[1] We report independent results from two subgroups of the Mars Express Radio Science (MaRS) team who independently analyzed Mars Express (MEX) radio tracking data for the purpose of determining consistently the gravitational attraction of the moon Phobos on the MEX spacecraft, and hence the mass of Phobos. New values for the gravitational parameter (GM = 0.7127 ± 0.0021 × 10 −3 km 3 /s 2 ) and density of Phobos (1876 ± 20 kg/m 3 ) provide meaningful new constraints on the corresponding range of the body's porosity (30% ± 5%), provide a basis for improved interpretation of the internal structure. We conclude that the interior of Phobos likely contains large voids. When applied to various hypotheses bearing on the origin of Phobos, these results are inconsistent with the proposition that Phobos is a captured asteroid.

3. Discussion [15] The bulk density of Phobos sheds some light on its internal composition, structure, and probable origin. Willner et al. [2009] derived a value of 5689 ± 60 km3 from MEX High Resolution Stereo Camera (HRSC) images. With this, the bulk density is r Ph = 1876 ± 20 kg/m3 (±1.1%) which is an improvement by a factor of three compared to past values. The error in the bulk density is driven by the error in the volume. [16] The porosity, the ratio of the bulk density and the grain density of an object, represents the percentage of the volume occupied by voids. The porosity of Phobos is computed from its mass, its bulk density and known grain densities of the hydrous chondrites of the CM group and the Tagish‐Lake meteorite samples. The result of 30 ± 5 percent suggests that the interior of Phobos contains large voids. Similar large porosities and low bulk densities have been found in C‐type asteroid such as the asteroid Mathilde [Yeomans et al., 1997] and the Jupiter's small inner moon Amalthea [Anderson et al., 2005]. A similar formation process of these porous bodies, however, is totally unclear. [17] The interior structure of Phobos could well be the result of its complete shattering and subsequent reassembly, as is thought to have occurred in the history of many asteroids subjected to violent collisions [Richardson et al., 2002]. The existence of the Stickney crater on Phobos would support the conclusion that Phobos contains large voids throughout its interior. [18] The origin of Phobos can be discussed in terms of its orbital history. Several scenarios ranging from possible to speculative have been proposed. The surface of Phobos shows some spectral similarities to those of various asteroid types. Based on these similarities it was suggested that Phobos is a former asteroid, formed in the outer asteroid belt and later captured by Mars [Burns, 1992]. This scenario, however, does not explain how the energy loss required to change the incoming hyperbolic orbit into an elliptical orbit bound to Mars is accounted for [Burns, 1992; Peale, 2007]. Models of orbit evolution based on tidal interactions between Mars and Phobos cannot account for the current near‐circular and near‐equatorial orbit [Mignard, 1981]. Scenarios of evolution to the current circular orbit require an additional drag by, e.g., the primitive planetary nebulae or the Martian atmosphere [Sasaki, 1990]. An alternative is a Phobos formed in an orbit around Mars. Phobos and Deimos could be remnants of an early, larger body that was broken into parts by gravitational gradient forces during Mars capture [Singer, 2007]. Or Phobos could have formed by the re‐accretion of impact debris lifted into Mars’ orbit [Craddock, 1994]. If Phobos were a remnant of a larger moon, it is not expected to be as porous as is reported here. If Phobos were formed from the re‐accretion of impact debris lifted into Mars’ orbit, the disc would be composed of a mixture of Martian crust and impactor material. The spectral properties, however, of the Phobos surface and the Martian crust do not match very well. [19] This inconsistency is resolved by a collision between a body already orbiting Mars but formed from the debris disc remaining after formation of Mars and a second body originating from the asteroid belt [Peale, 2007]. This scenario is consistent with the high porosity of Phobos and its spectral properties.

4. Conclusions [20] Figure 1 compares the Mars Express flyby solutions with previous determinations of GM Ph . The MEX results reported here have significantly reduced systematic uncertainties as compared with previous secular solutions. This is largely due to improvements in our knowledge of Mars gravity field, the ephemeris of Phobos, the ephemerides of the other planets important to the mass solution, and to improvements in the spacecraft radio system used for close flybys. [21] The Phobos’ mass reported here tightens the constraints on our knowledge of the physical structure of Phobos. The derived bulk density is consistent with a loosely consolidated body of 30% porosity, independent of its origin. It appears though highly unlikely that Phobos is a captured asteroid. [22] The three mass determinations from this work are derived from two close flybys using two different methods and two different software packages. The results are mutually consistent with each other as well as with the most recent secular solution [Rosenblatt et al., 2008]. We note that the latter is based on a different data set from the ‘close flyby’ solution employed here but makes use of the same models for the gravity field of Mars and similar ephemerides of Phobos. [23] A more precise volume estimate would not change our conclusions as the absolute volume estimate and therefore the bulk density is well constrained. The Phobos porosity value is safely within the range for loosely consolidated bodies. [24] The gravity coefficient J 2 derived from flybys at Phobos closer than 100 km which are planned for MEX in 2010 will give further insight into the interior. The comparison of theoretical J 2 assuming constant and homogenous bulk density with an observed J 2 also will give evidence as to mass enhancements or voids.

Acknowledgments [25] The Mars Express Radio Science Experiment MaRS is funded by DLR Bonn under grants 50QP9909 and 50QM0701. P. Rosenblatt is supported by the Belgian PRODEX program managed by the European Space Agency in collaboration with the Belgian Federal Science Policy Office. Support for Mars Express Radio Science at Stanford University is provided by NASA through JPL contract 1217744. J.C. Marty was supported by the CNES Programme Directorate and the Toulouse Space Center. We thank all persons involved in the Mars Express project at ESTEC, ESOC, ESAC, JPL and the ESTRACK and DSN ground station antennas for their continuous support.