Interstellar chemistry recorded by nitrogen isotopes in Solar System organic matter

The relationship between the organic 15N/14N and D/H ratios in small Solar System bodies (meteorites, interplanetary dust and comets) suggests that isotopic exchange reactions taking place at various temperatures are at the origin of the observed variations. These relationships are used to determine the exothermicity (ΔE) of ion–molecule reactions that fractionated the nitrogen isotopic ratio in the presolar molecular cloud; that is ΔE=43±10 K. Comparison with current models of interstellar chemistry suggests that such a value could be achieved by condensation of 15N-rich gas-phase precursors onto grain surfaces and their further isolation from the gas by incorporation into large macromolecular structures.