Effects of spacecraft outgassing and charging observed by BepiColombo at Mercury

During the flybys of Mercury and Venus the ion spectrometers onboard the BepiColombo spacecraft observed enhanced fluxes of ions with energies of less than 50eV. Often these fluxes showed a characteristic double-band spectral shape in energy. We interpret these ion fluxes as being caused by ionized water molecules outgassing from the spacecraft. The double-band structure may be caused by a specific feature of the spacecraft potential: for a negatively charged spacecraft electrons will be rejected from the spacecraft and may form a negative space charge at distance given by the Debye length. This space charge may contribute to a separation of the observed ion fluxes into an inner and an outer source resulting in a double-band energy distribution. We show results of a particle-in-cell simulation supporting this interpretation.