A Comprehensive Model for Pickup Ion Formation at the Moon

A. R. Poppe; J. S. Halekas; Y. Harada

doi:10.1029/2022JE007422

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. Plain Language Summary The Moon is surrounded by a thin neutral atmosphere comprised of multiple atomic and molecular species. This atmosphere, technically termed an “exosphere,” is produced by multiple processes that are either internal or external to the Moon. As exospheric atoms and molecules are exposed to space, they are continuously ionized by photons, protons, and electrons from the Sun. Once ionized, these particles are accelerated by electromagnetic fields in interplanetary space and often lost from the Moon's environment. These “pickup ions” (so called because they are “picked up” into and carried along with the flow of particles in space by electromagnetic fields) provide information about the composition and distribution of the neutral lunar exosphere. We have constructed a model of 18 neutral and 20 ionized species in the lunar exosphere based on previous observations and/or models. The model predicts that the dominant pickup ion species in the lunar exosphere include carbon monoxide (CO+), 40‐argon (40Ar+), aluminum (Al+), sodium (Na+), and potassium (K+). The model is compared to observations of lunar pickup ions by NASA's ARTEMIS mission. We find that the model is missing a component of the pickup ion flux and suggest that the Moon may harbor a dense carbon dioxide (CO2) exosphere. Key Points We constructed an observationally based model of 18 neutral and 20 ionized species from the lunar exosphere Dominant pickup ion species are H2+ ${\mathrm{H}}_{2}^{+}$, He+, CO+, 40Ar+, Al+, Na+, K+, Si+, Ca+, and O+ with asymmetric spatial distribution The model underestimates pickup ion fluxes when compared to ARTEMIS observations, and we suggest CO2+ ${\mathrm{C}\mathrm{O}}_{2}^{+}$ as the missing component

A Comprehensive Model for Pickup Ion Formation at the Moon

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