Primordial N-2 provides a cosmochemical explanation for the existence of Sputnik Planitia, Pluto

摘要

The presence of N-2 in the surface environment of Pluto is critical in creating Pluto's richness of features and processes. Here, we propose that the nitrogen atoms in the N-2 observed on Pluto were accreted in that chemical form during the formation of Pluto. We use New Horizons data and models to estimate the amounts of N-2 in the following exterior reservoirs: atmosphere, escape, photochemistry, and surface. The total exterior inventory is deduced to be dominated by a glacial sheet of N-2-rich ices at Sputnik Planitia, or by atmospheric escape if past rates of escape were much faster than at present. Pluto's atmosphere is a negligible reservoir of N-2, and photochemical destruction of N-2 may also be of little consequence. Estimates are made of the amount of N-2 accreted by Pluto based on cometary and solar compositions. It is found that the cometary model can account for the amount of N-2 in Sputnik Planitia, while the solar model can provide a large initial inventory of N-2 that would make prodigious atmospheric escape possible. These consistencies can be considered preliminary evidence in support of a primordial origin of Pluto's N2. However, both models predict accreted ratios of CO/N-2 that are much higher than that in Pluto's atmosphere. Possible processes to explain "missing CO" that are given quantitative support here are fractional crystallization from the atmosphere resulting in CO burial at the surface, and aqueous destruction reactions of CO subject to metastable thermodynamic equilibrium in the subsurface. The plausibility of primordial N-2 as the primary source of Pluto's nitrogen (vs. NH3 or organic N-2 can be tested more rigorously using future constraints on the N-14/N-15 ratio in N2 and the Ar-36/N-2 ratio. (C) 2018 Elsevier Inc. All rights reserved.