Image Credit: NASA / JHUAPL / U. Michigan
MESSENGER's Fast Imaging Plasma Spectrometer (FIPS) scoops up ions from Mercury's exosphere and measures their composition as a ratio of their mass to their charge. Each atomic or molecular ion shows up as a distinct peak in this spectrum. Most ions are singly charged (missing one or having one extra electron) so are visible as a peak at the atomic mass of the element (for example, sodium at 23). A few are present as multiply charged ions, for instance doubly chaged oxygen at a peak of 8. The surprising result is the detection of water-related ions like O+, OH-, and H2O+.
(MESSENGER Scientists 'Astonished' to Find Water in Mercury's Thin Atmosphere!)