Magnetic evidence for an extended hydrogen exosphere at Mercury

Abstract Remote sensing observations of the Mariner 10 and MESSENGER spacecraft have shown the existence of atomic (H) and molecular (H2) hydrogen in the exosphere around Mercury. However, to date the hydrogen number densities could only be estimated indirectly from exospheric models, based on Lyman-α radiances for H and the H2 detection threshold of the Mariner 10 occultation experiment. Here we show the first in-situ measured altitude-density profile of atomic H, derived from magnetic field observations by MESSENGER. Our results reveal an extended H corona with densities decreasing from 1000 − 100 cm−3 between 2 − 8 Mercury radii. These densities are 2-3 orders of magnitude larger than previously predicted, but in good agreement with the Lyman-α radiances. The large H densities result from the dissociation of H2. We can reproduce the observed H densities using an exospheric model which includes ionization, dissociation and recombination, that allows us to constrain the H2 surface density to ∼ 5×10^5 cm−3. This is two orders of magnitude smaller than the previously assumed upper limit.