Lunar soils have been thought to contain two solar noble gas components with distinct isotopic composition. One has been identified as implanted solar wind, the other as higher-energy solar particles. The latter was puzzling because its relative amounts were much too large compared with present-day fluxes, suggesting periodic, very high solar activity in the past. Here we show that the depth-dependent isotopic composition of neon in a metallic glass exposed on NASA's Genesis mission agrees with the expected depth profile for solar wind neon with uniform isotopic composition. Our results strongly indicate that no extra high-energy component is required and that the solar neon isotope composition of lunar samples can be explained as implantation-fractionated solar wind.

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During the Apollo 11 and 12 landings, solar wind ions were collected in a metal foil for periods of 77 min and 18 hours 42 min, respectively. Mass spectrometric analyses of the collected solar wind particles were made in the laboratory and give the following fluxes and relative abundances for the Apollo 11 and 12 exposure periods, respectively: 4He flux: (6.2±1.2)×106 and (8.1±1.0)×106 cm−2 sec−1; 4He/³He: 1860±140 and 2450±100; 4He/20Ne: 430±90 and 620±70; 20Ne/22Ne:13.5±1.0 and 13.1±0.6; 22Ne/21Ne: —— and 26±12. The differences in the 4He/³He and 4He/20Ne ratios are significant and are interpreted as time variations in the composition of the interplanetary plasma. From He/O ratios reported in the literature, an approximate solar wind O/Ne ratio of 10 is derived. Comparisons of the results of the solar wind composition experiment with data from implanted solar wind particles in lunar soil and with trapped gases in aubritic meteorites indicate a secular variation in the 4He/³He ratio at the solar surface.

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