Solar Wind Oxygen in Lunar Metal Grains

Trevor Ireland, Peter Holden, and Marc Norman

Research School of Earth Sciences, Australian National University, Canberra, ACT 0200, Australia

We have extended our studies on solar wind implanted oxygen in lunar soils. Previously we reported analyses of metal grains from lunar soil 10084 from the Apollo 11 landing site. Two grains have oxygen concentration profiles consistent with solar wind implantation. The oxygen concentration quickly decays over 10-20 nm and then stabilizes at a few wt % and decays slowly for a few hundred nanometers. The oxygen isotopic composition is enriched in the heavy isotopes of oxygen, 17 O and 18 O, by 5% relative to terrestrial oxygen. We have analyzed metal spherules from two further soils 61141 and 78481 from Apollo 16 and 17 respectively. The Apollo 17 soil has metal spherules that appear tarnished suggesting a surface oxidized layer. The oxygen signal is consistent with this interpretation with high and stable oxygen count rates, and the isotopic composition of the oxygen is normal. Some of the Apollo 16 spherules appear fresh and have low surface contributions of oxygen. However, the oxygen concentration falls monotonically with no suggestion of an implanted component. The oxygen isotope composition remains normal throughout the analysis. These new results do not provide us with another instance of solar wind oxygen, but they do provide additional information on the nature of exposure of lunar soils to solar wind. The gardening of lunar soil appears to be a stochastic process with variable exposure time to solar wind and variable oxidation of metal surfaces. The Apollo 16 spherules are particularly important because either they have never been exposed to solar wind, or the solar wind oxygen has diffused from them. At the temperatures expected for solar wind implantation O diffusion may be quite rapid and the oxygen lost on timescales of weeks to months.

A solar composition enriched in 17 O and 18 O is inconsistent with previous proposals suggesting either oxygen similar to terrestrial, or a composition enriched in 16 O by 5% as found in refractory inclusions and some chondrules. It possibly reflects distinct gaseous (predominantly CO) and dust (predominantly silicate) O isotopic compositions, and the domination of the dust component alone in the planetary system.