The Gravitational Field of Deimos

Abstract Deimos may be largely uniform in density. We find here the gravitational field of Deimos and its moments of inertia from the moon's shape and an assumed uniform density distribution. From these data we compute the expected orientation. The orientation of the model Deimos relative to Mars agrees with that observed to within the observation uncertainties (a few degrees). Hence Deimos may be homogeneous and not have significant density variations in its interior. We also find that the spherical harmonic coefficient J 2 is about 0.108 for the model, so that Deimos is about as gravitationally flattened as Phobos, whose J 2 is about 0.105.