Physics
Scientific paper
Dec 2011
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2011agufm.p12c..06d&link_type=abstract
American Geophysical Union, Fall Meeting 2011, abstract #P12C-06
Physics
[5464] Planetary Sciences: Solid Surface Planets / Remote Sensing
Scientific paper
Abundant spacecraft data now demonstrate the presence of features associated with H on the lunar surface. The origin of that lunar H, whether as OH or H2O, is some combination of endogenic (juvenile) sources in the interiors of planetary materials and those resulting from exogenic deposition such as from the solar wind or comets. The ability of mineral (rock) and glass surfaces to internally host and surficially adsorb H is a function of several interrelated variables -- composition, crystallinity, and texture -- all of which will have an effect on observed band depth in remote sensing measurements. Studies of terrestrial materials show that the ability of nominally-anhydrous minerals to host H is related to composition in ways that reflect partition coefficients for H between melt and mineral, variations in bond strengths, and defect densities. This is important because the ability of a mineral to adsorb water on its exterior surface (chemisorption) should be related to some of the same factors that govern 'solubility' of H in the interiors of different mineral groups and compositions. IR signatures of internal OH/H2O can easily be confused with those of adsorbed OH/H2O. No correlation between H solubility and surface adsorptivity is observed in pristine glasses, which generally have passivated bonds on the surface and are hydrophobic. However, on the Moon, glass 'matures' rapidly via micrometeorite bombardment, potentially exposing dangling bonds on the surface that provide sites for H to adsorb. Unlike glasses, crystalline materials provide both defect lattice sites and dangling bonds on freshly-fractured surfaces that may enhance H adsorption. For example, bonding on mineral surfaces ranges from hydrogen bonding at non-lattice oxygen atoms (electronegative sites) to chemisorption at electropositive surface sites, such as structural defects or unsatisfied cations. Moreover, glasses and different mineral species also have different optical absorption coefficients. For example, OH region absorption coefficients for forsterite and clinopyroxene are 3× higher than those for feldspar and orthopyroxene [1] and orders of magnitude different from silicate glasses. Thus bands with equal areas from different materials (e.g. anorthite vs. olivine/pyroxene/glass in lunar highlands vs. maria) do not imply equal H concentrations, further complicating the task of deriving H concentrations from remote-sensed spectra of mixed phases. Also, crystalline materials on the lunar surface are extremely rough, with an effective spherical particle size of ~ 1 μm, which means that the reflectance spectra will be dominated by H from the exterior of the grains by providing more surface area for bonding of adsorbates than on smooth surfaces, also enhancing band depth due to optical scattering. [1] Rossman, G.R. (2006) Revs. Mineral. Geochem., 62, 1-28.
Darby Dyar M.
Grieves Gregory A.
Hibbitts C.
Orlando Thomas M.
Poston M.
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