Observation of charged nanograins at comet 67P/Churyumov-Gerasimenko

Soon after the Rosetta Orbiter rendezvoused with comet 67P/Churyumov-Gerasimenko at a solar distance of similar to 3.5 AU and began to fly in triangular-shaped trajectories around it, the Ion and Electron Sensor detected negative particles at energies from about 100 eV/q to over 18 keV/q. The lower energy particles came from roughly the direction of the comet; the higher-energy particles came from approximately the solar direction. These particles are interpreted as clusters of molecules, most likely water, which we refer to as nanograins because their inferred diameters are less than 100 nm. Acceleration of the grains away from the comet is through gas drag by the expanding cometary atmosphere, while acceleration back to the vicinity of the comet is caused partly by solar radiation pressure but mainly by the solar wind electric field. These observations represent the first measurements of energetic charged submicron-sized dust or ice grains (nanograins) in a cometary environment.