Abstract
Abstract The analysis of solar wind material implanted within lunar soil has provided significant insight into the makeup and evolutionary history of the solar wind and, by extension, the Sun and protosolar nebula. These analyses often rely on the tacit assumption that the Moon has served as an unbiased recorder of solar wind composition over its 4.5 billion yr lifetime. Recent work, however, has shown that for a majority of its lifetime, the Moon has possessed a dynamo that generates a global magnetic field with surface field strengths of at least 5 μT. In turn, the presence of such a field has been shown to significantly alter the lunar–solar wind interaction via the formation of a lunar “paleomagnetosphere.” This paleomagnetosphere has implications for the flux of solar wind minor ions to the lunar surface and their subsequent implantation in lunar soil grains. Here we use a three-dimensional hybrid plasma model to investigate the effects of the lunar paleomagnetosphere on the dynamics and precipitation of solar wind minor ions to the lunar surface. The model results show that the lunar paleomagnetosphere can suppress minor ion fluxes to the lunar surface by more than an order of magnitude and strongly fractionates the precipitating solar wind in a complex, nonlinear fashion with respect to both the minor ion charge-to-mass ratio and the surface paleomagnetic field strength. We discuss the implications of these results with respect to both the analysis of trapped material in lunar grains and the semiquantitative 40Ar/36Ar antiquity indicator for lunar soils.
Highlights
The results showed no significant differences between the spatial densities and precipitation flux to the lunar surface between the two heavy ion species, as expected given the q/m scaling in the Lorentz force law.) In the hybrid model, we set the density of all eight heavy minor ion species to be 10−6 cm−3 such that they do not affect the plasma interaction of the core solar wind protons with the lunar paleomagnetic field and act only as “test particles.”
Using the three-dimensional Amitis hybrid plasma model, we have simulated the interaction of solar wind protons and heavy minor ions with the lunar paleomagnetosphere over a range of assumed paleomagnetic field strengths
The hybrid model results show the successive growth of a lunar paleomagnetosphere with increasing surface magnetic field strength, in line with previous hybrid simulations performed by Garrick-Bethell et al (2019)
Summary
Examination of lunar regolith samples returned by the Apollo missions has revealed the implantation of exogenous species within the regolith matrix that originate primarily from the solar wind (e.g., Pepin et al 1970, 1995, 1999; Reynolds et al 1970; Becker & Clayton 1975; Wieler et al 1986; Becker & Pepin 1989, 1994; Benkert et al 1993; Kerridge 1993; Nichols et al 1994; Wieler 1998), with some additional contributions from terrestrial ionospheric outflow (e.g., Wieler et al 1999; Ozima et al 2005; Poppe et al 2016; Terada et al 2017; Wei et al 2020). Motivated by the findings of Tikoo et al (2017) that established the lunar low-field paleomagnetic epoch, Garrick-Bethell et al (2019) investigated the generation of the “lunar paleomagnetosphere” via the use of a three-dimensional hybrid plasma model (hybrid refers to the use of fluid electrons and particle ions; see, e.g., Fatemi et al 2017) This study described both the overall magnetospheric interaction of the lunar paleomagnetosphere with the solar wind and the flux and energy distribution of precipitating protons for surface paleomagnetic field strengths of 0.5, 1, and 2 μT. We discuss the implications of these results for understanding exogenous species embedded in lunar soil and the 40Ar/36Ar antiquity indicator, in particular with respect to the process of “magnetospheric fractionation” that occurs when the lunar paleomagnetosphere alters the dynamics of solar wind ions before they reach the lunar surface
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