The end of interstellar chemistry as the origin of nitrogen in comets and meteorites

We describe a mechanism for enhanced nitrogen isotope fractionation in dense molecular gas where most of the molecules containing carbon and oxygen have condensed on grains but where N-2 remains in the gas. The lack of hydroxl molecules prevents the recycling of N atoms into N-2, and the nitrogen eventually becomes atomic. Ammonia is formed efficiently under these conditions and rapidly accretes as ice. We find that a significant fraction of the total nitrogen is ultimately present as solid NH3. This interstellar ammonia is enhanced in N-15 with (NH3)-N-15/(NH3)-N-14 almost 80% higher than the cosmic N-15/N-14 ratio. It is possible that a large part of the nitrogen available to the early solar system was highly fractionated ammonia ice and hence that the N-15 enhancements of primitive solar system material and the depletion of N-2 in comets are concomitant. Other implications of this theory for observations of dense molecular material and the nitrogen inventory available to the protosolar nebula are briefly discussed.