The lunar space weathering rate: dichotomies in agents and products

Abstract:

The lunar surface undergoes continuous physical and chemical alternation due to its exposure to the space environment, in a process known as space weathering. The relative contribution of micrometeorites and solar wind to lunar space weathering is actively debated. We present new empirical constraints on the accumulation rates of space weathering products broadly on the lunar surface and show that the solar wind and micrometeorites may drive differing scale-dependent effects.

Micrometeorites are micron-scale impactors which bombard the surface, melting exposed regolith and vapor-depositing an amorphous coating on individual grains. The solar wind consists of ions and electrons which irradiate the surface, sputtering off lighter elements while enriching the surface in hydrogen. Both micrometeorites and solar wind have been linked to the reduction of mineral iron in regolith grains to form nano- to micro-scale metallic iron on the rims of individual regolith grains. This nano- and microphase iron has been characterized in Apollo samples, laboratory simulated space weathering studies, and more recently in global remote sensing datasets due to their unique effects on the visible spectrum.

Using maps produced by Trang & Lucey (2019), we extract the nanophase and microphase iron abundance in the ejecta of craters with known/modeled ages and, by plotting submicroscopic iron abundance as a function of age, determine the rate at which submicroscopic iron accumulates over time. In a targeted study of the lunar highlands (chosen for its relatively homogeneous iron content), we show that the rate of accumulation of nanophase iron is decoupled from that of microphase iron. We also show that the microphase iron saturates on shorter timescales than nanophase iron. We hypothesize that solar wind is necessary to produce nanophase iron, while micrometeorites can produce microphase iron independently. This interpretation is consistent with recent studies of lunar swirls, which show a paucity of nanophase iron – possibly due to magnetic standoff of solar wind – but enough microphase iron to be indistinguishable from the background regolith. We conclude that the dichotomy in lunar space weathering agents produces distinct effects in the rate of accumulation and saturation timescale of nano- and microphase iron on the Moon.