Evidence for a predominantly non-solar origin of nitrogen in the lunar regolith revealed by single grain analyses

We measured the amounts of 14N and 36Ar in single mineral and glass grains from a lunar soil by laser extraction, with the goal of studying the controversial origin of trapped nitrogen in the lunar regolith. The average 14N/36Ar ratio of 29 ilmenite grains is 379, similar to the value determined previously on a large ilmenite separate from the same soil and 10 times larger than the solar ratio of 37. However, the 14N/36Ar ratios in the individual grains vary between 1 and 440 times the solar ratio. 36Ar amounts in the ilmenite grains scatter by more than two orders of magnitude, N amounts by less than a factor of 6. The variability of the 14N/36Ar ratio forms a striking contrast to the very uniform relative abundances of Ar, Kr, and Xe trapped from the solar corpuscular radiation observed earlier in ilmenite and other mineral grains from the same soil. This strongly suggests that, on average, some 90% of the N in the grains has a non-solar source, contrary to the often expressed view that essentially all N in the lunar regolith has been trapped from the solar wind. The conclusion that the lunar regolith testifies to a secular variation of the N isotopic composition in the solar wind of ∼30% becomes thus highly questionable. The origin of the bulk of trapped lunar nitrogen remains unknown.