NO CHANGE IN THE LUNAR IMPACT FLUX THROUGH THE COPERNICAN PERIOD FROM MODELING IMPACT GLASS SPHERULE AGE DISTRIBUTION IN LUNAR REGOLITH

Standard lunar crater chronology systems assume there has been little change in the impact flux over the past 3 billion years (Gy). However, the age distribution of lunar impact glass spherules derived from ⁴⁰Ar/³⁹Ar dating shows an apparent increase in the lunar impact rate in the last 0.4-0.5 Gy. It remains unclear whether this apparent increase in the impact flux is real, or whether it is the result of a young age bias in the sampled spherule populations.

Here we construct a model for the evolution of age distribution of spherule populations within the lunar regolith using the three-dimensional Cratered Terrain Evolution Model (CTEM) Monte Carlo cratering code. In this model, we simulate a region of the lunar surface that experiences 3 Gy of impact cratering under the assumption of a constant flux of impactors. Each simulated impact event in the model both produces new spherules and reworks pre-existing spherules, and tracks their distribution through the regolith column. We constrain the spatial distribution of spherules in the ejecta using data derived from the distribution of terrestrial microteketites, which are assumed here to be analogues of lunar impact glass spherules.

Our results suggest that the apparent increase in the impact flux in the last 0.4-0.5 Gy seen in the ⁴⁰Ar/³⁹Ar age distribution of lunar impact glass spherule population is likely the result of a young age bias. This bias results from the limited sampling depth of the regolith by the Apollo astronauts, rather than as a result of intrinsic change in the impact rate.