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Observations near comet 67P by Rosetta show evidence of charged nanograins. Specifically, the ion and electron sensor (IES) observed negative particles with energy/charge (E/q) consistent with charged nanograins. However, theory predicts that both polarities should be present. On 19 September 2014, IES detected positively and negatively charged particles with E/q up to the maximum detectable by IES (similar to 17 keV/q) coming roughly from the Sun. The arrival directions of the negative and positive particles were different and consistent with their different charges. This observation is the first simultaneous measurement of positively and negatively charged nanograins by a plasma detector in a cometary environment. As observed previously for negative nanograins coming from the comet, we find similar peaks for the picked up negative nanograins, but only one peak for the positive ones. These E/q peaks are interpreted as different charge states since they occur at discrete multiples (1:3 in this event).

Plain Language Summary Negatively and positively charged particles are observed simultaneously by the ion and electron sensor (IES) onboard the Rosetta spacecraft soon after encountering comet 67P/Churyumov-Gerasimenko. The two signals are shifted in opposite directions consistent with their charge sign (+/-). These signal features, together with their energies extending much higher than solar wind energies, strongly suggest that they are charged small dust grains. While negatively charged small dust particles have been detected near a comet, this is the first observation of their positively charged counterparts.