Characterization of Mercury's Atomic and Molecular Hydrogen Exosphere and the First Detection of H 2 Ions

Abstract From 1974 to 1975, the Mariner 10 spacecraft studied Mercury's environment during three flybys and detected hydrogen, helium, and possibly atomic oxygen in the exosphere using its ultraviolet spectrometer, but no molecular hydrogen. Based on the sensitivity of the occultation instrument, an upper limit for the surface density was estimated at . Despite this early constraint, the presence and behavior of in Mercury's exosphere remained uncertain for decades due to the lack of direct observations. In this study, we derive the first in situ radial density profile of hydrogen molecules in Mercury's extended exosphere by analyzing magnetic field and plasma measurements collected between 2011 and 2015 by the MErcury Surface, Space ENvironment, GEophysics, and Ranging (MESSENGER) spacecraft. We report the first indirect detection of molecular hydrogen in Mercury's exosphere through the identification of ion cyclotron waves, which are generated by the interaction of exospheric pick‐up ions with the interplanetary magnetic field. These wave signatures allow us to infer the presence and density of molecules in the exosphere. Our results suggest a significantly lower dayside surface density of approximately , nearly 7,000 times lower than the density based on the observational threshold of Mariner 10. Furthermore, the variability of the observed atomic hydrogen exosphere cannot be explained by dissociation alone. Instead, several processes contribute to the hydrogen densities in Mercury's exosphere, including thermal desorption, charge exchange, micrometeoroid impacts, space weather interactions, and related surface reactions.