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The presence of N2 in the surface environment of Pluto is critical increating Pluto's richness of features and processes. Here, we propose that thenitrogen atoms in the N2 observed on Pluto were accreted in that chemical formduring the formation of Pluto. We use New Horizons data and models to estimatethe amounts of N2 in the following exterior reservoirs: atmosphere, escape,photochemistry, and surface. The total exterior inventory is deduced to bedominated by a glacial sheet of N2-rich ices at Sputnik Planitia, or byatmospheric escape if past rates of escape were much faster than at present.Pluto's atmosphere is a negligible reservoir of N2, and photochemicaldestruction of N2 may also be of little consequence. Estimates are made of theamount of N2 accreted by Pluto based on cometary and solar compositions. It isfound that the cometary model can account for the amount of N2 in SputnikPlanitia, while the solar model can provide a large initial inventory of N2that would make prodigious atmospheric escape possible. These consistencies canbe considered preliminary evidence in support of a primordial origin of Pluto'sN2. However, both models predict accreted ratios of CO/N2 that are much higherthan that in Pluto's atmosphere. Possible processes to explain "missing CO"that are given quantitative support here are fractional crystallization fromthe atmosphere resulting in CO burial at the surface, and aqueous destructionreactions of CO subject to metastable thermodynamic equilibrium in thesubsurface. The plausibility of primordial N2 as the primary source of Pluto'snitrogen (vs. NH3 or organic N) can be tested more rigorously using futureconstraints on the 14N/15N ratio in N2 and the 36Ar/N2 ratio.

Primordial N2 provides a cosmochemical explanation for the existence of Sputnik Planitia, Pluto