The End of Interstellar Chemistry as the Origin of Nitrogen in Comets and Meteorites

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Abstract
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 N2 remains in the gas. The lack of hydroxl molecules prevents the recycling of N atoms into N2, 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 15N with 15NH3/14NH3 almost 80% higher than the cosmic 15N/14N 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 15N enhancements of primitive solar system material and the depletion of N2 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.