The Lunar Surface-Exosphere Connection

Bounded by the emptiness of space and the Moon’s surface, the lunar exosphere is an atmosphere so thin that atoms rarely collide. This environment, composed primarily of neutral atoms and molecules, is generated by the interaction of radiation from the sun, meteoritic bombardment and radioactive decay, balanced by losses into space and recycling back to the surface. Sputtering by solar H⁺ and He⁺⁺ ejects lunar material into the exosphere at low energies; a fraction of which are ionized species. Sputtered secondary ions carry direct information about the planetary surface composition and are easily observed by spacecraft ion mass spectrometers. Measurements from the AMPTE, LADEE, WIND, and SELENE missions to the Moon have identified photo-ionized neutrals and secondary  ions in the exosphere, including: H₂⁺, He⁺, C⁺, O⁺, Na⁺, Al⁺, Si⁺, CO⁺, K⁺, Ar⁺, Ca⁺, and Fe⁺ [Hilchenbach et al. 1993; Stern 1999; Yakota et al. 2009, 2014; Halekas, J. S., et al. 2015].

To better understand the relationship between solar-wind derived secondary ions and the lunar surface composition, as well as guide the design of spacecraft mass spectrometers, secondary ion mass spectra (Fig. 1) were obtained during 4 keV He⁺ irradiation of lunar soils returned by the Apollo missions. For lunar analogs, we use < 1mm grains of two mature soil samples, mare 10084 and highland 62231, as well as a Corning glass lunar-simulant and the silicate-mineral olivine. Our sample surface compositions were measured by X-ray photoelectron spectroscopy (XPS) and SIMS spectra were taken with a Hiden Analytical EQS. For this study, positive SIMS spectra were measured at ejected ion energies from 2 to 36 eV, a range where the instrument transmission is expected to be roughly constant as a function of energy. The extraction lens voltages were optimized for ions of mass 27 (Al⁺) before collecting spectra. Because the lunar soils charged significantly under ion bombardment, the sample surface was charge neutralized during SIMS data collection using low energy (≤ 4 eV) electrons.