NATURE GEOSCIENCE | ADVANCE ONLINE PUBLICATION | www.nature.com/naturegeoscience 1
The Solar System formed when a fraction of a dense molecu-lar cloud collapsed and a central star, the proto-Sun, started burning its nuclear fuel1. The surrounding disk made of gas and dust, the protosolar nebula (PSN), was thoroughly stirred and homogenized due to large-scale heating and mixing driven by loss of angular momentum, the energy delivered by the proto-Sun, and magneto-rotational turbulence. The efficiency of these processes is evident in primitive (carbonaceous) meteorites, which show a remarkable homogeneity in the isotopic compositions of their constituents down to the parts per million level for most elements of the periodic table2. Relics of the initial heterogeneous mixture of stellar debris can only be found in nano- to micrometre-sized presolar grains that were thermally resistant enough to survive high-enthalpy processing3. However, the light elements hydrogen, carbon, nitrogen, and oxygen, show significant, sometimes extreme, isotope variations among Solar System objects and reservoirs, from a few per cent for C and O, up to several hundred per cent for H and N (ref. 4). These light elements, by far the most abundant ones in the PSN, were all present predominantly in the gaseous state (including H2, CO and N2, as well as their ionized derivatives) in the presolar cloud and in the disk. Consequently, they were prone to efficient isotope exchange and interactions with stellar photons and cosmic rays, either in the interstellar medium (ISM)5, or in the presolar cloud or the PSN4,6. Thus, these isotope compositions convey a unique record of the processes that formed the Solar System.
The largest isotope variations are observed for hydrogen and nitrogen. The deuterium/hydrogen (D/H) ratio varies by a factor of ~35, from the PSN value of 21 ± 0.5 × 10–6 (ref. 7) to D-rich ‘hotspots’ in meteorites, with values up to 720 × 10–6 (ref. 8). Inner Solar System objects (~150 × 10–6; ref. 7) and comets (150–500 × 10–6; refs 9–11) show intermediate values, and possibly indicate an increase of the
D/H ratio with heliocentric distance. This suggests a scenario in which nebular H2 (poor in deuterium) exchanged isotopically with
H2O at low temperature, resulting in a preferential D-enrichment of the water. Deuterium-rich water then froze out onto grain surfaces and exchanged isotopically with organics and silicates as a result of turbulent transport and aqueous alteration on forming planetesimals12. Although this scenario is not without weaknesses and is still
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. a matter of debate, the D/H isotopic tracer offers the possibility of investigating the relationships between the different Solar System reservoirs. In particular, it is central in the debate on the origin of water (cometary or asteroidal) in the inner Solar System, including that of the terrestrial oceans10.
The relative proportion 14N and 15N also shows outstanding variability in the Solar System. For expressing the N isotope composition, geochemists and cosmochemists use the stable isotope delta notation: δ15N = ((15N/14N)sample/(15N/14N)standard–1) × 1,000 where δ15N expresses the deviation of the sample ratio relative to a standard in parts per thousand (‰). The nitrogen standard is the isotope composition of atmospheric N2 (15N/14N = 3.676 × 10–3; ref. 13). On Earth, most variations are of the order of 10 parts per thousand14. Because the range of extraterrestrial N isotope variations can be much larger than the level, cosmochemists use instead the absolute value of the 15N/14N ratio, following the stable isotope convention that the rare, heavy isotope is the numerator.
To complicate matters further, astronomers and astrophysicists instead use the 14N/15N notation (272 for atmospheric N2; despite using the D/H notation for hydrogen as cosmochemists do). Both notations are given here for the sake of understanding by both of these communities.
On Earth, the N isotope composition varies by no more than 2%, but variations can reach 500% on a Solar System scale (Figs 1 and 2).
Until recently, the causes of this variability were not understood, for two main reasons. First, the initial 14N/15N ratio of the Solar System was unknown. Second, nitrogen isotopes are more difficult to quantify than hydrogen isotopes because they are generally less abundant in cosmochemical material, and because they are difficult to measure from a distance by spectroscopic methods. The analysis of solar wind (SW) ions returned to Earth by the Genesis mission — together with advances in high-spatial-resolution, high-sensitivity isotope analysis in the laboratory as well as in high-resolution UV spectroscopy (Box 1) — have permitted major leaps of understanding in the cosmochemistry of this element. Here, we review recent advances