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

Abstract

The distributions of 40 Ar/ 39 Ar-derived ages of impact glass spherules in lunar regolith samples show an excess at <500 Ma relative to older ages. It has not been well understood whether this excess of young ages reflects an increase in the recent lunar impact flux or is due to a bias in the samples. We developed a model to simulate the production, transport, destruction, and sampling of lunar glass spherules. A modeled bias is seen when either (1) the simulated sampling depth is 10 cm, consistent with the typical depth from which Apollo soil samples were taken, or (2) when glass occurrence in the ejecta is limited to >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.

Temporal Evolution of S‐Band Circular Polarization Ratios of Kilometer‐Scale Craters on the Lunar Maria

“…Regardless, our measurements require that centimeter-to-decimeter-scale clasts within the upper meter or so of regolith are hard to destroy and therefore persist in craters' ejecta deposits for billions of years, a conclusion also reached by Ghent et al (2016) based on observations of larger craters. This is also consistent with recent regolith evolution modeling by Huang et al (2018).…”

“…For this reason, we interpret the persistent enhancement in CPR in the proximal ejecta as mostly from subsurface clasts (as also suggested by Bandfield et al, 2011, and Ghent et al, 2014. This is also consistent with recent regolith evolution modeling by Huang et al (2018). Regardless, our measurements require that centimeter-to-decimeter-scale clasts within the upper meter or so of regolith are hard to destroy and therefore persist in craters' ejecta deposits for billions of years, a conclusion also reached by Ghent et al (2016) based on observations of larger craters.…”

“…This vertical column of infilling material will remain quite coarse-grained for billions of years, as both our observations and models of regolith mixing (e.g., Huang et al, 2018) imply that once clasts are buried, they are hard to fragment. This vertical column of infilling material will remain quite coarse-grained for billions of years, as both our observations and models of regolith mixing (e.g., Huang et al, 2018) imply that once clasts are buried, they are hard to fragment.…”

Impact Gardening as a Constraint on the Age, Source, and Evolution of Ice on Mercury and the Moon

“…To calculate the colored contours we solve equation with values for and calculated using equation (which include assumptions about material properties, gravity, velocity, impact angle, and the scaling from impactor size to crater size) for impacts into lunar regolith and the flux of secondary impactors produced by a lunar‐scaled Brown et al () meteorite flux. We assume that the modern flux captured by Brown et al () has been stable for the last 1 Gyr (e.g., Neukum et al, ; Huang et al, ). The countours labeled “LROC CPF” are calculated using CPF values for and following values reported in Speyerer et al () for the production of splotches and scaled to transient crater diameter by a factor of 0.84 (Melosh, ; Methods section of Speyerer et al, ).…”

“…To calculate the secondary flux we assumes that the modern primary impact flux captured by Brown et al (2002) at the Moon has been stable over the last 1 Gyr (e.g., Neukum et al, 2001;Huang et al, 2018); however, recent work suggests that there may have been an order of magnitude uptick in the flux about 200 Myr ago (Mazrouei et al, 2019). Variations in the impact flux translate directly to variations in the rate of impact gardening.…”