Characteristics of ice grains in the Enceladus plume from Cassini observations

Ice grains in the Enceladus plume have been observed by several Cassini instruments during many Enceladus encounters. In this paper we study the ice grains ranging fromless than one nanometer to micrometers in size based on multiple instrument observations. We have analyzed the nanograin data from the E17 and E18 Cassini Plasma Spectrometer (CAPS) energy spectra using the approach of Hill et al. (2012) and studied the charging of the grains using the E3-E6 Radio and Plasma Wave Spectrometer (RPWS)-Langmuir Probe (LP) data presented by Morooka et al. (2011). To bridge the gap between CAPS nanograin observations and Cosmic Dust Analyzer and RPWS micrograin observations, we propose a composite size distribution and fit it to the E3, E5, E17, and E18 CAPS and RPWS data. The resulting size distribution peaks at similar to 2nm and provides a total grain mass density similar to 20% that of the water vapor measured by Ion and Neutral Mass Spectrometer at the densest part of the plume and a total grain charge density much smaller than that inferred from the RPWS-LP plasma data. Charge balance with the RPWS-LP plasma data would require many more grains than provided by the best fit composite distribution to the CAPS and RPWS data and would provide a grain mass density comparable to that of the water vapor. On the basis of these results, we study the subsequent motion of the ice grains and estimate the resulting grain current (similar to 10(4) to similar to 10(5)A) and grain mass production rate (similar to 15-65kg/s).