On deciphering the particle track record of lunar regolith history

Abstract

Detailed primary particle track densities and density gradients were obtained for ∼100 µm soil grains. Data combined from several characteristic soil samples are shown independently of soil models to be characterized by two distinguishable populations of tracks as follows. Population 1 consists of apparently younger tracks with low track density pc ≲ 108 cm−2 and a track density gradient distribution consistent with a relatively short exposure and simple orientation history within the top soil layer. Fresh soils tend to contain mostly population 1 tracks. Population 2 tracks, which are much more common, occur with high track density pc ≲ 3×108 cm−2 and gradients consistent with longer or repeated reoriented exposures within 1 mm of the lunar surface. Older soils contain predominantly population 2 tracks, although some population 1 tracks may be present as a result of recent surface exposure or admixture of fresh soil. A Monte Carlo model is outlined that is designed to simulate the near-surface history of ∼100 µm diameter grains during surface exposure episodes (SEE) lasting ∼106 yr each while they are within 0.5 cm of the surface. Applied to some soil samples, the only variable parameters are the fine-scale burial rate L, dependent on surface typography, and the number of SEE, related to maturity. Applied to primary particle tracks, we show detailed model results for L = 0.2, 0.4, and 0.8 cm/106 yr and 1, 10, and 50 SEE. The data are generally consistent with these results and imply an average L ∼ 0.4 cm/106 yr. Fresher soil is consistent with 1–10 SEE, and older soil with 10–50 SEE. Some applications are discussed.