Laboratory studies on the sputtering contribution to the sodium atmospheres of Mercury and the Moon

To ascertain the importance of sputtering by solar wind ions on the formation of a sodium exosphere around Mercury and the Moon, we have irradiated with 4 keV He ions, the Na bearing tectosilicates: albite, labradorite, and anorthoclase, as well as adsorbed Na layers deposited on albite and on olivine (a neosilicate that does not contain Na). Sodium at the surface and near surface (<40 Å) was quantified with X-ray photoelectron spectroscopy before and after each irradiation to determine the depletion cross section. We measured a cross section for sputtering of Na adsorbed on mineral surfaces, σs ≈ 1 × 10−15 cm2 atom−1. In addition, mass spectrometric analyses of the sputtered flux show that a large fraction of the Na is sputtered as ions rather than as neutral atoms. These results have strong implications for modeling the sodium population within the mercurian and the lunar exospheres.

Research highlights► XPS, SIMS studies of 4 keV He irradiated feldspars, olivine with/without adsorbed Na. ► Sputtered Na ion fraction close to 1 for albite. ► Sputtering cross section for Na adsorbed on minerals ≈ 1 × 10−15 cm2 atom−1. ► Surface Na decay in feldspars 100× slower due to radiation-induced diffusion.