The Nature of the Enceladus Plasma Cloud From the Cassini Plume Radio Occultation

The Cassini orbiter spacecraft flew behind the Enceladus plume, as observed from the Earth, on 26 January, 2010. At that time, Cassini was about 577,000 km behind Enceladus, and the radio line of sight passed through the plume about 52 km from the South pole. The occultation was observed by two Deep Space Net tracking stations near Canberra, Australia, one(DSS 43) instrumented for S-band (13.04 cm) and X-band (3.56 cm), and the other (DSS 34) with X-band and Ka-band (0.94 cm). Having two different coherent frequencies at two stations enabled us to obtain two independent measurements. The measurements were made well away from solar conjunction (Earth-Cassini-Sun angle of 122 deg.), where the effects of solar wind plasma were small, and the excellent stability of the Cassini USO (Ultra Stable Oscillator) of 10-13 could be fully exploited. The preliminary results reveal the presence of a plasma cloud around Enceladus,extending to a distance of about 7,000 km., and having an columnar electron content of about 0.2 hexem (1016 m-2 ). From this measurement, the radial distribution of electron density can be determined assuming a geometrical configuration, i.e. cylindrical symmetry.Using this assumption, and using the Abel transform to invert the observed frequency residuals, a nearly symmetrical electron density distribution is obtained, havind a density of about 500 cm-3 within about 2,500 km of the plume centerline, and gradually decreasibg to zero at 7,000 km. In the immediate vicinity of the plume the electron density decreases to almost zero. This work was performed at the Jet Propulsion Laboratory, California Institute of Technology; The University of Michigan;, and San Jose State University under NASA contracts.