Using polarized infrared spectroscopy to characterize surface adsorbed and internal water in planetary materials

The separation of surface adsorbed water and internal water at the grain level is important for understanding the behavior of water on the lunar surface. Polarized spectroscopy can distinguish surface specular reflection from volume scattering, thus has a great potential to be applied at infrared wavelengths to distinguish surface adsorbed water from internal water. Using an FTIR spectrometer, we measured the polarized infrared spectra of optical grade CaF2 containing zero internal water, and opal samples with internal water. We found that for both polarized parallel and perpendicular branches, the 3 mu m water absorption depth is stronger for smaller grain size fractions of CaF2 samples, but is weaker for smaller grain size fractions of opal samples. The parallel branch detected stronger water bands compared to the perpendicular branch for both samples. We also discovered that the parallel to perpendicular ratio spectra show minimal variation among different grain size fractions for CaF2 samples, while the opal samples exhibit an increase in 3 mu m absorption depth with increasing grain size. In addition, the ratio spectra can largely suppress organic contamination. These observations indicate that polarized infrared spectroscopy can detect variations in the relative content of surface-adsorbed water and internal water. Polarized infrared diffuse spectroscopy is promising for applications in remote sensing to distinguish between surface adsorbed water and internal water.