N2 accretion, metamorphism of organic nitrogen, or both processes likely contributed to the origin of Pluto's N2

Molecular nitrogen (N-2) plays a profound role in supporting processes on the surface and in the atmosphere of Pluto, yet the origin of Pluto's N-2 remains a mystery. However, this may begin to change as the N-14/N-15 ratio of N-2 was recently estimated based on a non-detection of (HCN)-N-15 in Pluto's atmosphere, while accounting for N-14/N-15 fractionation between HCN and N-2 using a photochemical model. Here, I show that, if this latter step of translating isotope ratios is adequately understood, then the derived N-14/N-15 ratio represents the first dis-tinguishing constraint on the origin of Pluto's N-2. One notable finding of the present study is that isotopic fractionation between atmospheric N-2 and N-2-rich ices on the surface of Pluto does not appear to be significant. I infer a lower limit of similar to 197 for the N-14/N-15 ratio of the dominant (solid) reservoir of N-2 on Pluto; i.e., mostly contained in Sputnik Planitia. From this lower limit, an endmember ammonia source of Pluto's N-2 can be ruled out. I perform N isotope mixing calculations that enable quantitative understanding of the relationships between contributions by primordial N-2, NH3, and nitrogen originally sourced in organic materials (N-org) to Pluto's observed N-2 inventory. These calculations also address how uncertainties in the isotopic composition of N-org and the history of atmospheric escape affect the allowed ranges of primordial N-2, NH3, and N-org contributions. While present uncertainties are substantial, I find that a contribution by primordial N-2, N-org, or both is implied, and the sum of their contributions should be at least similar to 45%. Hence, it is likely that Pluto formed from building blocks that were cold enough to trap N-2 (e.g., <30 K), or Pluto has a thermally processed and dynamic interior that supports generation of N-2 from N-org (at temperatures above similar to 350 degrees C) and N-2 transport to the surface. Furthermore, the lower limit on N-14/N-15 suggests that NH3 has been a less significant contributor to the origin of N-2 on Pluto than on Titan, which is indicative of a key difference in the origin and evolution of these worlds. Recommendations are given for future work that can continue to advance and contextualize understanding of the origin of Pluto's N-2. A new mission that can determine the origin of N-2 on Neptune's moon Triton should be a priority. Such a mission would offer an unprecedented opportunity for comparison of volatile origins on large Kuiper belt objects (past or present) with distinct histories.

Automated summary

The origin of molecular nitrogen on Pluto remains a mystery, but recent studies have provided the first constraint on the N-14/N-15 ratio of N-2, ruling out an ammonia source and suggesting contributions from primordial N-2 and nitrogen sourced in organic materials. Isotopic fractionation between atmospheric N-2 and N-2-rich ices on Pluto is not significant, and the lower limit on the N-14/N-15 ratio indicates a less significant role of NH3 compared to Titan. Future work and missions are recommended to further understand the origin of N-2 on Pluto.