Exospheres of Mercury and the Moon: an approach to investigate the different sources and depletion processes acting on airless bodies

The surface-bounded sodium exospheres of Mercury and the Moon are easily observed from the ground thanks to the fact that sodium is one of the brightest observable elements. Sodium act as crucial tracer for understanding exospheric dynamics in the inner Solar System.

While both celestial bodies share common release mechanisms—such as photon-stimulated desorption, micrometeoroid impact vaporization, and ion sputtering—their exospheric behaviors differ significantly due to their unique orbital parameters and space environments.

In fact, Mercury’s highly eccentric orbit, 3:2 spin-orbit resonance, and intrinsic global magnetic field lead to pronounced seasonal variations, longitudinal "cold-pole" accumulations, and intense interactions with the solar wind at the magnetic cusps.

In contrast, the Moon is tidally locked and lacks a global magnetic field, meaning its exospheric variations are more heavily influenced by its periodic passage through Earth's protective magnetotail, localized crustal magnetic anomalies, and transient meteoroid streams.

Furthermore, while both bodies exhibit massive, comet-like alkali tails driven by solar radiation pressure, Mercury's tail experiences extreme seasonal modulation based on its orbital true anomaly, whereas the lunar tail is uniquely observed via gravitational focusing by the Earth during the New Moon phase.

Comparing these two distinct environments highlights the complex interplay among surface processes, magnetospheric interactions, and space weather effects, and provide important hints on how the comparative planetology may act to disentangle processes and ambient to improve their comprehension.

We show how our long experience in the study of Mercury’s sodium exosphere morphology and dynamics can contribute a better understanding of the Moon’s exosphere and, more broadly, of the exospheres of airless bodies throughout the Solar System.