Seasonal Variability and Local Time Dependence of Mercury's Dayside Magnesium Exosphere

The Mercury Atmospheric and Surface Composition Spectrometer (MASCS) Ultraviolet and Visible Spectrometer (UVVS) on the MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) mission has made systematic dayside observations of magnesium (Mg) emission over the last 18 months. The dayside altitude profiles of magnesium usually reach altitudes less than 80 km and systematically cover all local times. The dataset covers five Mercury years beginning just after MESSENGER orbit insertion on March 29, 2011, and allows characterization of the seasonal and local time variability of the Mg column density near the surface. Initial findings indicate a seasonal variation at all dayside local times. In addition, Mg exhibits a local time dependence with the near-surface emission peaking near 16 h. This afternoon enhancement appears to be less energetic than the typical hot component observed at higher altitudes. The 16 h peak is a stable, consistent feature throughout the Mercury year. Comparisons of the variation of Mg with those for calcium and sodium versus dayside local time provide a basis for the evaluation of possible source mechanisms for this unexpected behavior.