Solar Wind Record on the Moon: Deciphering Presolar from Planetary Nitrogen

Abstract

Ion microprobe analyses show that solar wind nitrogen associated with solar wind hydrogen implanted in the first tens of nanometers of lunar regolith grains is depleted in ¹⁵ N by at least 24% relative to terrestrial atmosphere, whereas a nonsolar component associated with deuterium-rich hydrogen, detected in silicon-bearing coatings at the surface of some ilmenite grains, is enriched in ¹⁵ N. Systematic enrichment of ¹⁵ N in terrestrial planets and bulk meteorites relative to the protosolar gas cannot be explained by isotopic fractionation in nebular or planetary environments but requires the contribution of ¹⁵ N-rich compounds to the total nitrogen in planetary materials. Most of these compounds are possibly of an interstellar origin and never equilibrated with the ¹⁵ N-depleted protosolar nebula.