Tenuous Ring of Captured Dust at Saturn

Small dust particles, less than 300 nm in radius, on orbits with low eccentricity and small inclinations may be captured from the interplanetary medium through interactions with Saturn's magnetosphere. Previous work at Jupiter has shown that particles will be preferentially captured into retrograde orbits. We use simple two-dimensional numerical simulations to attribute this asymmetry to the conservation of the generalized Jacobi constant. Additionally, we use a more sophisticated fully three-dimensional code to predict the existence of a faint ring of captured dust particles in orbit about Saturn. In our two-dimensional code, dust grains with various radii and impact parameters are followed in the simulations until they crash into the planet, are ejected from orbit, or get captured into a circular orbit. We demonstrate that the range of values of the Jacobi constant for circular orbits is larger than retrograde orbits than for prograde ones. Since incoming particles will have a random distribution of Jacobi constants, the class of orbits with the largest available range should be the most populated. The three-dimensional code uses the Z3 model for the magnetic field and plasma parameters provided by Richardson [1995]. It includes solar radiation pressure, interplanetary magnetic field, planetary oblateness, and sputtering of the dust grains in the magnetosphere. The particles' initial conditions are calculated assuming a random distribution of eccentricities, semimajor axes, and inclinations about the sun. These particles are then input into the simulations until an equilibrium number of particles is reached. The predicted ring has a peak density of about 50 x 10^-15 cm^-3 and extends from just outside the main ring to about 10 planetary radii (R_S) and is about three R_S thick at its widest point. The Cassini spacecraft's dust detector will verify these predictions.