Sputtering of ice grains and icy satellites in Saturnʹs inner magnetosphere

Icy grains and satellites orbiting in Saturnʹs magnetosphere are immersed in a plasma that sputters their surfaces. This limits the lifetime of the E-ring grains and ejects neutrals that orbit Saturn until they are ionized and populate its magnetosphere. Here we re-evaluate the sputtering rate of ice in Saturnʹs inner magnetosphere using the recent Cassini data on the plasma ion density, temperature and composition [Sittler Jr., E.C., et al., 2007a. Ion and neutral sources and sinks within Saturnʹs inner magnetosphere: Cassini results. Planet. Space Sci. 56, 3–18.] and a recent summary of the relevant sputtering data for ice [Famá, M., Shi, J., Baragiola, R.A., 2008. Sputtering of ice by low-energy ions. Surf. Sci. 602, 156–161.]. Although the energetic (>10 keV) ion component at Saturn is much smaller than was assumed to be the case after Voyager [Jurac, S., Johnson, R.E., Richardson, J.D., Paranicas, C., 2001a. Satellite sputtering in Saturnʹs magnetosphere. Planet. Space Sci. 49, 319–326; Jurac, S., Johnson, R.E., Richardson, J.D., 2001b. Saturnʹs E ring and production of the neutral torus. Icarus 149, 384–396.], we show that the sputtering rates are sensitive to the temperature of the thermal plasma and are still robust, so that sputtering likely determines the lifetime of the grains in Saturnʹs tenuous E-ring.