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