THE ASYMMETRIC TIME-VARIABLE RINGS OF MARS

In this paper, we investigate the dynamics and steady-state behavior o f the hypothetical circumplanetary dust rings associated with the two tiny satellites of Mars, Phobos and Deimos. These moonlets are subject to a flux of micrometeoroids which erodes their surfaces and ejects m aterial into orbit around Mars. We study the detailed orbital dynamics of ejected material between a micrometer and a millimeter in radius a nd find that these grains are significantly perturbed by solar radiati on pressure and Mars' oblateness. The coupling between these two force s forms rings that are vertically and azimuthally asymmetric as well a s time-variable. Our analytic and numerical results show that material of all sizes launched from Deimos forms a ring that is displaced away from the Sun. Grains with radii smaller than approximate to 270 mu m launched from Phobos, however, form a ring that is displaced toward th e Sun. This effect, as well as surprisingly large orbital changes for Phobos grains, is due to a near resonance between Mars' orbital motion and the precession of pericenter due to the oblateness force. When vi ewed from along Mars' vernal equinox (the intersection between Mars' o rbital and equatorial planes), the ring formed by Deimos grains smalle r than approximate to 100 mu m is tilted out of the equatorial plane. We present a new analytical solution describing this vertical asymmetr y and interpret it in terms of the Laplace plane. Finally, we suggest that the martian rings may be sustained through the ejecta produced by energetic collisions between ring particles in the 20-50 mu m range a nd the small moonlets Phobos and Deimos. (C) 1995 Academic Press, Inc.