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

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 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 in comets are concomitant. Other implications of this N 2 theory for observations of dense molecular material and the nitrogen inventory available to the protosolar nebula are briefly discussed.

Formaldehyde and methylamine reactivity in interstellar ice analogues as a source of molecular complexity at low temperature

“…Indeed, some meteoritic materials and interstellar dust particles possess enhancements and anomalies in their 15 N 14 N ratios. The variability of the 15 N 14 N ratio in these materials suggest, as with deuterium, a formation induced by low-temperature interstellar chemistry (Charnley & Rodgers 2002;Kerridge et al 1987;Messenger & Walker 1997). To estimate the possible isotopic fractionation between the 14 N methylamine and the corresponding 15 N isotopologue in interstellar ices, we calculated with a reduced kinetic model (Eq.…”

Cometary Isotopic Measurements

“…(Charnley and Rodgers 2002). showed that when N atoms are initially abundant, as one might expect since dense clouds form from diffuse gas, then reaction (R12) efficiently incorporates 15 N into HC 15 NH + .…”

“…Ion-molecule chemistry in dense interstellar and/or protostellar material could induce 15 N enrichment in the ammonia ice produced in such environments (Charnley and Rodgers 2002). Alternatively, the 15 N enrichment observed in cometary ices could result from isotopic fractionation processes that occurred in the late protosolar nebula (Aléon 2010).…”

Constraints from Comets on the Formation and Volatile Acquisition of the Planets and Satellites

“…This differential between radio and visible measurements extended to Hale-Bopp (Arpigny et al 2003), which had already been shown to have a consistent isotopic ratio for carbon, as discussed above. Arpigny et al (2003) suggested that this difference was due to the known changing parentage of CN with heliocentric distance with the contributing parents at small distances having potentially been enriched in 15 N via an NH 3 ice-dust grain fractionation process in dense molecular clouds (Charnley and Rodgers 2002).…”