SECULAR CHANGES IN THE XENON AND KRYPTON ABUNDANCES IN THE SOLAR-WINDRECORDED IN SINGLE LUNAR GRAINS

NOBLE gases implanted into the lunar surface trace elemental and isoto pic abundances in the solar wind during the Sun's lifetime, and so pot entially provide a valuable record of changing physical processes on t he Sun. But the interpretation of this record is not straightforward, as it has proved difficult to discriminate the effects of solar variat ions from changes due to subsequent alteration processes on the lunar surface. Here we report analyses of krypton and xenon abundances in in dividual 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 th e 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 sever al billion years. Moreover, the measured enrichment of xenon, relative to argon, is similar to a well-known excess of easily ionizable eleme nts in the solar chromosphere. As xenon is not an easily ionizable ele ment, this finding supports the hypothesis that the selection effect a rises from element-specific ionization times in the solar chromosphere (2,3).