The second-degree gravity coefficients of Phobos from two Mars Express flybys

Abstract : Several close spacecraft flybys of Phobos have been performed over the past 40 yr in order to determine the gravity field of this tiny Martian moon. In this work, the second-degree coefficients of the gravity field of Phobos were derived from the radio tracking data of two combined Mars Express flybys (2010 and 2013), by applying a least squares regularized inverse technique, that introduces as an a priori the gravity field retrieved from a shape model based on constant density hypothesis. A gravitational mass estimate of (7.0765 ± 0.0075) × 10 5 m 3 s −2 and second-degree gravity coefficients C 20 = −0.1378 ± 0.0348 and C 22 = 0.0166 ± 0.0153(3σ) were derived. The estimated C 20 value, in contrast to the value of C 20 computed from the shape model under the constant density assumption, supports an inhomogeneous distribution inside Phobos at a confidence interval of 95 per cent (1.96σ). This result indicates a denser mass in the equatorial region or lighter mass in polar areas.