Nitrogen in solar energetic particles: Isotopically distinct from solar wind

Stepwise etching of lunar soil ilmenite grains reveals that the ¹⁵N/¹⁴N ratio of implanted nitrogen decreases with increasing implantation depth within the ilmenite grains, i.e., with increasing energy of implantation. These results show that N derived from solar energetic particles, N(SEP), is enriched in the light isotope, ¹⁴N, relative to solar-wind nitrogen, N(SW). This is in striking contrast to the neon isotopic record: Ne(SEP) is depleted in the light isotope, ²⁰Ne, relative to Ne(SW). These data suggest either distinct signatures in the respective solar source regions, or fractionation in the acceleration mechanism(s). However, the observed opposite fractionation trends for light N and Ne isotopes do not agree with model predictions.