Phobos mass determination from the very close flyby of Mars Express in 2010

The global geophysical parameters GM Ph = ( 0.7072 ± 0.0013 ) × 10 - 3 km 3 s - 2 , C 20 , C 22 and the bulk density 〈 ρ 〉 = ( 1862 ± 30 ) kg / m 3 have been determined from the closest Mars Express flyby at the Mars moon Phobos on 3rd March 2010 at a distance of 77 km. The second degree gravity field of Phobos ( C 20 , C 22 ) could not be solved for at sufficient accuracy. The low bulk density suggests a high porosity and an inhomogeneous mass distribution but the large errors of C 20 and C 22 are still consistent with a homogeneous as well as an inhomogeneous mass distribution. The modeling of the moon’s interior by a randomly selected mass distribution of given porosity and water ice content but constrained by the observed GM Ph and 〈 ρ 〉 let a simulated C 20 decrease with increasing porosity and water ice content indicating an increasingly inhomogeneous mass distribution. The high porosity together with an inhomogeneous mass distribution would be evidence that Phobos accreted in orbit about Mars from a debris disk and is not a captured asteroid.