First Detection of Nanoparticles in the Asteroid Belt from Spectral Analysis of Cassini/Rpws Radio Data

Nanodust grains of a few nanometer in size have been detected in the interplanetary medium at 1 AU and at the vicinity of giant planets Saturn and Jupiter. These are generally formed through collisional break-up of larger grains and are then picked-up by the planetary co-rotation electric field and/or the magnetized solar wind. Here we analyze the spectra measured by the radio and plasma wave instrument onboard Cassini during the cruise phase close to the asteroid belt (R~2.9 AU); they exhibit bursty signatures similar to those observed by the same instrument at 1 AU (Schippers et al., 2014; doi:10.1002/2014GL060566) and at Jupiter (Meyer-Vernet et al., 2009; doi:10.1029/2008GL036752). The observed wave level and spectral shape are consistent with impacts of fast nanoparticles (v~450 km/s). We discuss the origin of the particles by comparing the measured flux with the extrapolated flux from 1) the inner heliospheric source (0.2 AU), 2) the asteroid belt, and 3) Jupiter.