The E ring in the vicinity of Enceladus I: Spatial distribution and properties of the ring particles Sascha Kempf a,b Uwe Beckmann a Georg Moragas-Klostermeyer a Frank Postberg a Ralf Srama a Thanasis Economou d J¨ urgen Schmidt c Frank Spahn c Eberhard Gr ¨ un a,e a MPI f¨ ur Kernphysik, Saupfercheckweg 1, D-69117 Heidelberg, Germany b IGEP, Universit¨ at Braunschweig, Mendelssohnstr. 3, D-38106 Braunschweig, Germany c Institut f¨ ur Physik, Universit¨ at Potsdam, Karl-Liebknecht-Str. 24/25, D-14476 Potsdam-Golm, Germany d LASR, University of Chicago, USA e HIGP, University of Hawaii, Honolulu, USA appeared in Icarus 193, 420–437 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 ∼ 4200km. We show that the FWHM at 3.95R S is due to three-body interac- tions 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 225ms −1 and 235 m s −1 relative to the moon’s surface dominate the vertical distri- bution of dust. Particles with initial velocities exceeding the moon’s escape speed of 207ms −1 but slower than 225ms −1 re-collide with Enceladus and do not con- tribute 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 dens- Preprint submitted to Icarus 2 February 2009