Mapping Lunar Volatiles and Hydration in Light and Shadow via Orbital Lidar Reflectance Measurements from NIR to MWIR.

Abstract:

Permanently shadowed regions (PSRs) at the lunar poles may act as a “witness plate” to early solar system volatile history, and preserve a record of volatile species: (i) remaining after the formation of the Moon, (ii) delivered by impact, (iii) resulting from lunar volcanism, and (iv) formed by the interaction of the surface with solar wind protons. Water has been the main volatile of interest due to its vital role in many important chemical and physical processes. However, other volatiles [1] also play crucial roles and act as signatures of different volatile sources, such as CO₂ and NH₃ being indicative of a cometary origin [2, 3]. A comprehensive map of water ice and other volatiles in PSRs is required to characterize composition, abundance and distribution and detect any temporal variability.

Hydration across the lunar surface was detected remotely by infrared spectrometers [4-7], and the band-depth signatures near 3 µm have indicated that hydration varied with latitude, local time, and surface temperature. This variation was suggestive of an active “lunar water cycle” with migration along temperature gradients, including towards the poles, sourced by the reduction of oxygen-bearing minerals by solar wind protons [7].

These interconnected questions can now be simultaneously addressed with an orbital lidar that simultaneously measures the surface reflectance at several wavelengths in the 3 µm region. We have developed the Resolve lidar approach for mapping hydration (water and hydroxyl) and the ices of H₂O, NH₃, and CO₂ on the lunar surface globally from a polar orbit. The Resolve lidar is designed to continuously measure the reflectance of the lunar surface at six primary wavelengths between 2 and 3.1 µm.

This multiwavelength lidar enables unambiguous surface reflectance measurements in darkness and sunlight across the entire lunar surface. With its fixed illumination geometry, no complex photometric corrections are needed to compare the spectral reflectance signatures across the Moon [8]. The Resolve lidar approach is designed to provide global coverage every two weeks (half a lunar day), like that of LRO. This allows the lidar footprints to sample the surface at all local times, enabling study of the diurnal changes of surface hydration at all latitudes, including measurements at the dawn and dusk terminators.