Dynamical characteristics of Phobos and Deimos

Abstract

The orbital properties of the two small Martian satellites, Phobos and Deimos, are discussed, as well as those dynamical constants of Mars that can be determined from the satellite orbits. A secular acceleration of the mean motion of Phobos was once thought to exist, but now, on the basis of the recent work of G. A. Wilkins, the acceleration is shown to be very small. Of mechanisms that could cause any such acceleration, only tidal friction appears to be important. From the orbital evolution of the Martian satellites under tidal forces, Phobos and Deimos seem to have originated in nearly circular orbits of low inclination fairly close to the distance at which a satellite's orbital period is the planet's rotation period. It is proposed that the Martian satellites were born at the same time as Mars from equatorial dust clouds. The satellites are predicted to be locked in synchronous rotation, with their axes of minimum moment of inertia pointing on the average toward Mars, whereas their maximum axes are approximately normal to their orbit planes. Moderate amplitude physical librations may be observed by Mariner 9 because of resonance effects produced by forcing torques that have roughly the satellite free‐libration period. Observations of the rotational motion of the satellites will permit an evaluation of the ratios of differences in the satellite principal moments of inertia and thus will place constraints on density models.