QUANTIFYING H ABUNDANCES ON THE MOON: THE ROLES OF COMPOSITION, OPTICAL CONSTANTS, AND BAND SHAPE

Reports of differing hydrogen contents in the lunar highlands and mare are challenging to interpret due to difficulties with quantification of H using the 3 μm band. There is a need for a simple means to measure H based on fundamental properties. Studies in the related geologic literature now provide data needed to better interpret H bands. The extinction coefficient, ε, for the 3 μm band has been determined for a wide range of glass compositions. The ε parameter for transmission spectroscopy is related to the k parameter used in reflectance spectroscopy along with n, which is usually constant in the near infrared, using the Kramers-Kronig transform. Using data from King and Larsen [1] and references therein, we find that ε varies systematically with glass composition expressed as (Si+Al)/total cations (in transmission) and calculated n (in reflectance). Combining what is known from transmission and reflectance spectra, we apply the results to the study of the lunar surface, where space weathering effectively converts much of the outermost surface into glass.

Systematic analyses of glass spectra from lunar-analog compositions are combined here with further analysis of the spectra from [1], with particular emphasis on the shape and components that make up the 3 μm band in glass reflectance and transmission spectra. In each spectrum, the 3 μm band has a distinctive shape that is a function of composition; each can be deconvolved into 1-3 Lorentzian or Gaussian component bands with consistent energies. Because both band shape and optical constants are directly related to composition, a relationship between band shape and optical constants is seen for any glass. Thus the shape of the 3 um band can directly inform the choice of appropriate optical constants to use to interpret the H abundances it represents. Automating this process would enable accurate measurements of H on various terrains on the lunar surface.

Complications to this simple algorithm include the variable effects of grain size on reflectance measurements of H in varying types of lunar materials, such as pyroclastic ashes, highland anorthosites, and mare basalts. Composition also affects solubility and diffusion of H in lunar materials, and may influence their resistance solar wind.

[1] King, P.L., and Larsen, J.F. (2013) Amer. Mineral., 98, 1162-1171.