11
*Centre de Recherches Petrographiques et Geochimiques, CNRS, Rue Notre-Dame des Pauvres, BP 20,
54501 Vandoeuvre Cedex, France
Department of Earth and Space Sciences, Osaka University, Toyonaka, Osaka 560-043, Japan
^Museum National d'Histoire Naturelle, CNRS, 61 rue Buffon, 75005 Paris, France
Abstract. The isotopic composition of solar nitrogen is a long standing issue that received recently new impetus.
The analysis of nitrogen isotopes in lunar samples and in Jupiter show that solar nitrogen is depleted in
15
N by 30 % relative to terrestrial. The systematic enrichment of
15
N in terrestrial planets and bulk meteorites requires the contribution of
15
N-rich compounds to the total nitrogen in planetary materials. Most of these compounds are possibly of an interstellar origin that never equilibrated with the
15
N-depleted protosolar nebula N
2
.
The isotopic composition of nitrogen (
15
N/
14
N, expressed as (8
15
N = [(
15
N/
14
N) sample
/(
15
N/
14
N) air
-
1] x 1000, in parts per mil, or %o) is extremely variable among solar system objects such as meteorites, planetary atmospheres, interplanetary dust particles (IDPs) (e.g., [1,2] and refs. therein), see Fig. 1. Because nitrogen has only two isotopes, it is unclear if such variations are due to isotopic fractionation in the proto-solar nebula, or in the interstellar medium, or if they result from mixing of pre-solar components processed in different stars, or from a combination of such processes.
The basic problem with this element is that the isotopic composition of solar nitrogen, the main reservoir of N in the solar system, has not been measured directly with precision, preventing an assessment of the solar system reference value for
15
N/
14
N. We have recently proposed that solar N is drastically depleted in
15
N, based on newly developed analyses of lunar soils and we discuss briefly some of the implications of this isotope heterogeneity for the evolution of the solar system.
A direct determination of the solar wind nitrogen composition will be possible when samples having been exposed in space during the
14M/15N
220
E, metal ffiP-___
"CC
170
( > Earth
(Atm)
Jupiti ilileo
Jupiter ISO cc»"0— — — —
Cornet Hale-Bopj
•••••O"
Solar wind SOHO
-500 0 500 1000
FIGURE 1. Variation of 5
15
N in the solar system
Different types of chondrites (E, CR) are shown together with measurements of the atmosphere of Venus and of
Mars.
Genesis Discovery Mission will be returned to laboratories. Studies of the lunar regolith, which has been irradiated by the solar corpuscular emission for long periods of time, have allowed to identify and measure precisely the isotopic composition of solar wind rare gases (e.g., [3]), which agrees well with direct measurement of solar wind rare gases in aluminium foils exposed
CP598, Solar and Galactic Composition, edited by R. F. Wimmer-Schweingruber
© 2001 American Institute of Physics 0-7354-0042-3/017$ 18.00
41

on the Moon. The abundance of nitrogen (and carbon) of different soils correlates fairly well with that of rare gases, and this has been taken as a firm evidence for a solar origin of N.
However, the N/Ar ratio of lunar soils is on an average one order of magnitude higher than Solar, and the 8
15
N lunar soil values range over 300 %o
(e.g., [1] and refs. therein). These observations were interpreted following two different types of models, not necessarily exclusive : (i) the isotopic composition of nitrogen in solar emission varied with time [4], contrary to that of rare gases (e.g.,
[5]), or (ii) several exotic components contributed nitrogen to lunar soils [6]. The first possibility has been discarded by Geiss and Bochsler [6] on the ground that no known spallation or thermonuclear reaction could produce enough
15
N and these authors concluded that the N isotope ratio at the solar surface has been constant during the last 4 billion years. They instead proposed that variations of the
15
N/
14
N ratio is due to mixing between a
15
N-rich solar component and a very light N component admixed in various amount to planetary matter. The analysis of N abundance together with that of Ar isotopes in single lunar soil grains by laser extraction-static mass spectrometry developed in Nancy, France, showed that lunar soils contained two or more nitrogen components [7]. In such a case the overall correlation between N and rare gases in different soils does not imply a common origin but is due to the implantation at the grain surface of different N and Ar components. In addition to solar corpuscular radiation, possible candidates include meteoritic N since it has been shown that the lunar regolith contains 1-2 % of carbonaceous chondrite debris [8] and that some of lunar soil grains present vapor-deposited rims though to have originated from meteoritic impacts [9]. Primitive meteorites have N/Ar ratios 5 orders of magnitude higher than the solar value so a limited contribution of such material only be seen for nitrogen, leaving the solar rare gas component essentially unaltered. A cometary contribution would also be possible but such case would imply either contribution of cometary silicates, or preferential trapping of nitrogen relative to rare gases if these volatiles were supplied by cometary ice.
