1. Graduate School of Earth and Planetary Science, University of Tokyo, Tokyo 113-0033, Japan
2. Solar-Terrestrial Environment Laboratory, Nagoya University, Honohara 3-13, Toyokawa, Aichi 442-8507, Japan
3. Earthquake Research Institute, University of Tokyo, Tokyo 113-0032, Japan
4. Institute of Isotope Geology and Mineral Resources, ETH-Zentrum, 8092 Zürich, Switzerland, and Department of Earth Planetary Sciences, Washington University, St Louis, Missouri 63130-4893, USA
5. †Present address: National Institute of Information and Communications Technology, Tokyo 184-8795, Japan

Correspondence to: M. Ozima¹ Correspondence and requests for materials should be addressed to M.O. (Email: EZZ03651@nifty.ne.jp).

Abstract

The nitrogen in lunar soils is correlated to the surface and therefore clearly implanted from outside. The straightforward interpretation is that the nitrogen is implanted by the solar wind, but this explanation has difficulties accounting for both the abundance of nitrogen and a variation of the order of 30 per cent in the ¹⁵N/¹⁴N ratio. Here we propose that most of the nitrogen and some of the other volatile elements in lunar soils may actually have come from the Earth's atmosphere rather than the solar wind. We infer that this hypothesis is quantitatively reasonable if the escape of atmospheric gases, and implantation into lunar soil grains, occurred at a time when the Earth had essentially no geomagnetic field. Thus, evidence preserved in lunar soils might be useful in constraining when the geomagnetic field first appeared. This hypothesis could be tested by examination of lunar farside soils, which should lack the terrestrial component.

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