N$_2$ accretion, metamorphism of organic nitrogen, or both processes likely contributed to the origin of Pluto's N$_2$

Abstract

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 $^{14}$N/$^{15}$N ratio of N$_2$ was recently estimated based on a non-detection of HC15N in Pluto's atmosphere, while accounting for $^{14}$N/$^{15}$N fractionation between HCN and N$_2$. Here, I show that, if this latter step of translating isotope ratios is adequately understood, then the derived $^{14}$N/$^{15}$N ratio represents the first distinguishing constraint on the origin of N$_2$. One finding of the present study is that isotopic fractionation between atmospheric N$_2$ and N$_2$-rich ices on Pluto does not appear to be significant. I infer a lower limit of ~197 for the $^{14}$N/$^{15}$N ratio of the dominant 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$_2$, NH$_3$, and nitrogen originally sourced in organic materials (Norg) to Pluto's observed N$_2$ inventory. These calculations address how uncertainties in the isotopic composition of Norg and the history of atmospheric escape affect the allowed ranges of primordial N$_2$, NH$_3$, and Norg contributions. While present uncertainties are substantial, I find that a contribution by primordial N$_2$, Norg, or both is implied, and the sum of their contributions should be at least ~45%. Hence, Pluto likely formed from building blocks that were cold enough to trap N$_2$, or Pluto has a thermally processed and dynamic interior that supports generation of N$_2$ from Norg and N$_2$ transport to the surface. The lower limit on $^{14}$N/$^{15}$N suggests that NH$_3$ has been a less significant contributor to the origin of N$_2$ on Pluto than on Titan.