The occurrence of several N components was further confirmed when it became possible to determine also the N isotopic composition of single lunar soil grains [10, 11] for soil sampled at different sites. These analyses demonstrated that
£-100 e
-200
100001
100
10,
Q.
Q.
•i-H
00
100 ffi
10
50 100 150
200
-200
+800
+400
Q
CO
-400
Grain 71501 (ilmenite)
\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ ._
-800
1
50 100 150
200
FIGURE 2. Examples of depth profiles of N and H concentrations and isotope compositions (dots and squares for H and N, respectively) in grains from soil
79035 and soil 71501 (adapted fromref. [2]).
+800
+400
Q
CO
-400 l"l f, f i t
•I—I—I—I
Grain 79035 (silicate)
-800
42

solar wind nitrogen is not enriched in
15
N contrary to what was proposed by [6] since
15
N-rich grains are depleted in solar wind argon and S
15
N values become negative with increasing contribution of solar wind argon. A lower limit of - 200 %o for solar 8
15
N could be set based on 8
15
N-Ar/N relationships.
A new method of ion probe rastering analysis developed at CRPG, Nancy [12] allowed to measure the depth profile isotopic variation of H and N on soil grain surfaces with a depth resolution of 10 nm for two soils of different antiquities (the antiquity refers to the epoch at which a given soil was exposed at the Moon's surface, based on indirect, semi-quantitative soil characteristics).
Results [2] from soil 79035, a regolith presumably exposed for > 500 Ma, show that a light N component depleted in
15
N by at least 240
%o is implanted in the depth range of few tens of nanometers that corresponds to implantation of
SW ions. The light N component is associated with solar wind, D-free hydrogen (Fig. 2), confirming the suspicion from single grain analysis that solar wind N is strongly depleted in
15
N relative to planetary N.
Other measurements aimed to determine the solar N composition are sparse. Kallenbach et al.
[13] reported a 8
15
N value of 360
+520
.
290
%o for the present -day solar wind using the SOHO TOP mass spectrometer facility. Fouchet el al. (Icarus
143, 223, 2000) reported a range of -480
+240
.
28 o %o for ammonia in the Jovian atmosphere using the
ISO short wavelength spectrometer. Since the
N/C/Ar ratios of the Jupiter atmosphere are close to solar [14], the Jovian N isotopic composition should reflect closely that of the proto-solar nebula and the discrepancy between the two measurements may be analytical. Recently, the
Galileo team has published the results of the mass spectrometric measurements of N isotopes during the Jovian atmosphere entry and proposed a 8
15
N value of -375+/-80 %o for Jupiter [14], in good agreement with the Fouchet et al's determination and with the upper limit of -240 %o for solar wind nitrogen on the Moon proposed by Hashizume et al. [2].
Since Jupiter and comets have sampled protosolar N 4.5 Ga ago, and soil 79035 has been presumably exposed to the solar wind several hundreds of Ma ago, this would still leave space in principle for secular evolution of solar wind N towards a more positive 8
15
N value at present as proposed by [4]. We think that this is not the case, for the reasons exposed by [6] and because we have recently analyzed a Luna-24 soil showing evidence for recent exposition (notably a low
40
Ar/
36
Ar ratio of 0.5) which shows also depletion of
15
N with increasing solar wind
36
Ar contribution
[H].
All solar system objects analyzed so far present enrichment in
15
N relative to solar and Jupiter (Fig.
1). Such enrichment could be in some cases the result of atmospheric processes (e.g., Mars and
Titan) but a contribution of
15
N-rich component(s) is needed for primitive objects as well as planetary interiors. Mixing between solar N and other(s) non-solar N source(s) is evident at the Moon's surface. Analysis of grains from soil 71501 which was presumably exposed recently [5] display a
15
N-rich component having deuterium-rich (nonsolar) hydrogen. Positive 8
15
N values were also found at the very surface of 71501 grains as well as grains and data taken together define a range of values for non-solar N between -50 and +130 %o
[2], essentially similar to that defining the upper limit of bulk soil spallation-corrected data (e.g.,
[1]). The depth profile analysis of major elements of 71501 ilmenite grains showed that the
15
N-rich component is associated with silicon coatings [2].
The association of Si-rich coatings with
15
N-rich nitrogen and D-rich hydrogen strongly suggests that such coatings originated from exotic material having impacted the Moon's surface. We can evaluate comets and meteorites as potential sources. The single existing cometary N isotope measurement was done on HCN in comet Hale-
Bopp [15] and gave a 8
15
N value of -157
+136
.
109
%o.
