MESSENGER Observations of the Spatial Distribution of Planetary Ions Near Mercury

Global measurements by MESSENGER of the fluxes of heavy ions at Mercury, particularly sodium (Na⁺) and oxygen (O⁺), exhibit distinct maxima in the northern magnetic-cusp region, indicating that polar regions are important sources of Mercury's ionized exosphere, presumably through solar-wind sputtering near the poles. The observed fluxes of helium (He⁺) are more evenly distributed, indicating a more uniform source such as that expected from evaporation from a helium-saturated surface. In some regions near Mercury, especially the nightside equatorial region, the Na⁺ pressure can be a substantial fraction of the proton pressure.