Characteristics of dust particles detected near Saturnʹs ring plane with the Cassini Radio and Plasma Wave instrument

During the inbound and outbound passes of the Cassini spacecraft through Saturnʹs ring plane on July 1, 2004, the Radio and Plasma Wave Science (RPWS) instrument detected many small particles striking the spacecraft. When a small particle strikes the spacecraft at a high velocity, it is instantly vaporized and produces a small cloud of plasma that expands radially outward from the impact site. As the plasma cloud expands away from the spacecraft it produces a voltage pulse on the RPWS electric field antennas, the amplitude of which is proportional to the mass of the impacting particle. Two types of measurements are made: waveform measurements from the x-axis dipole antenna, and spectrum measurements from the w-axis monopole antenna. The waveform measurements from the dipole antenna provide a determination of the impact rate and the relative mass distribution, and the spectrum measurements from the monopole antenna provide a determination of the root-mean-square particle mass. The impact rate at both ring plane crossings provides a good fit to the sum of two Gaussians, with an average impact rate of about 1200 per second (the exact value depends on the voltage threshold used), and a north–south thickness of about 300 km. The mass distribution depends on the distance from the ring plane, varying from about m−2 near the ring plane at z = 0 ± 100 km , where z is the north–south distance from the ring plane, to as steep as m−4 well away from the ring plane at z = 500 ± 100 km . The mechanisms involved in the impact detection are discussed and a formula relating the root-mean-square particle mass to the root-mean-square voltage on the w-axis monopole is derived. Using this formula, the root-mean-square mass is estimated to be 7.7×10−11 g, which for water ice particles with a density of 0.92 g cm−3 gives a root-mean-square radius of about 2.6 μm.