The lunar 40Ar/36Ar antiquity indicator in the presence of a lunar paleomagnetosphere

A.R. Poppe, I. Garrick-Bethell, S. Fatemi, C. Grava

In Press, Journal Pre-proof, Available online 6 April 2024



• Model the effects of a lunar paleomagnetosphere on argon flux to the lunar surface.
• Paleomagnetosphere suppresses 36Ar flux by an order-of-magnitude.
• Paleomagnetosphere enhances 40Ar flux by factor of 3.
• Paleomagnetosphere required to balance increased solar wind 36Ar flux at earlier epochs.”

“The ratio of 40Ar/36Ar trapped within lunar grains, commonly known as the lunar antiquity indicator, is an important semi-empirical method for dating the time at which lunar samples were exposed to the solar wind. The behavior of the antiquity indicator is governed by the relative implantation fluxes of solar wind-derived 36Ar ions and indigenously sourced lunar exospheric 40Ar ions. Previous explanations for the behavior of the antiquity indicator have assumed constancy in both the solar wind ion precipitation and exospheric ion recycling fluxes; however, the presence of a lunar paleomagnetosphere likely invalidates these assumptions. Furthermore, most astrophysical models of stellar evolution suggest that the solar wind flux should have been significantly higher in the past, which would also affect the behavior of the antiquity indicator. Here, we use numerical simulations to explore the behavior of solar wind 36Ar ions and lunar exospheric 40Ar ions in the presence of lunar paleomagnetic fields of varying strengths. We find that paleomagnetic fields suppress the solar wind 36Ar flux by up to an order-of-magnitude while slightly enhancing the recycling flux of lunar exospheric 40Ar ions. We also find that at an epoch of 2 Gya, the suppression of solar wind 36Ar access to the lunar surface by a lunar paleomagnetosphere issomewhat fortuitouslynearly equally balanced by the expected increase in the upstream solar wind flux. These counterbalancing effects suggest that the lunar paleomagnetosphere played a critical role in preserving the correlation between the antiquity indicator and the radioactive decay profile of indigenous lunar 40K. Thus, a key implication of these findings is that the accuracy of the 40Ar/36Ar indicator for any lunar sample may be strongly influenced by the poorly constrained history of the lunar magnetic field.”