Dust distribution around Neptune: Grain impacts near the ring plane measured by the Voyager Planetary Radio Astronomy Experiment

Abstract

During the Voyager 2 flyby of Neptune the planetary radio astronomy (PRA) experiment recorded an intense noise near the equatorial plane around 3.4 and 4.2 R N , as already observed during previous Voyager ring plane crossings at Saturn and Uranus. This noise is interpreted as being due to impact ionization of dust grains striking the spacecraft. We deduce a power law index of the grain mass distribution of about 2. The PRA system is sensitive to particles with radii larger than ∼1.6 μm, and the largest particles, detected near the ring plane, are evaluated to have a radius of ∼10 μm. The spatial dust distribution along the spacecraft trajectory around the two equatorial crossings is found not to be symmetrical with respect to the ring plane and spread over wide regions: over ∼2 R N perpendicularly to the equatorial plane with the densest part concentrated within ∼700 km. The vertical optical depth τ of this dense region is found to be 10 −6 – 10 −8 .