ON THE DUST BELTS OF MARS

The dust complex around Mars formed by the impact ejecta from the surf aces of its satellites is studied. As found by Juhasz et al. (1993), t he different-sized particles of each moon generate several populations with quite dissimilar features. Relatively large grains are concentra ted in a toroidal belt along the satellite's orbit (''torus'' or ''dis k''). The intermediate-sized debris, though also fill a disklike volum e, are rapidly lost by the collisions with martian surface and hence a re not abundant (''subdisk''). The fine submicron-sized dust is strong ly affected by the electromagnetic forces and solar wind streams and w ould form an extended envelope around Mars (''halo''). Of these popula tions, the first should dominate the mass density and perhaps the opti cal depth of the dust complex. We constructed a model of the disks/tor i around both satellites, considering successively: incoming meteoroid al flux, ejecta size and velocity distributions, dynamics of orbiting particles, and reaccretion of grains onto the moons' surfaces. The min imal radius of torus' particles is found to be 14 mu m for Phobos and 12 mu m for Deimos. The fraction of the total ejecta yield maintaining the disk/torus is estimated as 40 to 70%. The dust density reaches a maximal value at the moon's orbit. For both satellites, the peak mass density of torus' particles is 10(5) to 10(6) times the background val ue near Mars. The number density of the grains between 15 and 100 mu m is of order 10(4)-10(5) km(-3), being 2-3 orders of magnitude greater than that of larger fragments. The mass and number densities decrease slowly with distance from the satellite's orbit, so that the ''effect ive'' semi-sizes of the torus' section are half the radius of the moon 's orbit. The torus of Deimos should be more extended than that of Pho bos.