Is there a record of interstellar pick-up ions in lunar soils?

Solar wind noble gases and nitrogen implanted in the surface layers of lunar grains have frequently been studied to infer the history of the solar wind. In sub-surface layers, and thus presumably from particles with higher energies than solar wind, a mysterious population, dubbed “SEP,” accounts for most of the implanted gas. This “SEP” population is mysterious for at least four reasons: i) In the case of neon it accounts for several tens of percent of the total amount of implanted gas, completely disproportionate from what is expected from solar wind particles; ii) its isotopic composition is distinct from solar; iii) while the heavy neon isotopes are enriched relative to 20Ne, 15N is depleted relative to 14N, signatures which are unexpected from known fractionation processes in particle acceleration; iv) the elemental abundance of N with respect to the noble gases (e.g., Ar) is inconsistent with solar abundances. Many attempts to explain the origin and nature of this mysterious component seem unsatisfac...