A low fraction of nitrogen in molecular form in a dark cloud

Abstract

Nitrogen is the fifth most abundant element in the Universe. In the interstellar medium, it has been thought to be mostly molecular (N₂)¹. However, N₂ has no observable rotational or vibrational transitions, so its abundance in the interstellar medium remains poorly known. In comets, the N₂ abundance is very low^2,3, while the elemental nitrogen abundance is deficient with respect to the solar value. Moreover, large nitrogen isotopic anomalies are observed in meteorites and interstellar dust particles⁴. Here we report the N₂H⁺ (and by inference the N₂) abundance inside a cold dark molecular cloud. We find that only a small fraction of nitrogen in the gas phase is molecular, with most of it being atomic. Because the compositions of comets probably reflect those of dark clouds⁵, this result explains the low N₂ abundance in comets. We argue that the elemental nitrogen abundance deficiency in comets can be understood if the atomic oxygen abundance is lower than predicted by present chemical models. Furthermore, the lack of molecular nitrogen in molecular clouds explains the nitrogen anomalies in meteorites and interstellar dust particles, as nitrogen fractionation is enhanced if gaseous nitrogen is atomic⁶.