Solar oxygen and calcium in Mercuryʹs exosphere

In situ and remote sensing observations have provided a glimpse of the space environment at Mercury and the interaction of its magnetic field with both the planetary surface and its exosphere. In this work we suggest that O 6 + and Ca 11 + supplied by the solar wind are adequate to explain the observations of emissions from neutral O and Ca in the Mercury environment, or exosphere. The actual observational data for O are sparse with only a single detection by instrumentation on Mariner 10 with no altitude profile. For Ca the observations are more thorough and show Ca to be present at thousands of km off the high latitude limbs of Mercury. We discuss the abundance and distribution of O and Ca in Mercuryʹs exosphere with the solar wind as the sole source and loss exclusively by ionization. Results are compatible with the observed abundances. We present a new model of solar wind Ca and O implantation into Mercuryʹs regolith and suggest observations that could be made by Mercury atmospheric and surface camera spectrograph (MASCS) and the energetic particle and plasma spectrometer (EPPS) on MErcury, Surface, Space ENvironment, GEochemistry, Ranging spacecraft (MESSENGER) to differentiate between solar wind delivery of O and Ca vs. a true crustal source as may be measured by the neutron spectrometer (NS) and gamma-ray spectrometer (GRS) and possibly the X-ray spectrometer (XRS).