Morphological and Spectral Characterization of Lunar Regolith Breakdown Due to Water Ice

Remote sensing observations of the lunar surface suggest that the regolith at the lunar poles is affected by several natural processes that create distinctly more porous and fine-grained regolith. One such process may be the mechanical breakdown of regolith particles by frost wedging. As the natural cracks and pores within a regolith particle are infiltrated by water, the pressure generated by the freezing of this water diminishes the particle's strength. After repeated exposures, this process may break the particle apart, producing an increase in fine-particulate material within permanently shadowed regions at the lunar surface. To evaluate the extent to which lunar regolith may be weathered by ice-regolith interactions in the Moon's polar regions, we present morphological and spectral analyses of high-fidelity lunar regolith simulants LHS-1 (lunar highlands simulant-1) and LMS-1 (lunar mare simulant-1) that have been exposed to various concentrations of water ice over extended periods of time. Alteration of regolith particle morphology is explored by analyzing changes in individual grain size and shape parameters and quantifying the abundance and behavior of clinging fine particulate material. Visible-to-near infrared and mid-infrared reflectance spectra taken under ambient laboratory conditions from 1 -25 μm (400-10,000 cm^-1) are used to identify and characterize any differences in spectral features that may occur as a result of the proposed regolith breakdown mechanism. With increasing water concentration and length of freezing time, all analyses display trends that indicate changes in particle shape and increased generation of sub-micron fine particulate material. The results of our experiment are presented with a focus on how they are impacted by simulant composition and particle size. Particles of Mg-rich composition display more weathering than particles of Ca/Al-rich composition. Shape and size are altered for particles < 63 μm in diameter, while particles > 63 μm in diameter remain relatively unchanged. This work demonstrates that the lunar regolith is susceptible to alteration in the presence of water ice and that water ice is likely a contributor to the weathering environment within permanently shadowed regions on the lunar surface.