Metallic species, oxygen and silicon in the lunar exosphere: Upper limits and prospects for LADEE measurements

Abstract

[1] The only species that have been so far detected in the lunar exosphere are Na, K, Ar, and He. However, models for the production and loss of species derived from the lunar regolith through micrometeoroid impact vaporization, sputtering, and photon-stimulated desorption, predict that a host of other species should exist in the lunar exosphere. Assuming that loss processes are limited to ballistic escape, photoionization, and recycling to the surface, we have computed column abundances and compared them to published upper limits for the Moon. Only for Ca do modeled abundances clearly exceed the available measurements. This result suggests the relevance of some loss processes that were not included in the model, such as the possibility of gas-to-solid phase condensation during micrometeoroid impacts or the formation of stable metallic oxides. Our simulations and the recalculation of efficiencies for resonant light scattering show that models for other species studied are not well constrained by existing measurements. This fact underlines the need for improved remote and in situ measurements of the lunar exosphere such as those planned by the Lunar Atmosphere and Dust Environment Explorer (LADEE) spacecraft. Our simulations of the LADEE neutral mass spectrometer and visible/ultraviolet spectrometer indicate that LADEE measurements promise to provide definitive observations or set stringent upper limits for all regolith-driven exospheric species. We predict that observations by LADEE will constrain assumed model parameters for the exosphere of the Moon.

Lunar exospheric argon modeling

“…Other sources for argon include surface sputtering, but Wurz et al (2007) demonstrated that it is a negligible process, at least for argon (Sarantos et al (2012) showed that it is not so for refractories and metals). While an important source for the lunar helium exosphere, the solar wind delivers very little 40 Ar compared to 36 Ar, as demonstrated by Yaniv and Heymann (1972).…”

Imaging Plasma Density Structures in the Soft X-Rays Generated by Solar Wind Charge Exchange with Neutrals

“…modeled the soft X-ray intensities produced by charge exchange at the Moon and predicted them to be about 10 keV cm −2 s −1 sr −1 , comparable to the diffuse cosmic X-ray background (Lumb et al 2002). presented observations of limb brightening in the ROSAT lunar data consistent with the expected signal from solar wind charge exchange with the lunar exosphere and compared these observations to the predictions of hybrid simulations for solar wind ion access to regions behind the terminator of the Moon Farrell et al 2008;Fatemi et al 2012) and models of the lunar exosphere (Sarantos et al 2012;Tenishev et al 2013). Of particular note is that the soft X-rays produced by charge exchange can provide diagnostics on the entire neutral exospheric density including all species, whereas other techniques are spectroscopic in nature and observe particular lines, for example Na (Potter et al 2000).…”

Structure and composition of the distant lunar exosphere: Constraints from ARTEMIS observations of ion acceleration in time‐varying fields

“…A number of observational studies have constrained neutral source densities [Feldman and Morrison, 1991;Flynn and Stern, 1996;Stern et al, 1999;Cook et al, 2013]; however, these studies placed only upper limits on many species. We therefore compare our inferred production rates to modeled distributions [Wurz et al, 2007;Sarantos et al, 2012aSarantos et al, , 2012bPoppe et al, 2016].…”