SpIRRL: Spectroscopic Infrared Reflectance LIDAR

We propose to mature and demonstrate a highly sensitive six-wavelength surface reflectance lidar for orbital measurements of lunar surface hydroxyl and water ice absorption bands at 1.6 and 3-microns. The use of laser illumination provides a uniform lighting that enables pole-to-pole global observations during lunar day and night, as well as in permanently shadowed regions. Using pulsed laser signals and highly sensitive infrared detectors enable independent measurement and removal of the background solar illumination and thermal emission from the surface. We propose a three-year effort to build a brass board instrument and test it to bring the TRL from 4 to 6. The remote detection of water at the lunar surface by infrared spectroscopy by three spacecraft (Cassini, Deep Impact and Chandrayaan-1; Clark, 2009, Pieters et al. 2009; Sunshine et al. 2009) showed strong absorption in the 3-micron region, with equally strong variations in this band with lunar time of day. These observations show water or hydroxyl on the surface of the Moon can be detected by spectroscopy in the 3-micron region. In addition to this unambiguous remote detection of water-related chemistry, LCROSS detected remote detection of water from its impact into permanently shadowed terrain, Several techniques indirectly sensitive to water have reported potential detections in permanently shadowed regions at the lunar poles via radar, neutron spectroscopy, UV albedo, and 1064 nm laser reflectance, (Nozette et al. 1996; Feldman et al. 1998, 2001; Colaprete et al. 2010; Gladstone et al. 2012; Thompson et al. 2012, Zuber et al. 2012). However, none of these measurement techniques can measure with 100-m level spatial resolution in regions of permanent shadow. There continue to be model dependent uncertainties in the corrections of the background thermal emissions in passive remote sensing measurements. Our lidar, named Spectroscopic Infra-Red Reflectance Lidar (SpIRRL) uses an intra-cavity optical parametric oscillator (IOPO) laser. The pump laser source uses the same Nd:YAG lasers used in the Lunar Orbiter Laser Altimeter (LOLA). A potassium titanyl phosphate (KTP) crystal is used to convert the 1.064-micron pump laser light into two longer wavelength laser lights, one in the 1.6-micron band and one in the 3-micron band, depending on the crystal cut and phase matching. The receiver uses two single photon sensitive linear mode HgCdTe avalanche photodiode arrays to give high measurement precision at these laser wavelengths. There are three IOPO lasers with six output wavelengths. Performance calculations show that ~1% precision in surface reflectance at each wavelength can be obtained with ~1 km along track resolution from 50 km altitudes. From the six wavelength samples the surface abundance of water ice will be estimated via spectral matching calculations. All subsystems of the lidar are currently at TRL-5 or higher. We plan to build a brass board instrument with all the six laser wavelengths, demonstrate spectroscopic reflectance measurements to the required precision in the lab, and conduct environmental tests. Our objective is to demonstrate TRL-6 at the instrument level to enable us to propose SpIRRL to a near term lunar opportunity such as PRISM or Discovery. Our new measurement capability and the scientific impact of high sensitivity water abundance mapping are directly traced to questions raised in the Decadal Survey about lunar volatile composition, distribution, and transportation. It can also provide a global survey of water on the Moon for In Situ Resource Utilization and the Artemis program.

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