The electrostatic potential of E ring particles

Abstract

Saturn's E ring is remarkable not only for its huge size. Based on optical measurements the ring was surprisingly found to mainly consist of ice particles of uniform size (radii between 0.3 and 2 microns) [Nicholson et al., 1996. Observations of Saturn's ring-plane crossing in August and November. Science 272, 509–516], though the source of the ring, the icy moon Enceladus, feeds the ring with grains having a much broader mass distribution [Spahn et al., 1999. Dynamics of dust ejected from enceladus: application to the cassini dust detector. J. Geophys. Res. 104, 24111–24120]. Because of this contradiction, the mass distribution of the ring must be closely connected to the dynamics of the ring particles which is governed by the competition of various disturbing forces acting on the grains. It was realised early that the dynamics of the charged ring particles is strongly affected by electromagnetic forces as well as by plasma drag. Understanding the implications on the ring dynamics thus requires knowledge of the electrostatic equilibrium potential of the dust grains. The charge-sensitive grid system of the cosmic dust analyser (CDA) on the Cassini spacecraft allows an in situ determination of the charge of grains bigger than 2–3 microns. Since the CDA detector determines the grain mass simultaneously with the dust charge the electrostatic potential φ d can be estimated as well. Here we report the first measurement of the electrostatic potential of E ring particles. Particles detected inside the orbit of Saturn's moon Rhea were always found to be negatively charged, whereas grains outside Rhea's orbit were on a positive potential of about 3 V. Our results are in good agreement with measurements of the spacecraft potential by Cassini's radio and plasma wave science (RPWS) investigation [Wahlund et al., 2005. The inner magnetosphere of Saturn: Cassini RPWS cold plasma results from the first encounter. Geophys. Res. Lett. 32]. Our analysis implies that the CDA dust analyser (DA) mass calibration by Göller and Grün [1998. Calibration of the Galileo/Ulysses dust detectors with different projectile materials and at varying impact angles. Planet. Space Sci. 37, 1197–1206] is applicable for ice particles. We achieved a good match between our findings and the model calculations of the grain potential when using new Cassini measurements of plasma properties within Saturn's inner plasma sphere. Based on this we provide a realistic description for the grain potential needed for modelling the dynamics of Saturn's E ring.