The Mystery of the Missing Solar Wind Oxygen in Lunar Soil\

Trevor Ireland¹, Michelle Bannister², and Masahiko Honda¹

1 Research School of Earth Sciences, Australian National University, Canberra, ACT 0200, Australia
2 Research School of Astronomy and Astrophysics, Australian National University, Canberra, ACT 0200, Australia

 

We have previously reported an isotopically heavy component (5% enriched in ¹⁷O and ¹⁸O) in the lunar soil that is preserved in the top few hundred nanometers of lunar metal grains.  The location of this component appears consistent with the site expected for solar wind oxygen.  However, McKeegan and colleagues reported at the Lunar and Planetary Science Conference this year that solar wind appears enriched in ¹⁶O.  We have carried out measurements on a larger number of grains from a variety of soils in an attempt to elucidate this discrepancy.  Many more grains have been discovered with ¹⁷O ¹⁸O enrichments with surprisingly few having ¹⁶O-rich compositions, and none around the 6% enriched composition reported by McKeegan et al. (2008).  Further analyses by Hashizume and Chaussidon (2008) also reveal ¹⁷O, ¹⁸O-rich compositions in modern lunar soils, while ¹⁶O rich compositions in ancient soils are typically mass-dependently fractionated.  The question then arises, what happened to the solar wind oxygen on the surface of the Moon.

We have measured Ne isotopic compositions in olivine grains from the lunar soils and these are consistent with solar wind implantation.  It should therefore be expected that solar wind oxygen should be present in the surfaces of these grains.  We cannot measure olivine grains for implanted solar-wind oxygen because of the high intrinsic concentration of oxygen in olivine.  The issue is either the high density of the lunar metals biasing metal grains from exposure to the solar wind, or an issue of preservation.  To address this, we will attempt to measure Ne in these metal grains to ascertain their exposure history.  The solar wind exposure takes place at lunar surface temperatures of around 100°C.  At such temperatures, oxygen diffusion is probably a major issue in retention in the lunar grains, particularly with the high flux of hydrogen carried by the solar wind.  These issues will be examined with experiments on oxygen diffusion in metals under appropriate physical conditions.

While solar wind appears poorly represented in the metal grains, the component enriched in ¹⁷O and ¹⁸O is quite widespread.  This component may be carried in cometary water, which is expected to have an isotopically heavy composition. The preservation of this component may be related to subsurface reaction of water with Fe metal.