During the first flyby of the BepiColombo composite spacecraft at Mercury in October
2021 ion spectrometers observed two intense spectral lines with energies between 10 and
70eV. The spectral lines persisted also at larger distances from Mercury and were ob-
served again at lower intensity during cruise phase in March 2022 and at the second and
third Mercury flyby as a single band. The ion composition indicates that water is the
dominant gas source. The outgassing causes the composite spacecraft to charge up to
a negative potential of up to -50V. The distribution and intensity of the lower energy
signal depends on the intensity of low energy electron fluxes around the spacecraft which
again depend on the magnetic field orientation. We interpret the observation as being
caused by water outgassing from different source locations on the spacecraft being ion-
ized in two different regions of the surrounding potential. The interpretation is confirmed
by two dimensional particle-in-cell simulations.