Magnetic Evidence for an Extended Hydrogen Exosphere at Mercury

Remote observations by the Mariner 10 and MErcury Surface, Space ENvironment, GEophysics, and Ranging (MESSENGER) spacecraft have shown the existence of hydrogen in the exosphere of Mercury. However, to date the hydrogen number densities could only be estimated indirectly from exospheric models, based on the remotely observed Lyman-α radiances for atomic H, and the detection threshold of the Mariner 10 occultation experiment for molecular H2. Here, we show the first on-site determined altitude-density profile of atomic H, derived from in situ magnetic field observations by MESSENGER. The results reveal an extended H exosphere with densities that are ∼1–2 orders of magnitude larger than previously predicted. Using an exospheric model that reproduces the H altitude-density profile allows us to constrain the so far unknown H2density at the surface, which is ∼2–3 orders of magnitude smaller than previously assumed. These findings demonstrate the importance of (a) in situ measurements supporting remote observations of Mercury's exosphere that will be realized in the near future by the BepiColombo mission and (b) that dissociation processes play a crucial role in Mercury's exosphere