LONG-TERM COMPOSITIONAL VARIATION IN SOLAR CORPUSCULAR RADIATION - EVIDENCE FROM NITROGEN ISOTOPES IN THE LUNAR REGOLITH

Implantation of solar corpuscular radiation into the lunar surface gen erates a population of solar atoms in the rims of lunar regolith grain s. Laboratory analysis of those atoms can yield a measure of solar com position. Nitrogen trapped in the lunar regolith consists of at least two components, putatively originating in the Sun, differing in releas e temperature and therefore probably in implantation energy. The highe r-energy component is depleted in N-15 relative to the lower-energy co mponent by amounts that range up to at least 20%. These components sup erficially resemble those identified previously in the solar-derived l ight noble gases, though with several marked differences. Thus the hig her-energy noble gas components are depleted in the lighter isotope. U nlike the noble gas case, the N-15/N-14 ratios of both N components va ry with antiquity in a complex fashion; the lower-energy component ech oes the variations in the higher-energy component which dominate the i sotopic evolution of the bulk samples. The magnitude of the bulk sampl e variation exceeds 30%; the higher-energy component varies by at leas t 25%. The bulk long-term trend in N-15/N-14 does not result from vari ations in mixing ratio of the two components. Both the compositional d ifference between the components and the long-term variations within t hem apparently originate in the Sun, though this conclusion is inconsi stent with current understanding of solar structure and evolution. The nitrogen isotopic record therefore appears to represent a major chall enge to solar physics.