The Need for Plasma Measurements on the Lunar Surface

To date, plasma observations in the energy range of a few eV to 10 keV on the lunar surface have been very limited. Most of these observations were conducted during the Apollo program by sensors in the Apollo Lunar Surface Experiments Package (ALSEP)—namely, the Solar Wind Spectrometer (SWS), the Suprathermal Ion Detector Experiment (SIDE) with Cold Cathode Ion Gauge, and the Charged Particle Lunar Environment Experiment (CPLEE). While some of these observations—such as plasma measurements on the nightside over the full lunar night—remain unique, the overall sensor performance was highly limited by modern standards. Instruments on the Chinese landers have been focused on specific objectives, such as the measurement of negative ions and energetic neutral atoms (ENAs). Therefore, our knowledge of the lunar 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 lunar exploration efforts that are about to commence. Improved plasma measurements are needed for:

• Proper modeling of surface and man-made object charging and potentials;
• Investigation of dust dynamics, including dust release and dust–plasma interactions;
• Study of space weathering processes that modify surface characteristics and composition;
• Understanding the formation, release, and dynamics of volatiles in the lunar exosphere;
• Exploring surface-level plasma dynamics and interactions with various plasma domains as the Moon moves along its orbit.

The measurement need to span over four fundamental scales:

• Microscale (kinetic, 10⁻⁴–10⁻² cm):
  To resolve the microphysics of particle–surface interactions
• Mesoscale (sub-Debye, 10 cm–10 m):
  To study plasma processes where quasi-neutrality breaks, generating strong electric fields
• Macroscale (MHD, 10 m–1 km):
  To explore connections between plasma dynamics and lunar topography
• Global scale (MHD, 1 km–1,000 km):
  To reveal the influence of large-scale structures, such as magnetic anomalies and the terminator, on local plasma populations

In this presentation, we:

• Demonstrate the limitations of current surface-level plasma knowledge
• Highlight the critical importance of advancing this understanding
• Provide an overview of the measurement requirements and techniques needed to address these scientific and operational gaps