Upper limits to the fractionation of isotopes due to atmospheric escape: Implications for potential 14N/15N in Pluto’s atmosphere

Abstract:

Formation and evolution of the solar system is studied in part using stable isotope ratios that are presumed to be primordial, or representative of conditions in the protosolar Nebula. Comets, meteorites and giant planet atmospheres provide measurements that can reasonably be presumed to represent primordial conditions while the terrestrial planets, Pluto and Saturn’s moon Titan have atmospheres that have evolved over the history of the solar system. The stable isotope ratios measured in these atmospheres are, therefore, first a valuable tool for evaluating the history of atmospheric escape and once escape is constrained can provide indications of conditions of formation. D/H ratios in the atmosphere of Venus provide indications of the amount of water lost from Venus over the history of the solar system, while several isotope ratios in the atmosphere of Mars provide evidence for long-term erosion of the atmosphere. We have recently demonstrated that the nitrogen ratios, ¹⁴N/¹⁵N, in Titan’s atmosphere cannot evolve significantly over the history of the solar system and that the primordial ratio for Titan must have been similar to the value recently measured for NH₃ in comets. This implies that the building blocks for Titan formed in the protosolar nebula rather than in the warmer subnebula surrounding Saturn at the end of its formation. Our result strongly contrasts with works showing that ¹⁴N/¹⁵N in the atmosphere of Mars can easily fractionate from the terrestrial value to its current value due to escape processes within the lifetime of the solar system. The difference between how nitrogen fractionates in Mars and Titan’s atmospheres presents a puzzle for the fractionation of isotopes in an atmosphere due to atmospheric escape. Here, we present a method aiming at determining an upper limit to the amount of fractionation allowed to occur due to escape, which is a function of the escape flux and the column density of the atmospheric constituent. Through this approach, we demonstrate that fractionation on Titan is more limited than on Mars. When applied to Pluto, we find that any potential measurement of ¹⁴N/¹⁵N in Pluto’s atmosphere can constrain the type of escape occurring from Pluto’s atmosphere and possibly the source of nitrogen for Pluto.