Plasma on the lunar surface: Terra Incognita to be explored

To date, plasma observations on the lunar surface have been very limited. Most of these observations were conducted during the Apollo program, relying on technology that is now over 50 years old, or by a few individual sensors with narrowly defined scientific objectives on Chinese landers. Our knowledge of the plasma environment and the processes that govern it remains sparse, relying heavily on orbital measurements and theoretical models. This current level of understanding is insufficient to support the large-scale exploration efforts that are about to commence. In essence, we are navigating a "Terra Incognita" in this domain.

To characterize and understand near-surface plasma and its interactions with the lunar surface are required for:

• Properly modeling surface and man-made object charging and potentials;
• Investigating dust dynamics, including dust release and dust-plasma interactions;
• Studying weathering processes that modify surface characteristics and compositions;
• Establishing the role of the space environment in the formation, release, and dynamics of volatiles within the lunar exosphere;
• Understanding plasma dynamics at the surface and interactions with various plasma domains as the Moon moves along the orbit.

Plasma and its interaction processes need to be studied across four fundamental scales:

• Microscale (kinetic, 10^-4 – 10^-2 cm) to address microphysics of the particle – surface interaction
• Mesoscale (sub-Debye, 10 cm–10 m) to investigate plasma process when quasi-neutrality breaks creating strong electric fields
• Macroscale (MHD, 10 m–1 km) to explore connections between plasma dynamics and topography
• Global scale (MHD, 1 km–1,000 km) to reveal effects of large-scale structures, such as magnetic anomalies and the terminator, on local plasma populations.

In this presentation, we demonstrate the limitations of our current knowledge, highlight the critical importance of advancing it, and outline steps to explore the "Terra Incognita" of plasma on the lunar surface.