Secular changes In the xenon and krypton abundances in the solar wind recorded in single lunar grains

NOBLE gases implanted into the lunar surface trace elemental and isotopic abundances in the solar wind during the Sun's lifetime, and so potentially provide a valuable record of changing physical processes on the Sun. But the interpretation of this record is not straightforward, as it has proved difficult to discriminate the effects of solar variations from changes due to subsequent alteration processes on the lunar surface. Here we report analyses of krypton and xenon abundances in individual grains of lunar soil (in contrast to the multi-grain samples used previously), which permit such discrimination. The abundances of the heavy noble gases have not been altered on the Moon, confirming the view(1) that there have been modest variations in the composition of solar wind, on timescales of 10(8)-10(9) years, during the past several billion years. Moreover, the measured enrichment of xenon, relative to argon, is similar to a well-known excess of easily ionizable elements in the solar chromosphere. As xenon is not an easily ionizable element, this finding supports the hypothesis that the selection effect arises from element-specific ionization times in the solar chromosphere(2,3).