If representative of the cometary reservoir, this measurement would rule out comets as the unique source of non-solar N on the Moon.
Micrometeorites and meteorites are better candidates. The range of 8
15
N values observed in most meteorites bracket the terrestrial ratio (Fig. 1) and micrometeorites share similarities with the
CM carbonaceous chondrite clan which contains on an average 10
3
ppm of nitrogen with 8
15
N around +40 %o. Since heavier N is observed in the
Moon, other sources of N are required and IDPlike matter could fit since IDPs have 8
15
N values up to 480 %o [16]. In such case the secular N isotopic variations for soils having different antiquities reflect variations in the solar versus meteoritic/cometary contributions through time and would provide an unique way to investigate
43

the variability of meteoritic bombardment of the
Earth through geological time. Preliminary mass balance calculations are very encouraging with this respect as they show a good consistancy between the required metoritic flux on the Moon to account for N isotope variability in lunar regolith and the micrometeoritic flux estimated for the Earth (Hashizume et al., in prep.).
The systematic enrichment of
15
N in terrestrial planets and bulk meteorites requires the contribution of
15
N-rich compounds to the total nitrogen in planetary materials. Most of these compounds are possibly of an interstellar origin that never equilibrated with the
15
N-depleted protosolar nebula N
2
. Isotopic fractionation of nitrogen will enrich HCN in
15
N by up to 30% relative to N
2
following ion-molecule reactions in interstellar clouds, qualitatively consistent with the difference in
15
N/
14
N between solar and planetary
N.
The 8
15
N value at the Earth's surface
(atmosphere plus sediments plus crust) is + 1.5 %o and the upper mantle value is around - 5 %c. Thus terrestrial nitrogen does not show evidence of solar gas origin, contrary to mantle neon. Taken at face value, terrestrial nitrogen could have been supplied by both cometary- and meteorite-like matter, however the terrestrial D/H ratio rules out significant contribution from comets [17]. In addition, the volatile element pattern of the terrestrial mantle is chondritic [18], which leaves little doubt that nitrogen has been supplied to the
Earth under the form of meteoritic compounds rather than N
2
.
Discussions with Rainer Wieler, Paul Mahaffy,
John Kerridge, Kurt Marti, Richard Becker and
Toby Owen were greatly appreciated, although this work does not necessarily reflects the view of some of these scientists. This work was supported by grants from the Programme National de
Planetologie, Institut National des Sciences de
1'Univers, the Japanese Ministry of Education,
Science, Sports and Culture, the Centre National de la Recherche Scientifique and the Region
Lorraine.
1. Kerridge, J.F., Rev. Geophys. 31,423-437, (1993).
2. Hashizume, K., Chaussidon, M., Marty, B., and Robert, F.,
Science 290, 1142-1145 (2000).
3 Benkert, J.P., Baur, H., Signer, P., and Wieler, R., /.
Geophys. Res. 98, 13,147-13,162 (1993).
4 Kerridge, J.F., Science 188,162-164 (1975)
5 Wieler, R., and Baur, H., Meteoritics 29, 570-580 (1994).
6 Geiss, J., and Bochsler, P., Geochim. Cosmochim. Acta 46,
529-548 (1982).
7 Wieler, R., Humbert, F., and Marty, B., Earth Planet. Sci.
Lett. 167,47-60 (1999).
8 Keays, R.R., Ganapathy, R., Laul, J.C., Anders, E.,
Herzog, G.F., and Jeffery, P.M., Science 167, 490-493
(1970).
9 Keller, L.P., and McKay, D.S., Geochim. Cosmochim. Acta
61,2331-2342(1997).
10 Hashizume, K., Marty, B., and Wieler, R., in LPS XXX
CD-ROM, pp. 1567, LPI, Houston (1999).
11 Assonov, S.S., Marty, B., Shukolyukov, Y.A., and
Semenova, A.S., in LPS XXXII, CD-ROM, pp. 1798, LPI,
Houston (2001).
12 Chaussidon, M., and Robert, F., Nature 402, 270-273
(1999).
13 Kallenbach, R., et al., Astrophys. J. 507, L185-L188
(1998).
14 Owen, T., Mahaffy, P.R., Niemann, H.B., Atreya, S., and
Wong, M., Astrophys. J. 553, L77-L79 (2001).
15 Jewitt, D.C., Matthews, H.E., Owen, T., and Meier, R.,
Science 278, 90-93 (1997).
16 Messenger, S., Nature 404, 968-971 (2000).
17 Dauphas, N., Robert, F., and Marty, B., Icarus 148, 508-
512(2000).
18 Marty, B., and Zimmermann, J.L., Geochim. Cosmochim.
Acta 63, 3619-3633 (1999).
44