In situ Investigation of Neutrals Involved in the Formation of Titan Tholins

TL;DR

This study uses laboratory plasma simulations to investigate neutral chemical species involved in Titan's aerosol formation, identifying key precursors like ethylene under Titan-like conditions.

Contribution

It introduces a cryogenic trapping method in plasma experiments to detect neutral precursors of Titan aerosols, providing new insights into their formation mechanisms.

Findings

Identification of NH3, HCN, C2H2, and C2H4 as key neutral products.

Detection of heavy molecules such as C7 during plasma experiments.

Ethylene and other volatiles are confirmed as crucial solid-phase precursors.

Abstract

The Cassini Mission has greatly improved our understanding of the dynamics and chemical processes occurring in Titan's atmosphere. It has also provided us with more insight into the formation of the aerosols in the upper atmospheric layers. However, the chemical composition and mechanisms leading to their formation were out of reach for the instruments onboard Cassini. In this context, it is deemed necessary to apply and exploit laboratory simulations to better understand the chemical reactivity occurring in the gas phase of Titan-like conditions. In the present work, we report gas phase results obtained from a plasma discharge simulating the chemical processes in Titan's ionosphere. We use the PAMPRE cold dusty plasma experiment with an N2-CH4 gaseous mixture under controlled pressure and gas influx. An internal cryogenic trap has been developed to accumulate the gas products during their production and facilitate their detection. The cryogenic trap condenses the gas-phase precursors while they are forming, so that aerosols are no longer observed during the 2h plasma discharge. We focus mainly on neutral products NH3, HCN, C2H2 and C2H4. The latter are identified and quantified by in situ mass spectrometry and infrared spectroscopy. We present here results from this experiment with mixing ratios of 90-10% and 99-1% N2-CH4, covering the range of methane concentrations encountered in Titan's ionosphere. We also detect in situ heavy molecules (C7). In particular, we show the role of ethylene and other volatiles as key solid-phase precursors.