Modeling the production of submicroscopic iron in the lunar highlands

Abstract

Micrometeoroid impacts, solar wind bombardment, and dielectric breakdown driven by solar energetic particles all potentially alter the optical properties of the lunar regolith by creating submicroscopic metallic iron (smFe0), which includes both nanophase (<33nm) and microphase (>33nm) iron. We create a simple model that describes the time-dependent accumulation of optically active smFe0 in the lunar highlands. Our model synthesizes recent analyses of how space weathering processes form smFe0-bearing agglutinates and rims on soil grains and how impact gardening controls the exposure time of these grains. It successfully reproduces the results of a prior analysis of the formation of smFe0 in the highlands, particularly in regard to nanophase iron, showing that there is consistency among diverse analyses of Apollo samples and of orbital observations. We find that the results of our model are not consistent with the solar wind directly forming smFe0 (although the solar wind may play a role in optical maturation via hydrogen implantation). Our model results are consistent with smFe0 in the lunar highlands being created mainly by micrometeoroid impacts, with a possible contribution from dielectric breakdown weathering.