A Model of Titan-like Chemistry to Connect Experiments and Cassini Observations

Abstract

A numerical model is presented for interpreting the chemical pathways that lead to the experimental mass spectra acquired in the Titan Haze Simulation (THS) laboratory experiments and for comparing the electron density and temperature of the THS plasma to observations made at Titan by the Cassini spacecraft. The THS plasma is a pulsed glow-discharge experiment designed to simulate the reaction of N 2 /CH 4 -dominated gas in Titan's upper atmosphere. The transient, one-dimensional model of THS chemistry tracks the evolution of more than 120 species in the direction of the plasma flow. As the minor species C 2 H 2 and C 2 H 4 are added to the N 2 /CH 4 -based mixture, the model correctly predicts the emergence of reaction products with up to five carbon atoms in relative abundances that agree well with measured mass spectra. Chemical growth in Titan's upper atmosphere transpires through ion–neutral and neutral–neutral chemistry, and the main reactions involving a series of known atmospheric species are retrieved from the calculation. The model indicates that the electron density and chemistry are steady during more than 99% of the 300  μ s long discharge pulse. The model also suggests that the THS ionization fraction and electron temperature are comparable to those measured in Titan's upper atmosphere. These findings reaffirm that the THS plasma is a controlled analog environment for studying the first and intermediate steps of chemistry in Titan's upper atmosphere.