Investigating Molecular Water and Hydroxyl in Granular Material through 0.14 - 8 micron Bidirectional Reflectance Measurements

Water, as H₂O or OH, is ubiquitous in the surfaces of airless solid bodies through-out the inner solar system. The 3-mm absorption feature provides insight into the hydration states of minerals [e.g. 1,2], meteorites [e.g. 3,4,5], and has been used to infer water on asteroids [e.g. 6,7]. The 6-mm absorption feature is uniquely indicative of molecular water [e.g. 8,9] providing the first unequivocal detection of molecular water on the surface of the Moon [10]. The UV region, although possessing a strong absorption feature in water ice, has so far provided equivocal information on granular material hydration [11]. The 6-mm band is proving able to detect the presence of molecular water that does not clearly affect the spectrum of the 3-mm region [12]. Here we compare and contrast bidirectional reflectance measurements of powdered samples of extra-terrestrial samples and analogs over these wavelengths in which water and hydroxyl are spectrally active in an attempt to understand the relationship between the respective hydration signatures. We are exploring the possibility that the combination of these features can provide robust information on the relative abundance of OH and H₂O, and whether the OH and/or H₂O is contained within the regolith grains or is (chemi)-adsorbed onto the grains' surfaces.

References: [1] Bishop et al., 1994, Clays and Clay Minerals, 42, 6, 702-716. [2] Milliken and Mustard, 2005, J. Geophys. Res., 110, doi:10.1029/2005JE002534. [3] Sato et al., 1997, Met. & Planet. Sci., 32:503-507. [4] Takir et al., 2013, Met. & Planet. Sci., 48, 9, 1618-1637. [5] Takir et al., 2019, Icarus, 333, 243-251. [6] Rivkin et al., 2006, Met. & Planet. Sci., 38, 9, 1383-1398. [7] Takir et al., 2012, Icarus, 219, 641-654. [8] Bartholemew et al., 1980, J. Amer. Ceramic Soc, 63, 481-485. [9] Newman et al., 1986, Amer. Min., 71, 1527-1541. [10] Honniball, C., et al., 2020, LPI, (2326), 1422. [11] Hibbitts et al., 2017, 417-10. [12] Hibbitts et al., 2020, LPI, (2326), 2526