A Comprehensive Model for Pickup Ion Formation at the Moon

Abstract

The lunar exosphere is an ensemble of multiple overlapping, noninteracting neutral distributions that reflect the primary physical processes acting on the lunar surface. While previous observations have detected and constrained the behavior of some species, many others have only circumstantial evidence or theoretical modeling suggesting their presence. Many species are so tenuous as to be unobservable by direct neutral sampling, yet in comparison, measurements in their ionized form provide a particularly sensitive method of detection. To better aid the interpretation of past measurements and planning of future observations, we present a model for the production of lunar pickup ions from the Moon consisting of two components: An analytic model for the distributions of 18 neutral species produced by various mechanisms and an analytic model for the ionization and subsequent acceleration of 20 exospheric and surface‐sputtered pickup ion species. The dominant lunar pickup ions in the model are H2+ ${\mathrm{H}}_{2}^{+}$, He+, CO+, 40Ar+, Al+, Na+, K+, Si+, Ca+, and O+ with an asymmetric distribution favoring the positive interplanetary electric field hemisphere of the Moon. We compare the model predictions to statistically averaged pickup ion fluxes around the Moon as observed by the ARTEMIS spacecraft over the past decade. By filtering for interplanetary electric field‐aligned, high‐energy observations, we find that the pickup ion model lacks an additional source of heavy species. We suggest that a dense CO2 exosphere of 3 × 104 − 1 × 105 cm−3 could account for the missing pickup ion flux as part of the recycling of solar wind carbon ions incident to the Moon.

Limits on the Density of the Lunar Ionosphere: ARTEMIS Observations

“…However, the very low density of solar wind plasma in the lunar wake should highly suppress these charge exchange reactions, casting doubt on charge exchange as a major source of ionization at midnight. Meanwhile, works by Halekas et al (2018) and Poppe et al (2022) have revealed no non-solar-wind ion densities higher than 0.3 and 0.1 cm −3 , respectively, within the sunlit portion of the lunar environment, in disagreement with high ion densities predicted by the model of .…”

Space plasma physics science opportunities for the lunar orbital platform - Gateway

“…The low number densities of this very tenuous atmosphere, particularly of the minority species, and the complexity and multiplicity of the source and loss mechanisms (including thermal release, photon stimulated desorption, electron stimulated desorption, sputtering, micrometeorite impact vaporization, etc.) have resulted in a poor understanding of this system (Wurz et al, 2007;Poppe et al, 2022;Wurz et al, 2022). Figure 9 provides the altitude density profiles of the major species, separately for the atoms and molecules released thermally from the regolith and for the atoms released through sputtering.…”

“…− In-situ measurement of freshly ionized pickup ions, originating from the lunar exosphere neutral species. (Hartle and Killen, 2006;Yokota et al, 2009;Wang et al, 2011;Poppe et al, 2022). At high altitudes above the lunar surface, as those of the Gateway orbit (cf.…”

“…At high altitudes above the lunar surface, as those of the Gateway orbit (cf. section 4.1), this method can provide higher sensitivity in the detection of low number density species than the direct sampling of the parent neutrals (Halekas et al, 2015;Poppe et al, 2022).…”

Energetic Neutral Atom (ENA) Emission Characteristics at the Moon and Mercury From 3D Regolith Simulations of Solar Wind Reflection

“…Similar to its predecessors, BepiColombo does not include a lander and our understanding of Mercury is thus further strongly connected to remote analysis methods. As it is done for the Moon (Poppe et al., 2022), one important method for investigating Mercury's surface has been the analysis of the exosphere around the planet (Killen et al., 2007), allowing insights into composition and surface processes. It is populated by release of atoms from the surface and here sputtering by ion impacts represents a key driver for some species (Wurz et al., 2022).…”