Solar wind nitrogen and indigenous nitrogen in lunar material

Abstract

There is ample evidence that the main source of the abundant light elements in lunar, fine-grained regolith samples in the solar wind. The extra-lunar contribution is easy to detect, because lunar igneous rocks are, in general, strongly depleted in light volatile elements, such as H, He, C, N and Ne. We report on chemically bound nitrogen contents in various lunar samples from the Apollo 11 through Apollo 17 landing sites by using the Kjeldahl method. This technique detects only nitrogen in a bound state and discriminates against molecular nitrogen N 2 of atmospheric origin. We have found by the analysis of grain-size fractions, by leach-experiments, by comparison with trapped noble gases and with agglutinate contents that the main amount of the nitrogen in the fines is derived from the solar wind by ion implantation and is present primarily in a bound state. Leach-experiments on Apollo 15 and Apollo 17 fines indicate two chemical forms of nitrogen: ammonium- and nitride-nitrogen. Nitrogen and carbon chemistry in lunar fines appear to be analogous in respect to the hydrogen compounds ammonia and methane, and the presence of nitrides and carbides. Hot atom chemistry probably causes the synthesis of those compounds on the surface area of fine grains. Implanted nitrogen is better retained in fine-grained material than carbon and the light noble gases and is, therefore, a useful reference in terms of solar elemental abundances. Carbon to nitrogen ratios of lunar fines are interpreted as lower limits for the abundance ratio of these elements in the sun. Bound nitrogen contents have also been determined in several lunar igneous rocks and U.S.G.S. standard rocks using an optimized, program-controlled Kjeldahl method. This technique is superior to the previously applied conventional Kjeldahl method with respect to sensitivity and reproducibility for nitrogen analysis. Most of the lunar igneous rocks analyzed in this work are quite uniform and low in bound nitrogen contents; the range is only about 3 to 8 ppm N for samples of various types. Lunar fines and soil breccias contain, however, a factor of 10 to 20 more nitrogen than igneous rocks. The small nitrogen contents of the rocks imply that indigenous nitrogen and/or ammonia were at very low concentrations at the time of mineral and rock formation in the early history of the moon. This indicates either severa a priori nitrogen depletion of the moon's parent material or rigorous loss of nitrogen and other volatile elements in the early history of the moon.