The role of meteoritic delivery to Mercury’s surface-exosphere environment

Because of Mercury’s close orbital position to the Sun and its low mass, the planet cannot retain a dense atmosphere and instead possesses an exosphere. The gaseous species detected so far - such as H, (Mariner 10, MESSENGER), He (Mariner 10, MESSENGER, BepiColombo flyby), (possibly) O (Mariner 10), Na, K, Ca (ground-based and MESSENGER), Mg (MESSENGER), and Li and H₂ (inferred from MESSENGER magnetic field data) - yield a column content in the order of about 10¹² cm⁻², which is lower than the expected column content of an exosphere which is in the order of about 10¹⁴ cm⁻². In addition to these elements, MESSENGER’s FIPS instrument also detected C, OH, H₂O, Ne, Al, Si, O₂, Ar, CO₂, Ti, and Fe ions at the downstream of Mercury. Atmospheric measurements from the Mariner 10 occultation experiment indicate that the electron density near the planet's surface is less than 10³ cm^-3 on both sides of the planet, implying an upper limit to the dayside surface gas number density of about 10⁶ cm^-3. The observed elements are expected to represent only a small fraction of Mercury’s exosphere,  as the total surface pressure contributed by these known species is far lower than the upper limit for the exospheric surface pressure of 10^-11 - 10^-12 bar. While H and He appear to originate from the solar wind (i.e., thermal release of implanted solar wind ions), heavier elements are sourced from the planet’s regolith and may be delivered by meteoroids of various sizes. In this study, we investigate the role of meteoroids and their contribution to the elemental input on Mercury's surface and exosphere. We also discuss the potential effects of meteoritic material on the volatile components of the planet's surface composition and examine which currently undetected elements, potentially delivered from meteoroids, may contribute to the still-unknown surface pressure.