The Lunar Paleo‐Magnetosphere: Implications for the Accumulation of Polar Volatile Deposits

Abstract

Analyses of lunar samples suggest the Moon once possessed a dynamo from ~4.25 Ga until perhaps as recently as 1 Ga, with surface field strengths between ~5 and 100 μT. While the exact timing, strength, and structure of these paleomagnetic fields are not precisely known, such relatively strong fields imply that the Moon also likely possessed a magnetosphere. Here, we present hybrid plasma simulations of the structure and morphology of the putative lunar “paleo‐magnetosphere” for varying surface field strengths and orientations, using ambient solar wind conditions representative of the early Sun. The presence of the paleo‐magnetosphere reduces total solar wind fluxes to the lunar surface overall yet, for a spin‐aligned dynamo, increases the relative solar wind flux to the lunar polar regions. In turn, the paleo‐magnetosphere may have altered the rate of volatile accretion to the Moon over its history.

Evidence for an Ancient Near‐Equatorial Lunar Dipole From Higher Precision Inversions of Crustal Magnetization

“…Garrick‐Bethell et al. (2019) studied how the ancient dynamo field would have interacted with the early solar wind. They showed that an equatorial dipole would channel solar wind hydrogen over a substantial fraction of the surface as the Moon spins, in contrast to a spin‐aligned dipole, which would block solar wind access from most of the Moon.…”

“…Therefore, assuming the gravity and hydrogen studies provide reasonable constraints, any verified near‐equatorial magnetic paleopoles would suggest that the Moon's dynamo dipole axis was offset from the spin axis. This configuration could have implications for the dynamo mechanism (Takahashi et al., 2009), as well as implications for the formation of polar volatile deposits (Garrick‐Bethell et al., 2019). A major goal of this study is to reassess, in the context of new uncertainty estimation methods, the locations of previously reported near‐equatorial paleopoles.…”

Hybrid Simulations of Solar Wind Proton Precipitation to the Surface of Mercury

“…The model self-consistently couples the interior magnetic response to the ambient plasma environment using a semi-implicit method (model details are extensively explained in Fatemi et al, 2017). Amitis has been previously used to study the plasma interaction with the Moon (Fatemi et al, 2017;Fuqua Haviland et al, 2019;Garrick-Bethell et al, 2019;Poppe, 2019), asteroid 16 Psyche , and Mercury and our simulation results have been previously validated through comparison with analytical theories (Fuqua Haviland et al, 2019) and with ARTEMIS and MESSENGER observations (Fatemi et al, 2017Poppe, 2019).…”

A Double Disturbed Lunar Plasma Wake

“…This allows the hybrid model to provide a more accurate description of the ion physics at this scale while not being as computationally expensive as a purely kinetic model. Numerous studies have used Amitis to model solar wind interactions with planetary bodies ranging in size from asteroids to Mercury (Fatemi et al., 2017, 2018, 2020 Fatemi & Poppe, 2018; Fuqua Haviland et al., 2019; Garrick‐Bethell et al., 2019; Poppe, 2019).…”

“…This allows the hybrid model to provide a more accurate description of the ion physics at this scale while not being as computationally expensive as a purely kinetic model. Numerous studies have used Amitis to model solar wind interactions with planetary bodies ranging in size from asteroids to Mercury (Fatemi et al, 2017Fatemi & Poppe, 2018Fuqua Haviland et al, 2019;Garrick-Bethell et al, 2019;. Amitis runs were performed assuming a lunar conductivity of 10 −7 S/m and using two different sets of time-dependent upstream boundary conditions, each running until a steady-state is obtained.…”