: Observed bulk properties

This work is a review of the mass determinations of the Mars moon Phobos by spacecraft close flybys, by solving for the Martian gravity field and by the analysis of secular orbit perturbations. The absolute value and accuracy is sensitive on the knowledge and accuracy of the Phobos ephemeris, of the spacecraft orbit, other perturbing forces acting on the spacecraft and the resolution of the Martian gravity field besides the measurement accuracy of the radio tracking data. The mass value and its error improved from spacecraft mission to mission or from the modern analysis of “old” tracking data but these solutions depend on the accuracy of the ephemeris at the time of observation. The mass value seems to settle within the range of GMPh=(7.11±0.09)×10−4 km3 s−2 which covers almost all mass values from close flybys and “distant” encounters within its 3−σ error (1.5%). Using the volume value determined from MEX HRSC imaging, the bulk density is (1873±31) kg m−3 (3−σ error or 1.7%), a low value which suggests that Phobos is either highly porous, is composed partially of light material or both. The determination of the gravity coefficients C20 and C22 from the Mars Express 2010 close flyby does not allow to draw conclusion on the internal structure. The large errors do not distinguish whether Phobos is homogeneous or not. In view of theories of the Phobosʹ origin, one possibility is that Phobos is not a captured asteroid but accreted from a debris disk in Mars orbit as a second generation solar system object.