No Change in the Recent Lunar Impact Flux Required Based on Modeling of Impact Glass Spherule Age Distributions

The distributions of 40Ar/39Ar‐derived ages of impact glass spherules in lunar regolith samples show an excess at 10 crater radii from the crater, consistent with terrestrial microtektite observations. We suggest that the observed excess of young ages for lunar impact glasses is likely due to limitations of the regolith sampling strategy of the Apollo program, rather than reflecting a change in the lunar impact rate. Plain Language Summary Lunar regolith samples collected by the Apollo astronauts contain impact glass spherules that record the age of formation in the Ar‐Ar isotope dating system. There are as many spherules with measured ages within the last 500 million years as there is in the previous 4 billion years of lunar history, and it has remained a mystery as to whether this is because the impact rate was higher in the recent past, or if there was some process that was biasing these samples toward a young age. We have developed a three‐dimensional computer model that simulates the production, transport, destruction, and sampling of impact‐generated glass spherules on the Moon. Using reasonable assumptions that are backed up from data on Earth craters, we are able to reproduce the observed excess of young spherule ages seen in the Apollo samples assuming that impact rate has not changed over the last three billion years. We find that the young age bias is only seen because the Apollo samples were collected in the upper few centimeters of the lunar surface. Future glasses collected from the upper few meters of the surface should have ages that better reflect the true rate of impacts over time. Key Points An excess of young lunar impact glass spherules