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 NH 3 . This interstellar ammonia is enhanced in 15 N with 15 NH 3 / 14 NH 3 almost 80% higher than the cosmic 15 N/ 14 N 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 15 N 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.