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

Abstract

Molecular nitrogen (N2) plays a profound role in supporting processes on the surface and in the atmosphere of Pluto, yet the origin of Pluto's N2 remains a mystery. However, this may begin to change as the 14N/15N ratio of N2 was recently estimated based on a non-detection of HC15N in Pluto's atmosphere, while accounting for 14N/15N fractionation between HCN and N2 using a photochemical model. Here, I show that, if this latter step of translating isotope ratios is adequately understood, then the derived 14N/15N ratio represents the first distinguishing constraint on the origin of Pluto's N2. One notable finding of the present study is that isotopic fractionation between atmospheric N2 and N2-rich ices on the surface of Pluto does not appear to be significant. I infer a lower limit of ∼197 for the 14N/15N ratio of the dominant (solid) reservoir of N2 on Pluto; i.e., mostly contained in Sputnik Planitia. From this lower limit, an endmember ammonia source of Pluto's N2 can be ruled out. I perform N isotope mixing calculations that enable quantitative understanding of the relationships between contributions by primordial N2, NH3, and nitrogen originally sourced in organic materials (Norg) to Pluto's observed N2 inventory. These calculations also address how uncertainties in the isotopic composition of Norg and the history of atmospheric escape affect the allowed ranges of primordial N2, NH3, and Norg contributions. While present uncertainties are substantial, I find that a contribution by primordial N2, Norg, or both is implied, and the sum of their contributions should be at least ∼45%. Hence, it is likely that Pluto formed from building blocks that were cold enough to trap N2 (e.g., <30 K), or Pluto has a thermally processed and dynamic interior that supports generation of N2 from Norg (at temperatures above ∼350 °C) and N2 transport to the surface. Furthermore, the lower limit on 14N/15N suggests that NH3 has been a less significant contributor to the origin of N2 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 N2. A new mission that can determine the origin of N2 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.