Dayside nitrogen and carbon escape on Titan: the role of exothermic chemistry

¹ Space Science Institute, Macau University of Science and Technology, Macau, PR China
e-mail: cuijun7@mail.sysu.edu.cn
² School of Atmospheric Sciences, Sun Yat-Sen University, Zhuhai, Guangdong, PR China
³ National Astronomical Observatories, Chinese Academy of Sciences, Beijing, PR China
⁴ Chinese Academy of Sciences Center for Excellence in Comparative Planetology, Hefei, Anhui, PR China
⁵ Université de Reims Champagne Ardenne, CNRS, GSMA UMR 7331, 51097 Reims, France
⁶ Yunnan Astronomical Observatory, Chinese Academy of Sciences, Kunming, Yunnan, PR China
⁷ Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing, PR China

Received: 2 October 2019
Accepted: 16 November 2019

Abstract

Context. Atmospheric escape has an appreciable impact on the long-term climate evolution on terrestrial planets. Exothermic chemistry serves as an important mechanism driving atmospheric escape and the role of such a mechanism is of great interest for Titan due to its extremely complicated atmospheric and ionospheric composition.

Aims. This study is devoted to a detailed investigation of neutral N and C escape on the dayside of Titan, which is driven by exothermic neutral–neutral, ion–neutral, and dissociative recombination (DR) reactions. It was carried out based on the extensive measurements of Titan’s upper atmospheric structure by a number of instruments on board Cassini, along with an improved understanding of the chemical network involved.

Methods. A total number of 14 C- and N-containing species are investigated based on 146 exothermic chemical reactions that release hot neutrals with nascent energies above their respective local escape energies. For each species and each chemical channel, the hot neutral production rate profile is calculated, which provides an estimate of the corresponding escape rate when combined with the appropriate escape probability profile obtained from a test particle Monte Carlo model.

Results. Our calculations suggest a total N escape rate of 9.0 × 10²³ s⁻¹ and a total C escape rate of 4.2 × 10²³ s⁻¹, driven by exothermic chemistry and appropriate for the dayside of Titan. The former is primarily contributed by neutral-neutral reactions, whereas the latter is dominated by ion–neutral reactions; however, contributions from neutral–neutral and DR reactions to the latter cannot be ignored either. Our calculations further reveal that the bulk of N escape is driven by hot N(⁴S) production from the collisional quenching of N(²D) by ambient N₂, while C escape is mainly driven by hot CH₃ and CH₄ production via a number of important ion–neutral and neutral–neutral reactions.

Conclusions. Considered in the context of prior investigations of other known escape mechanisms, we suggest that exothermic chemistry is likely to contribute appreciably to non-thermal C escape on the dayside of Titan, although it plays an insignificant role in N escape.