Correlation Between Lunar Surface and Exospheric Sodium: Effects of Albedo-Driven Temperature on Multilayer Sodium Reservoirs Rather Than Surface Abundance Variations

TL;DR

This study shows that lunar sodium exosphere variability is mainly driven by surface temperature and albedo effects on multilayer reservoirs, not by surface sodium abundance differences.

Contribution

It provides new observational evidence linking surface temperature and albedo to sodium release, emphasizing thermo-physical processes over compositional variations.

Findings

Exospheric sodium correlates with surface temperature and albedo.

Surface sodium shows diurnal modulation due to thermal desorption.

No significant compositional difference between mare and highland terrains.

Abstract

Sodium (Na) is a moderately volatile element in the lunar exosphere, released from the surface through thermal and non-thermal processes. We present a combined analysis of Chandrayaan-2 CLASS surface Na abundance, LADEE-UVS exospheric measurements, and DIVINER surface temperature data to investigate the coupling between surface and exospheric Na. Surface Na exhibits a pronounced diurnal modulation, with depletion during lunar daytime and enhancement at dawn and dusk, consistent with thermal desorption from weakly bound multilayer reservoirs (>1 ML). Exospheric Na shows longitudinal enhancements above low-albedo mare regions, whereas CLASS-derived surface abundances reveal no significant compositional differences between mare and highland terrains. The observed exospheric structure correlates strongly with surface temperature and albedo, implicating temperature-dependent thermal desorption as the dominant release mechanism at low altitudes. These findings indicate that the spatial variability of Na release efficiency, rather than surface Na abundance, primarily governs the distribution of lunar exospheric sodium. This study places a new observational evidence on sodium retention on the lunar surface and release mechanisms and demonstrates the dominant influence of surface thermo-physics in controlling the near-surface lunar sodium exosphere.