Multiwavelength Lidar for Remote Spectroscopic Surveys of Volatiles on the Lunar Surface

Our understanding of the history of water on the lunar surface is tied to the formation and early evolution of silicate bodies in the solar system. In addition, the current water cycle on the moon, including origination, migration, and deposition, should be investigated to yield insights into the nature of volatile processes on planetary surfaces. Since they provide their own light source, lidar measurements of surface reflectance are particularly useful because they view the surface with a uniform illumination geometry. To date the surface reflectance measurements from planetary laser altimeters have used only a single laser wavelength. Although informative, these albedo-only measurements suffer from an inherent ambiguity because many physical and chemical characteristics can cause surface albedo to vary.

To resolve these ambiguities, we plan to develop a multi-wavelength lidar to TRL 6 for a lunar lander or rover that will remotely measure the surface reflectance at eight mid-infrared wavelengths - spanning a range from 1.06 µm to 3.5 µm - to probe volatiles on the local lunar surface. The lidar will measure at carefully-selected wavelengths in the 3-micron region where water-bearing phases are strongly absorbing at very low (tens of ppm) abundances, and where organics also have strong absorption features. The eight lidar beams will be simultaneously emitted from a small lens assembly mounted on the lander's camera mast, allowing measurements to be directed to any surface within 10-m to 1-km range from the lander. The reflectivity measurements will have high sensitivities for surfaces at all solar illumination angles as well as in darkness (i.e. during the lunar night or in shadowed regions). The lidar wavelengths will be targeted at key spectral features that allow resolving ambiguities in the volatile composition of the surface and changes in local volatile abundance over time. A rover or lander mounted lidar enables localization of volatiles/organics to several-meter accuracy, allowing the spectroscopic study of individual crater walls, central uplifts, boulders, and local slopes.