Photochemistry on Pluto: part II HCN and nitrogen isotope fractionation

Abstract

We have converted our Titan one-dimensional photochemical model to simulate the photochemistry of Pluto's atmosphere and include condensation and aerosol trapping in the model. We find that condensation and aerosol trapping are important processes in producing the HCN altitude profile observed by the Atacama Large Millimeter Array (ALMA). The nitrogen iso-tope chemistry in Pluto's atmosphere does not appear to significantly fractionate the isotope ratio between N 2 and HCN as occurs at Titan. However, our simulations only cover a brief period of time in a Pluto year, and thus only a brief portion of the solar forcing conditions that Pluto's atmosphere experiences. More work is needed to evaluate photochemical fractionation over a Pluto year. Condensation and aerosol trapping appear to have a major impact on the altitude profile of the isotope ratio in HCN. Since ALMA did not detect HC 15 N in Pluto's atmosphere, we conclude that condensation and aerosol trapping must be much more efficient for HC 15 N compared to HC 14 N. The large uncertainty in photochemical fractionation makes it difficult to use any potential current measurement of 14 N/ 15 N in N 2 to determine the origin of Pluto's nitrogen. More work is needed to understand photochemical fractionation and to evaluate how condensation, sublimation and aerosol trapping will fractionate N 2 and HCN.

Chemical composition of Pluto aerosol analogues

“…At lower altitudes, between 200 and 400 km, the condensation of volatiles onto photochemical aerosols dominates (Luspay-Kuti et al, 2017;Mandt et al, 2017;Wong et al, 2017). This condensation concerns C2 hydrocarbons -C2H2, C2H4, C2H6 - HCN, CH2NH, C3H4, C3H6, CH3CN, C4H2, HC3N, C2H3CN, C2N2, CH3C2CN and C6H6 (Gao et al, 2017;Krasnopolsky and Cruikshank, 1999;Lara et al, 1997;Luspay-Kuti et al, 2017;Mandt et al, 2017;.…”

Pluto’s atmosphere observations with ALMA: Spatially-resolved maps of CO and HCN emission and first detection of HNC

“…Similarly, from a neutral-only model, Wong et al (2017) examined the sensitivity of the HCN mixing profile to the sticking coefficients from their photochemical model (with K 𝑧𝑧 = 1 × 10 3 cm 2 s −1 ) and also concluded to 𝛾 = 0.01. Rather different results were found by Luspay-Kuti et al (2017) and Mandt et al (2017) from their own ion-neutral model, who used a much higher 10 https://lasp.colorado.edu/home/see/ (Woods et al, 2005).…”

“…They modeled separately the processes of gas condensation and sticking onto aerosols, the latter being required to limit the HCN abundance in the lower atmosphere, finding that sticking and condensation coefficients of 7 × 10 −4 and 4.5 × 10 −3 , respectively, are required to provide reasonable agreement with observations from Paper I. A further finding of Mandt et al (2017) was that the non-detection of HC 15 N from ALMA and associated upper limit (HC 15 N/HC 14 N < 1/ 125, Paper I) implies that condensation and aerosol trapping are (unexpectedly) much more efficient for HC 15 N compared to HC 14 N. Although not fully consistent in their approaches and results, these studies illustrate how the determination of the HCN profile can constrain its condensation and adsorption on the aerosol processes.…”

Triton’s Variable Interaction With Neptune’s Magnetospheric Plasma

“…To represent Triton’s atmosphere within AIKEF, we apply the coupled Ion‐Neutral‐Photochemical (INP) model (de la Haye et al., 2008; Luspay‐Kuti et al., 2015, 2016; Mandt et al., 2012). This model has been used to provide realistic, quantitative representations of the atmospheres of various solar system objects, including Titan and Pluto (e.g., Luspay‐Kuti et al., 2017; Mandt et al., 2017), and is therefore highly suitable to generate a valid model of Triton’s atmosphere. Including over 1,500 reactions between 50 neutral and 34 ion species, INP is a one‐dimensional model that couples ion‐neutral chemistry to solve the continuity equation throughout Triton’s atmosphere and ionosphere.…”