Estimates for uncharged nanograins in Enceladus' plume

Abstract:

Enceladus' plume provides a unique laboratory for dust-plasma interactions. Negatively charged nanograins, which represent the vast majority of grains, have been measured by Cassini Plasma Spectrometer (CAPS). Even a small fraction of positively charged nanograins has been detected by CAPS. However, there is a crucial lack of information on uncharged grains. Thus, no information on the total grain production rate of Enceladus or its total contribution to Saturn's E-ring are available yet. We present an estimation of uncharged grains as well as a total grain production rate by an analytical model and simulations. First, we derive an analytical model from basic equations of grain charging and quasi-neutrality connecting the amount of uncharged grains to negatively and positively charged grains. A first estimate for uncharged grains then results from the ratio of negatively-to-positively charged nanograins. For more accurate estimations we compare results from combined dust and plasma simulations with our analytical ones and CAPS data for charged nanograins to determine Enceladus' total grain production rate and a global profile of uncharged nanograins in the plume. The dust simulations of the plume and the plasma simulations with A.I.K.E.F. for plasma-plume interactions are performed iteratively allowing for the different time scales for dust and plasma dynamics.