The relative proportion of the two isotopes of nitrogen, N-14 and N-15, 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 N-15 by tens to hundreds of per cent; organic matter in primitive meteorites records the highest N-15/N-14 isotopic ratios. The measurements indicate that the protosolar nebula, inner Solar System, and cometary ices represent three distinct isotopic reservoirs, and that the N-15 enrichment generally increases with distance from the Sun. The N-15 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.
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