Equipotential Figure of Phobos Suggests Its Late Accretion Near 3.3 Mars Radii

Abstract

The ellipsoidal figure of Phobos has been tentatively interpreted as resulting from accretion in a tidal environment close to its parent planet Mars. The issue is compounded by the rapid tidal decay of the moon's orbit. Consequently, when and where the moon actually formed remains elusive. We determined the gravity equipotential ellipsoid of Phobos during its orbital decay and studied the evolution of surface ellipsoidal heights. We found that the equipotential ellipsoid closely fits the body shape near 3.6 Mars radii. Meanwhile, the height distribution resembles Gaussian at 3.3 Mars radii, suggesting equilibrium of topography. The ellipsoidal shape may have accreted here under appreciable tidal influence of roughly half the magnitude at Phobos' current position (at 2.76 Mars radii). This lends evidential support to the hypothesis that Phobos is not a primordial object but has reaccreted from a debris ring after a previous disruption event.