A statistical study of impact ejecta distribution around Phobos and Deimos

Abstract

Impact-ejecta dynamics around Phobos and Deimos are considered for dust particles of the order of 1 mm and greater. Only the gravitational forces of the planet and the satellite as well as the centrifugal and Coriolis forces are taken into account. In the case of Phobos the ejecta generally either reimpact the surface in tens of minutes after their launch or escape rapidly away from the gravitational attraction of the satellite. Deimos, being subjected to a relatively smaller perturbation force from Mars, can keep its ejecta on reimpacting and temporary satellite orbits for hours. Phobos shows large variability in its escape velocity as a function the launch point and the direction of launch. The statistical properties of large numbers of particles moving around the satellites are determined in a series of Monte Carlo simulations. Two factors, important for the regolith-formation process, are investigated: the escape probability from a given site on the surface and the reimpact probability. Both of these functions vary appreciably over the surfaces of Phobos and Deimos. The differences of the escape probabilities between adjoining regions are greater for Phobos than for Deimos, while the reimpact probability variations are substantial for both satellites. The density patterns for Phobos exhibit an interesting dependence on the particles' launch velocities. For small velocities (4–5 m sec −1) we observe two curved tubes of enhanced density that start from the sub-Mars and anti-Mars points on the surface. The density tends to become isotropic for greater launch velocities. Deimos shows a rather regular, ellipsoidal dust halo. The overall density of dust particles—with the meteoroid flux intensity, cratering laws, and velocity distribution of ejecta taken into account—is estimated to be of about a few hundred particles per kilometer cubed close to the surface of Phobos and drops down to about tens of particles per kilometer cubed at a distance of 40 km from the satellite's center. The density of dust around Deimos is two to three times larger.