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

Alessandro Mura, Anna Milillo, Valeria Mangano, Francois Leblanc, Martina Moroni, Christina Plainaki, Stefano Massetti, Stefano Orsini, Mirko Stumpo,

Pierpaolo Dibartolomeo

► To cite this version:

Alessandro Mura, Anna Milillo, Valeria Mangano, Francois Leblanc, Martina Moroni, et al.. The effect of asymmetrical MMIV flux on the yearly variability of Mercury's Sodium exosphere. EPSC-DPS Joint Meeting 2025, Sep 2025, Helsinki, Finland. 10.5194/epsc-dps2025-1538 . hal-05281457

HAL Id: hal-05281457

https://hal.science/hal-05281457v1

Submitted on 25 Sep 2025

**HAL** is a multi-disciplinary open access
archive for the deposit and dissemination of sci-
entific research documents, whether they are pub-
lished or not. The documents may come from
teaching and research institutions in France or
abroad, or from public or private research centers.

L'archive ouverte pluridisciplinaire HAL, est
destinée au dépôt et à la diffusion de documents
scientifiques de niveau recherche, publiés ou non,
émanant des établissements d'enseignement et de
recherche français ou étrangers, des laboratoires
publics ou privés.

<figure><img src="image_1.png" /></figure>

Distributed under a Creative Commons Attribution 4.0 International License
<figure><img src="image_2.png" /></figure>

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

**Alessandro Mura**<fnref n="1" />, **Anna Milillo**<fnref n="1" />, **Valeria Mangano**<fnref n="1" />, **François Leblanc**<fnref n="2" />, **Martina Moroni**<fnref n="1" />, **Christina Plainaki**<fnref n="3" />, **Stefano Massetti**<fnref n="1" />, **Stefano Orsini**<fnref n="1" />, **Mirko Stumpo**<fnref n="1" />, and **Pierpaolo Dibartolomeo**<fnref n="1" />

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.

<footnote n="1">¹INAF, IAPS, Rome, Italy (alessandro.mura@inaf.it)</footnote>

<footnote n="2">²LATMOS, Paris, France</footnote>

<footnote n="3">³ASI, Rome, Italy</footnote>
<figure><img src="image_3.png" /></figure>

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.