Insights into Mercury's Hydrogen Exosphere: Characterization and First Detection of H₂ Molecules

Between 1974 and 1975, the Mariner 10 spacecraft investigated Mercury's environment during three flybys. By using its ultraviolet spectrometer, it identified helium, atomic oxygen, and hydrogen atoms in Mercury’s exosphere. Interestingly, no H₂ molecules were detected during these flybys. Based on data from the occultation instrument, an upper limit for H₂ surface density was established from the detection threshold of about 1.4 x 10⁷ cm^-3. Here, we present the first in-situ detection of H₂ molecules in the Hermean Exosphere, identified through magnetic field and plasma measurements obtained from the MESSENGER spacecraft. The data was analyzed for ion cyclotron waves produced by H₂⁺ pick-up ions. Our findings reveal a much lower dayside surface density of approximately 2000 cm^-3, significantly below the Mariner 10 detection threshold. Furthermore, the observed atomic hydrogen in the exosphere cannot be entirely attributed to H₂ dissociation. Instead, it likely arises from a combination of thermal hydrogen atoms, charge exchange processes, space weather effects, H₂ dissociation and micrometeorite impacts.