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

Authors: R.F. Wimmer-Schweingruber and P. Bochsler
Reference: ACE-2000 Workshop Proceedings (AIP) (in press)

Abstract: 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 unsatisfactory. 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 1 AU by orders of magnitude. If this scenario is correct, lunar soils serve 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.

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Last Update: May 5, 2000 by James Weygand