Deciphering diverse sources of the nitrogen in lunar soil

A team from the Centre de recherches pétrographiques et géochimiques
(CRPG/ CNRS – Vandœuvre les Nancy ; Petrographic and Geochemical Research Center) and the laboratoire de pétrologie, minéralogie, métallogénie (CNRS - Universités Paris 6 et Paris 7 - Muséum national d’histoire naturelle ; Petrology, Mineralogy and Metallogeny Laboratory, National Natural History Museum), has deciphered the diversity of the sources of the nitrogen present on the lunar surface, employing a method it has been developing. Using static mass spectrometry coupled with laser extraction and the ims 1270 ion probe, this team has shown that the sun is not the sole source of the nitrogen deposited on the moon. This nitrogen originates not only from the solar wind but also from meteorite or comet material deposited on the moon during impacts. Although samples of lunar soil were returned to earth 30 years ago, such analyses have not been possible despite the international efforts of many laboratories. These results are important both as regards our knowledge of the sun and as regards the possible sources of nitrogen in the solar system with respect to the debate on the origin of organic material essential for the appearance of life.
This work has recently been published in Science.
One of the team, CNRS research director Marc Chaussidon, receives the regional research prize for the Lorraine region on Thursday November 23rd in respect of his work on the 1270 ion microprobe, used to study meteorites, lunar soil and ancient terrestrial samples.

Lunar soil is an accumulation of debris from meteorites and probably from comets that have fallen onto our satellite since its creation 4.4 billion years ago. Depending on their provenance, these soil fragments are of different ages and compositions. Added to this is fine dust and gases from interstellar space and especially from the solar wind, which strikes the lunar soil where it becomes embedded. The moon has registered these extra-terrestrial add-ons since its creation, while on earth they have been completely erased from the surface due to erosion and plate tectonic movement.

The moon thus constitutes a unique natural experiment for cosmic irradiation, cast over geological time.

Analyses carried out over the last three decades have shown that the rare gases contained in the lunar soil are implanted into the grains constituting the soil by the sun’s radiation. Their chemical and isotopic composition is constant and similar to that directly measured in the solar " wind " (particle radiation). In contrast, an analysis of the nitrogen in lunar soil has shown a very substantial variability in the isotopic composition, of the order of 30%. Until now, this variability has been interpreted as due to a modulation in the sun’s composition over time, but that does not explain the apparent absence of variation for rare gases.

The new method developed by the team at CRPG and the Petrology, Mineralogy and Metallogeny Laboratoy has enabled grain-by-grain analysis of the isotopic compositions of the rare gases and nitrogen contained in about ten samples of lunar soil provided to CRPG by NASA to be carried out. This method has demonstrated the diversity of nitrogen sources present on the lunar surface: the solar wind on the one hand and meteorite or comet material on the other hand, added to the Moon during impacts. Analyses carried out with CRPG’s 1270 ion probe have lifted the veil over these sources. They provide access to profiles of the variation in isotopic composition with depth from the surface of lunar grains to the interior, with a resolution of the order of 10 nanometers. This latter is essential as the relatively low energy of solar radiation means that it can only penetrate about the first 50 nanometers of lunar grain thickness. These profiles demonstrate the presence of solar nitrogen, as it is associated with purely solar hydrogen, i.e., containing no deuterium. This nitrogen is depleted in 15N (nitrogen isotope 15) by more than 24%, while the grain surface carries a further nitrogen component enriched in 15N resulting from bombardment of the moon by micrometeorites or comets. Thus the fine surface film of a single lunar grain bears witness to external additions to the moon’s surface.

This discovery has two important implications. Firstly, it can qualify and quantify the extraterrestrial flux of matter over time. The moon’s soil bears witness to cosmic bombardment over many epoques and the team hopes to reconstruct previous bombardment rates. Such a bombardment has, of course, also affected the Earth, but at a higher intensity since the Earth’s surface is larger. This bombardment may have been a vector for elements essential to the development of life on Earth (volatile elements such as water, carbon, nitrogen and organic material) and its quantification is clearly very important. In addition, the great heterogeneity of nitrogen isotopes in the solar system implies the transport of material on a solar system scale, which would have enabled exchange between the central regions such as that where the Earth was formed, and the outermost regions where lower temperature phases such as organic material were most probably to be found.

Space probe measurements of the solar wind are, of course, very important in this context and will enable these discoveries to be confirmed or otherwise. Results from the SOHO European probe suggest enrichment of the solar wind in 15N, while those from the team at CRPG and the Petrology, Mineralogy and Metallogeny Laboratory indicate a depletion with respect to planetary nitrogen. These measurements will have to be carried out again with greater accuracy. Determination of the isotopic nitrogen composition in the solar wind constitutes one of the major aims of a future NASA Discovery mission known as Genesis. A further method for determining the composition of solar gases is to analyse the atmosphere of gas giants such as Jupiter, the atmospheres of which were directly captured from gas from the solar nebula. 4 years ago, the Galileo mission sent a mass spectrometer into Jupiter’s atmosphere to measure its composition. The Galileo mission team is publishing fresh analyses of the nitrogen composition, which are completely compatible with those carried out by CRPG, since their measurements indicate a 15N depletion of 36% in Jupiter’s atmosphere while the team from the CRPG and the Petrology, Mineralogy and Metallogeny Laboratory have fixed their lower limit at 24%.

Reference:
Solar wind record on the Moon : Contrasting nitrogen isotopic composition of protosolar gas and planetary bodies by K. Hashizume, M. Chaussidon, B. Marty et F. Robert, Science, 11th November 2000, vol 290, pp. 1142-1145.

Research contact:
Bernard Marty and Marc Chaussidon
Centre de recherches pétrographiques et géochimiques
(CRPG/ CNRS – Vandœuvre les Nancy)
Tel : +33 3 83 59 42 14
e-mail: bmarty@crpg.cnrs-nancy.fr
e-mail: chocho@crpg.cnrs-nancy.fr

INSU-CNRS contact:
Christiane Grappin
Tel: +33 1 44 96 43 37
e-mail: christiane.grappin@cnrs-dir.fr

Press contact:
Carine Noël
Tel: +33 1 44 96 46 06
e-mail: carine.noel@cnrs-dir.fr

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