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 Mercuryʼs 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.