The E ring in the vicinity of Enceladus: I. Spatial distribution and properties of the ring particles

Abstract

Saturn's diffuse E ring is the largest ring of the Solar System and extends from about 3.1 R S (Saturn radius R S = 60 , 330 km ) to at least 8 R S encompassing the icy moons Mimas, Enceladus, Tethys, Dione, and Rhea. After Cassini's insertion into her saturnian orbit in July 2004, the spacecraft performed a number of equatorial as well as steep traversals through the E ring inside the orbit of the icy moon Dione. Here, we report about dust impact data we obtained during 2 shallow and 6 steep crossings of the orbit of the dominant ring source—the ice moon Enceladus. Based on impact data of grains exceeding 0.9 μm we conclude that Enceladus feeds a torus populated by grains of at least this size along its orbit. The vertical ring structure at 3.95 R S agrees well with a Gaussian with a full-width–half-maximum (FWHM) of ∼ 4200 km . We show that the FWHM at 3.95 R S is due to three-body interactions of dust grains ejected by Enceladus' recently discovered ice volcanoes with the moon during their first orbit. We find that particles with initial speeds between 225 and 235 m s −1 relative to the moon's surface dominate the vertical distribution of dust. Particles with initial velocities exceeding the moon's escape speed of 207 m s −1 but slower than 225 m s −1 re-collide with Enceladus and do not contribute to the ring particle population. We find the peak number density to range between 16 × 10 −2 m −3 and 21 × 10 −2 m −3 for grains larger 0.9 μm, and 2.1 × 10 −2 m −3 and 7.6 × 10 −2 m −3 for grains larger than 1.6 μm. Our data imply that the densest point is displaced outwards by at least 0.05 R S with respect of the Enceladus orbit. This finding provides direct evidence for plume particles dragged outwards by the ambient plasma. The differential size distribution n ( s d ) d s d ∼ s d − q s d s d for grains > 0.9 μm is described best by a power law with slopes between 4 and 5. We also obtained dust data during ring plane crossings in the vicinity of the orbits of Mimas and Tethys. The vertical distribution of grains > 0.8 μm at Mimas orbit is also well described by Gaussian with a FWHM of ∼ 5400 km and displaced southwards by ∼ 1200 km with respect to the geometrical equator. The vertical distribution of ring particles in the vicinity of Tethys, however, does not match a Gaussian. We use the FWHM values obtained from the vertical crossings to establish a 2-dimensional model for the ring particle distribution which matches our observations during vertical and equatorial traversals through the E ring.