The effect of asymmetrical MMIV flux on the yearly variability of Mercury's Sodium exosphere

Observations of Mercury’s sodium exosphere reveal a distinctive annual variability, with two intensity peaks occurring near aphelion and perihelion. We identify a "hot pole" of micrometeoroid impact vaporization (MMIV) as a critical source of sodium, which is active for all Mercury orbit, with prevalence on the dawn side, but whose effect is evident to explain Mercury’s aphelion Sodium peak. On a yearly cycle, the MMIV is more enhanced in two regions, located at 90° and 270° longitude at the equator. Using new data from the Themis telescope, we confirmed this hypothesis by employing a Markov Chain Monte Carlo (MCMC) method to rigorously test the relevance of this asymmetric source. This model accurately captures both the magnitude and the seasonal patterns of Mercury’s Na exosphere. The results highlight the combined effect of Mercury’s rotation and orbital position in modulating sodium source and loss processes, explaining the observed dawn-side enhancements. Additionally, the model underscores the necessity of a persistent sodium supply on the nightside, potentially driven by plasma interactions or micrometeoroid precipitation, while also shedding light on the still-puzzling dusk-side sodium increases observed by MESSENGER during its inbound passes.

Figure: Yearly-averaged MMIV precipitation onto the surface of Mercury (from Mura et al., 2023 https://doi.org/10.1016/j.icarus.2023.115441). The two regions at 90 and 270 of longitudes are "hot poles" of MMIV precipitation.