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

Saturn's diffuse E ring is the largest ring of the Solar System and extends from about 3.1R (Saturn radius R=60,330 km) to at least 8R 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 3.95R agrees well with a Gaussian with a full-width-half-maximum (FWHM) of ∼4200 km. We show that the FWHM at 3.95R 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 m and 21×10 m for grains larger 0.9 μm, and 2.1×10 m and 7.6×10 m for grains larger than 1.6 μm. Our data imply that the densest point is displaced outwards by at least 0.05R 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)ds∼sd-qds 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.