REVIEW ARTICLE PUBLISHED ONLINE: 15 JUNE 2015 | DOI: 10.1038/NGEO2451 Nitrogen isotope variations in the Solar System Evelyn Füri and Bernard Marty The relative proportion of the two isotopes of nitrogen, 14N and 15N, varies dramatically across the Solar System, despite little variation on Earth. NASA’s Genesis mission directly sampled the solar wind and confirmed that the Sun — and, by inference, the protosolar nebula from which the Solar System formed — is highly depleted in the heavier isotope compared with the reference nitrogen isotopic composition, that of Earth’s atmosphere. In contrast, the inner planets, asteroids, and comets are enriched in 15N by tens to hundreds of per cent; organic matter in primitive meteorites records the highest 15N/14N isotopic ratios. The measurements indicate that the protosolar nebula, inner Solar System, and cometary ices represent three distinct isotopic reservoirs, and that the 15N enrichment generally increases with distance from the Sun. The 15N enrichments were probably not inherited from presolar material, but instead resulted from nitrogen isotope fractionation processes that occurred early in Solar System history. Improvements in analytical techniques and spacecraft observations have made it possible to measure nitrogen isotopic variability in the Solar System at a level of accuracy that offers a window into the processing of early Solar System material, large-scale disk dynamics and planetary formation processes. he Solar System formed when a fraction of a dense molecu- a matter of debate, the D/H isotopic tracer offers the possibility of lar cloud collapsed and a central star, the proto-Sun, started investigating the relationships between the different Solar System Tburning its nuclear fuel1. The surrounding disk made of gas reservoirs. In particular, it is central in the debate on the origin of and dust, the protosolar nebula (PSN), was thoroughly stirred and water (cometary or asteroidal) in the inner Solar System, including homogenized due to large-scale heating and mixing driven by loss that of the terrestrial oceans10. of angular momentum, the energy delivered by the proto-Sun, and The relative proportion 14N and 15N also shows outstanding magneto-rotational turbulence. The efficiency of these processes variability in the Solar System. For expressing the N isotope com- is evident in primitive (carbonaceous) meteorites, which show position, geochemists and cosmochemists use the stable isotope a remarkable homogeneity in the isotopic compositions of their delta notation: constituents down to the parts per million level for most elements 2 15 15 14 15 14 of the periodic table . Relics of the initial heterogeneous mixture δ N = (( N/ N)sample/( N/ N)standard–1) × 1,000 of stellar debris can only be found in nano- to micrometre-sized presolar grains that were thermally resistant enough to survive where δ15N expresses the deviation of the sample ratio relative to a high-enthalpy processing3. However, the light elements hydrogen, standard in parts per thousand (‰). The nitrogen standard is the 15 14 –3 carbon, nitrogen, and oxygen, show significant, sometimes extreme, isotope composition of atmospheric N2 ( N/ N = 3.676 × 10 ; isotope variations among Solar System objects and reservoirs, from ref. 13). On Earth, most variations are of the order of 10 parts per a few per cent for C and O, up to several hundred per cent for H thousand14. Because the range of extraterrestrial N isotope vari- and N (ref. 4). These light elements, by far the most abundant ones ations can be much larger than the level, cosmochemists use in the PSN, were all present predominantly in the gaseous state instead the absolute value of the 15N/14N ratio, following the stable (including H2, CO and N2, as well as their ionized derivatives) in isotope convention that the rare, heavy isotope is the numerator. the presolar cloud and in the disk. Consequently, they were prone To complicate matters further, astronomers and astrophysicists 14 15 to efficient isotope exchange and interactions with stellar photons instead use the N/ N notation (272 for atmospheric N2; despite and cosmic rays, either in the interstellar medium (ISM)5, or in the using the D/H notation for hydrogen as cosmochemists do). Both presolar cloud or the PSN4,6. Thus, these isotope compositions con- notations are given here for the sake of understanding by both of vey a unique record of the processes that formed the Solar System. these communities. The largest isotope variations are observed for hydrogen and On Earth, the N isotope composition varies by no more than 2%, nitrogen. The deuterium/hydrogen (D/H) ratio varies by a factor of but variations can reach 500% on a Solar System scale (Figs 1 and 2). ~35, from the PSN value of 21 ± 0.5 × 10–6 (ref. 7) to D-rich ‘hotspots’ Until recently, the causes of this variability were not understood, for in meteorites, with values up to 720 × 10–6 (ref. 8). Inner Solar System two main