Constraints on Lunar Space Weathering Rates from Landscape Evolution Modeling

The chemical evolution of the lunar surface is dominated by the effects of micrometeorite impacts and energetic ions from the solar wind. These processes, collectively known as space weathering, alter the optical properties and microstructure of surface grains through the deposition of nanometer-sized reduced metallic Fe particles [1,2]. Lab measurements have characterized space weathering in Apollo sample grains, using proxies like rim thickness and Fe oxidation state [3]. However, to constrain the rate at which lunar soil grains are chemically altered it is necessary to understand how space weathering processes operate at macroscopic scales. The amount of time a given grain spends on the surface undergoing space weathering depends on the rate at which the regolith is being overturned by impacts and diffusive mass-wasting. In this work, we link measurements of the microscopic chemical alteration of soil materials to the macroscopic physical evolution of the lunar surface using a numerical landscape evolution model. We calibrate this model by matching the topographic statistics of our synthetic surfaces to those of the real lunar surface, specifically the slope and amplitude of the topographic power spectrum calculated from LOLA-derived digital terrain models [4]. With this calibrated model we track the 3D position of tracer particles as the surface evolves and evaluate the distribution of surface residence times experienced by lunar soil grains. Finally, we compare this distribution to the distribution of grain weathering proxies in order to deduce the lunar space weathering rate. [1] Hapke, (2001), JGR: Planets, https://doi.org/10.1029/2000JE001338. [2] Noble et al., (2005), Meteoritics and Planetary Science, https://doi.org/10.1111/j.1945-5100.2005.tb00390.x. [3] Thompson et al., (2016), Meteoritics and Planetary Science, https://doi.org/10.1111/maps.12646. [4] Smith et al., (2010), Space Science Reviews, https://doi.org/10.1007/s11214-009-9512-y.