A missing link in the nitrogen-rich organic chain on Titan

¹ Université Paris-Saclay, UVSQ, CNRS, LATMOS, 78280 Guyancourt, France
e-mail: nathalie.carrasco@latmos.ipsl.fr
² Université Paris-Saclay, CNRS, Institut de Chimie Physique, 91405 Orsay, France
³ Sorbonne Université, Laboratoire de Chimie Théorique, UMR 7616 CNRS, 75005 Paris, France
e-mail: riccardo.spezia@sorbonne-universite.fr

Received: 8 April 2021
Accepted: 5 April 2022

Abstract

Context. The chemical building blocks of life contain a large proportion of nitrogen, an essential element. Titan, the largest moon of Saturn, with its dense atmosphere of molecular nitrogen and methane, offers an exceptional opportunity to explore how this element is incorporated into carbon chains through atmospheric chemistry in our Solar System. A brownish dense haze is consistently produced in the atmosphere and accumulates on the surface on the moon. This solid material is nitrogen-rich and may contain prebiotic molecules carrying nitrogen.

Aims. To date, our knowledge of the processes leading to the incorporation of nitrogen into organic chains has been rather limited. In the present work, we investigate the formation of nitrogen-bearing ions in an experiment simulating Titan’s upper atmosphere, with strong implications for the incorporation of nitrogen into organic matter on Titan.

Methods. By combining experiments and theoretical calculations, we show that the abundant N₂⁺ ion, produced at high altitude by extreme-ultraviolet solar radiation, is able to form nitrogen-rich organic species.

Results. An unexpected and important formation of CH₃N₂⁺ and CH₂N₂⁺ diazo-ions is experimentally observed when exposing a gas mixture composed of molecular nitrogen and methane to extreme-ultraviolet radiation. Our theoretical calculations show that these diazo-ions are mainly produced by the reaction of N₂⁺ with CH₃ radicals. These small nitrogen-rich diazo-ions, with a N/C ratio of two, appear to be a missing link that could explain the high nitrogen content in Titan’s organic matter. More generally, this work highlights the importance of reactions between ions and radicals, which have rarely been studied thus far, opening up new perspectives in astrochemistry.