R.F. Wimmer-Schweingruber and Peter Bochsler

Physikalisches Institut
University of Bern
Sidlerstrasse 5
CH 3012 Bern, Switzerland

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, thus completely disproportionate from what is expected from normal solar 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 accompanying particle acceleration; iv) the elemental abundance ratio of N with respect to the noble gases (e.g. Ar) is inconsistent with solar abundance pattern. Many attempts have been made to explain the origin and nature of this mysterious component. In this work we propose that pick-up ions from interstellar neutrals, accelerated in the heliosphere and subsequently implanted into grains of the lunar regolith might account for the large amount of non-solar "SEPs". The solar system must have encountered various dense interstellar clouds throughout its history. Some of them must have compressed the heliosphere considerably, and they must have increased the ACR flux at 1AU by orders of magnitude. Lunar soil might have served as a 'travel diary' for the voyage of the solar system through the galaxy, preserving records of the isotopic and elemental composition of dense interstellar clouds.