Mercury's Helium Exosphere determined by Ion Cyclotron Waves

Mariner 10 detected the existence of an exosphere around Mercury in 1974-1975 by remote spectrometric observations during flybys. More than four decades later the MErcury Surface, Space ENvironment, Geochemistry and Ranging (MESSENGER) spacecraft confirmed the existence of the exosphere. So far, the neutral helium (He) number density around Mercury’s exosphere was based on assumptions related to the spectroscopic observations, which are applied to exospheric models to derive an altitude-dependent density profile of the neutral helium around the planet. Here, we present the first on-site measured density profile of He, using in-situ magnetic field measurements from MESSENGER. These data were analyzed for the identification of Ion-Cyclotron Waves (ICWs) that originated from exospheric pick-up He+ ions. The results reveal an extended He-exosphere with a lower surface number density as expected by previous studies. To provide further context, the results are compared with measurements obtained by Mariner 10 and BepiColombo (first flyby), which shows that the measurements of the PHEBUS UV-instrument onboard of the MPO align very well with the determined density from this study.