On the methylacetylene abundance and nitrogen isotope ratio in Pluto’s atmosphere

Abstract

To bring our photochemical model of Pluto’s atmosphere and ionosphere (Krasnopolsky, 2020) into agreement with the recent detection of 5 ​× ​1015 ​cm−2 methylacetylene C3H4 (Steffl et al., 2020), rate coefficients of the three key reactions of C3H4 production and loss have been changed to values calculated by Vuitton et al. (2019). This change reduces the abundance of cyanoacetylene HC3N close to the measured ALMA upper limit (Lellouch et al., 2017), increases the model abundances of H2 and C3H8 by factors of 1.3, and reduces the abundances of H and C4H2 by factors of 1.3, while other species are changed less than 5% in the model. The predicted abundance of atomic hydrogen agrees with that derived from the observed Lyman-beta emission by Steffl et al. (2020).The observed HC14N/HC15N ​> ​125 on Pluto (Lellouch et al., 2017) looks puzzling compared to HC14N/HC15N ​= ​60 on Titan. Our analysis confirms the predissociation of N2 at 80–100 ​nm as the main process of nitrogen isotope fractionation. The observed twofold difference is partially caused by the diffusive depletion of the heavy isotope in HCN and in the predissociation of N2. On Pluto, the mean altitudes of HCN and predissociation of N2 are 500 and 860 ​km, well above the homopause at 96 ​km. On Titan, observations of HC14N/HC15N refer to 90–460 ​km (Vinatier et al., 2007), the predissociation occurs near 985 ​km, both below the homopause at 1000 ​km, and diffusive depletion does not occur. Therefore the observed limit corresponds to 14N/15N ​> ​253 for N2 in the lower atmosphere and 14N/15N ​> ​228 in the upper layers of the N2 ice. These limits reflect the conditions on Pluto in the last two million years, which is the lifetime of N2 in Pluto’s atmosphere with the current N2 loss of 37.5 ​g ​cm−2 Byr−1 primarily for photodestruction. The calculated isotope fractionation factor of 1.96 accounts for the formation and condensation of nitriles, diffusive separation, and fractionation in thermal escape. Variations of 14N/15N in the N2 ice are related to the evolution of the solar EUV, mixing processes in the N2 ice, and possible periods of hydrodynamic escape, which are poorly known and not considered here.