Full IR Remote Sensing of the Moon’s Highland Crust for Water Definition

The isotopic content of water is a key indicator of its quality and suitability for the human use. The full IR interval, from far- to near-IR, is considered to define spectral features of the various isotopic forms of water (–OH/–OD and H2O/HDO/D2O) contained in anorthite. Herewith, water can be included in the crystal structure and/or adsorbed on the surface. Anorthite is important as the main rock-forming mineral of lunar polar crust which covers more than 70% of the lunar surface. The results of numerical modeling with ALT/CUSTEP module (Biovia Materials Studio) and experimental studies by IR Fourier spectrometer with plug-ins for transmission/diffuse reflection as well as Raman InVia Renishaw Reflex spectrometer are presented. Numerical calculations demonstrated that the crystal lattice of anorthite remains stable when hydroxyl groups and molecular water are included in it. Raman spectra indicated complex network of water clusters on the anorthite surface after wetting. The calculated IR spectra of anorthite containing water isotopologues were compared with the observations of SOFIA IR observatory (NASA/DLR). The presence of ‒OH/–OD and HDO isotopic forms of water is well confirmed, but additional probing is needed to detect pure D2O water more precisely. Suggestions on the most efficient orbital measurements for water mapping on the lunar highland by IR spectroscopy are provided.