^1,2Chanud N. Yasanayake,¹Brett W. Denevi,³Takahiro Hiroi,⁴Brad. L. Jolliff,¹Anna C. Martin,^3,5Annabelle L. Gao,^6,7Margaret L. Zhang,^8,9Lucas M. Bloom,¹⁰Samuel J. Lawrence
Journal of Geophysical Research (Planets) (in Press) Open Access Link to Article [https://doi.org/10.1029/2023JE008115]
¹Johns Hopkins University Applied Physics Laboratory, Laurel, MD, USA
²Johns Hopkins University, Baltimore, MD, USA
³Brown University, Providence, RI, USA
⁴Washington University in St. Louis, St. Louis, MI, USA
⁵Marriotts Ridge High School, Marriottsville, MD, USA
⁶University of North Carolina at Chapel Hill, Chapel Hill, NC, USA
⁷Mount Hebron High School, Ellicott City, MD, USA
⁸University of Alabama, Tuscaloosa, AL, USA
⁹Severna Park High School, Severna Park, MD, USA
¹⁰NASA Johnson Space Center, Houston, TX, USA
Published by arrangement with John Wiley & Sons

The lunar surface evolves over time due to space weathering, and the visible–near-infrared spectra of more mature (i.e., heavily weathered) soils are lower in reflectance and steeper in spectral slope (i.e., darker and redder) than their immature counterparts. These spectral changes have traditionally been attributed to the space-weathered rims of soil grains (and particularly nanophase iron therein). However, understudied thus far is the spectral role of agglutinates—the agglomerates of mineral and lithic fragments, nanophase iron, and glass that are formed by micrometeoroid impacts and are ubiquitous in mature lunar soils. We separated agglutinates and non-agglutinates from six lunar soils of varying maturity and composition, primarily from the 125–250 μm size fraction, and measured their visible–near-infrared reflectance spectra. For each soil, the agglutinate spectra are darker, redder, and have weaker absorption bands than the corresponding non-agglutinate and unsorted soil spectra. Moreover, greater soil maturity corresponds to darker agglutinate spectra with weaker absorption bands. These findings suggest that agglutinates (rather than solely the space-weathered rims) play an important role in both the darkening and reddening of mature soils—at least for the size fractions examined here. Comparisons with analog soils suggest that high nanophase iron abundance in agglutinates is likely responsible for their low reflectance and spectrally red slope. Additional studies of agglutinates are needed both to more comprehensively characterize their spectral properties (across size fractions and in mixing with non-agglutinates) and to assess the relative roles of agglutinates and rims in weathering-associated spectral changes.