Long‐term compositional variation in solar corpuscular radiation: Evidence from nitrogen isotopes in the lunar regolith

Abstract

Implantation of solar corpuscular radiation into the lunar surface generates a population of solar atoms in the rims of lunar regolith grains. Laboratory analysis of those atoms can yield a measure of solar composition. Nitrogen trapped in the lunar regolith consists of at least two components, putatively originating in the Sun, differing in release temperature and therefore probably in implantation energy. The higher‐energy component is depleted in 15 N relative to the lower‐energy component by amounts that range up to at least 20%. These components superficially resemble those identified previously in the solar‐derived light noble gases, though with several marked differences. Thus the higher‐energy noble gas components are depleted in the lighter isotope. Unlike the noble gas case, the 15 N/ 14 N ratios of both N components vary with antiquity in a complex fashion; the lower‐energy component echoes the variations in the higher‐energy component which dominate the isotopic evolution of the bulk samples. The magnitude of the bulk sample variation exceeds 30%; the higher‐energy component varies by at least 25%. The bulk long‐term trend in 15 N/ 14 N does not result from variations in mixing ratio of the two components. Both the compositional difference between the components and the long‐term variations within them apparently originate in the Sun, though this conclusion is inconsistent with current understanding of solar structure and evolution. The nitrogen isotopic record therefore appears to represent a major challenge to solar physics.