Nitrogen Isotopes on the Moon: Archives of the Solar and Planetary Contributions to the Inner Solar System

The two isotopes of nitrogen, 14N and 15N, have relative abundances extremely variable among solar system reservoirs such as planets and their atmospheres, primitive and differentiated meteorites, and comets. Expressed in the delta notation (δ15N = {[15N/14N]sample/[15N/14N]standard−1} × 1000, in parts per mil, or ‰., where the standard is atmospheric N having 15N/14N = 0.003676), δ15N ranges from −250‰. (the lower limit of lunar soil values) up to 1600‰. (measured in the meteorites benccubinites). The lunar surface constitutes a unique archive of the past corpuscular (solar and meteoritic) contributions to planetary surfaces. Nitrogen trapped in the lunar regolith presents a highly variable isotopic composition, which represents either secular variation of the solar wind composition although this possibility conflicts with the apparent isotope stability over time of other solar wind volatile elements, or more likely different contributions from solar corpuscular radiation and non-solar sources. In this case, the solar nitrogen component is depleted by more than 24% in 15N, whereas non-solar, planetary sources (meteorites, micrometeorites, possibly comets) are enriched in the heavy isotope of nitrogen by ≥10% on average. Variations in the nitrogen isotopic composition of lunar soils are explained by a secular change in the strength of the planetary flux, and a correlation between N isotopic compositions and surface exposure age for different soils suggest that the planetary contribution to the inner solar system might have increased in the last 0.4 Gy. The variability of the N isotope composition among solar system objects might be due to incomplete equilibration of nitrogen isotopes from different host phases of pre-solar origin. Alternatively, it could result from mixing between 15N-depleted protosolar nitrogen originally present in the gas and presolar solid (organic?) compounds enriched in 15N.