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

Christopher R. Glein

doi:10.1016/j.icarus.2023.115651

paper

N $_{2}$ accretion, metamorphism of organic nitrogen, or both processes likely contributed to the origin of Pluto's N $_{2}$

arXiv:2306.06350 · doi:10.1016/j.icarus.2023.115651

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.

23 pages, 3 figures, 3 tables

N $_{2}$ accretion, metamorphism of organic nitrogen, or both processes likely contributed to the origin of Pluto's N $_{2}$

Abstract

References in corpus (7)

Cited by in corpus (3)

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

Abstract

References in corpus (7)

Cited by in corpus (3)

N $_{2}$ accretion, metamorphism of organic nitrogen, or both processes likely contributed to the origin of Pluto's N $_{2}$