reasons. First, the initial 14N/15N ratio of the Solar System objects (~150 × 10–6; ref. 7) and comets (150–500 × 10–6; refs 9–11) was unknown. Second, nitrogen isotopes are more difficult to quan- show intermediate values, and possibly indicate an increase of the tify than hydrogen isotopes because they are generally less abundant D/H ratio with heliocentric distance. This suggests a scenario in in cosmochemical material, and because they are difficult to meas- which nebular H2 (poor in deuterium) exchanged isotopically with ure from a distance by spectroscopic methods. The analysis of solar H2O at low temperature, resulting in a preferential D-enrichment of wind (SW) ions returned to Earth by the Genesis mission — together the water. Deuterium-rich water then froze out onto grain surfaces with advances in high-spatial-resolution, high-sensitivity isotope and exchanged isotopically with organics and silicates as a result of analysis in the laboratory as well as in high-resolution UV spectros- turbulent transport and aqueous alteration on forming planetesi- copy (Box 1) — have permitted major leaps of understanding in the mals12. Although this scenario is not without weaknesses and is still cosmochemistry of this element. Here, we review recent advances Centre de Recherches Pétrographiques et Géochimiques, CNRS-Université de Lorraine, 15 Rue Notre Dame des Pauvres, BP20, 54501 Vandoeuvre-lès-Nancy, France. e-mail: [email protected]; [email protected] NATURE GEOSCIENCE | ADVANCE ONLINE PUBLICATION | www.nature.com/naturegeoscience 1 REVIEW ARTICLE NATURE GEOSCIENCE DOI: 10.1038/NGEO2451 0 during dredge-up from the carbon layer (primary production from 4000 3 alpha particles and two protons). During the cold CNO cycle 15N is 100 2000 destroyed, requiring other mechanisms such as 15N production dur- 800 ing the hot CNO cycle in novae and, possibly, neutrino spallation 200 200 on 16O in type II supernovae, both secondary types of production. δ 0 15 N N 300 Consequently, a nucleosynthetic isotope composition cannot be 15 –200 ( ‰) predicted from theory, and estimates for the N isotope production N/ 400 14 in the Galaxy rely on (limited) observations. –400 In molecular clouds, the N isotope composition is mainly 500 + –500 measured in CN, HCN, HNC, and, more recently, in NHD mol- 17–22 CN ecules . For molecular N2, the signal-to-noise ratio is too low for 600 HNC HCN a spectroscopic measurement. The data seem to define an isotope 700 –600 gradient with a decrease of 15N relative to 14N with increasing dis- 024681012 tance from the galactic centre (Fig. 1; refs 17,18). Because stars in Distance from the galactic center (kpc) the galaxy centre tend to be more metal-rich (where metallicity is the proportion of elements heavier than helium), secondary pro- Figure 1 | Nitrogen isotope variation in molecular clouds from our galaxy duction of 15N that presumably takes place in novae and superno- as a function of distance from the galactic centre Isotope compositions . vae should decrease more rapidly with galactocentric distance than are given in both absolute 14N/15N ratios and 15N values (deviation from the δ the primary component of 14N production, in qualitative agree- atmospheric 15N/14N ratio). The observed correlation (dashed line) appears ment with the observation (Fig. 1). Strikingly, the Solar System, to be consistent with secondary 15N production in massive stars. The total represented by the PSN value (see next subsection), does not fit N isotope range within the Solar System (blue shading), defined by the most the correlation. The observed offset could result from a localized 15N-rich hotspot in the Isheyevo meteorite24 (red diamond) and the PSN N isotope evolution since the Solar System isolated nitrogen from (yellow circle) composition31, is comparable to that in our galaxy. The range the local ISM 4.56 billions years ago (Ga). However, a 14N/15N value for the inner Solar System and comets are shown by dark blue and green of ≥400 has been proposed for the local ISM23 where the Sun was boxes, respectively. Error bars indicate 1 uncertainties. Data from refs 17,18. σ born, consistent with the solar ratio and implying little N isotope evolution with time. Alternatively, the apparent anomaly may be that are particularly relevant in the context of the measurements explained by the uncertainty in the data defining the correlation currently being made of Comet 67P/Churyumov-Gerasimenko by depicted in Fig. 1. CN or HCN, the molecules for which the nitro- the Rosetta spacecraft instruments. gen isotopic compositions are measured in dense cores, could have been isotopically fractionated with respect to the source composi- Nucleosynthesis of N isotopes and galactic evolution tion, and may not be representative of the local ISM values. Indeed, The production paths of nitrogen isotopes and their corresponding measurements of the N isotope compositions in three starless, 15–17 14 + rates are not fully understood .