Structural elucidation of soluble organic matter: Application to Titan's haze

Abstract

The origin and evolution of organic matter in the solar system intertwines astrobiology and planetary geochemistry issues. To observe the contribution of atmospheric processes in the formation of complex organic matter, one of the most intensively studied objects in the outer solar system is Titan, the largest moon of Saturn. Its reducing atmosphere of methane and nitrogen hosts a thick, permanent, nitrogen-rich, organic haze, whose complex composition remains largely unknown. Due to the measurement of species at large mass-to-charge ratio and infrared information from the Cassini-Huygens mission, polyaromatic hydrocarbons (PAHs) based structure for this haze has been suggested. Here, we propose a snapshot of the global chemical structure based on the analysis of laboratory analogue of Titan's haze with ion mobility spectrometry coupled with mass spectrometry. This robust analysis, validated with other geochemical complex mixtures, such as petroleum, allows for the observation of the size and three-dimensional structure of detected species. By comparison, with standards molecules, we exclude several structures such as pure PolyHCN and pure polycyclic aromatic hydrocarbons to be present in the principal trend of the laboratory tholins. Using theoretical calculations, we propose a plausible structure consistent with our results, which is a branched triazine-pyrazole. We observe that the larger the aerosols molecules are, the more they tend towards a structure containing small aromatics cores linked together by short chains. We suggest that the use of such an analytical approach could help advance our understanding of other complex organic compounds in the Solar System such as soluble organic matter in meteorites.

Suggested plausible structures for Titan's haze analogs using tandem mass spectrometry

“…This work confirms structural hypotheses proposed in previous works performed by ion mobility concerning the presence of bulky structures in the soluble molecules (Maillard et al, 2020;Ruger et al, 2019). Such part of these structure were identified in several NMR studies (Derenne et al, 2012;He and Smith, 2014a, b) and liquid-chromatography-mass spectrometry analyses (Gautier et al, 2016).…”

“…The more aromatic one will be called series 1 and the other series 2. To illustrate this observation, putative structural formulas of compounds representative of the two major series were added in the Figure. These putative structures are based on hypotheses made in previous works (Gautier et al, 2017;Maillard et al, 2020;Ruger et al, 2019). Two types of aromatic materials are observed by the series 1 and series 2.…”

“…However, the isomeric and structural diversity remained unknown. In order to obtain an overview of the global structure present in tholins sample, ion mobility coupled with mass spectrometry was performed (Maillard et al, 2020;Ruger et al, 2019). These studies allowed for a narrowing of the possibilities of structures by excluding several families of compounds.…”

Imaging Titan’s Organic Haze at Atomic Scale

“…Molecules B3, B12, and B8 are very close to structures identified in previous solid-state NMR analyses (He & Smith 2014;McGuigan et al 2006). Structures such as A1, B5, and B9 were hypothesized in recent works (Maillard et al 2020;Ruger et al 2019). Furthermore, as stated above, the observed structures could contain even more nitrogen atoms that are not revealed because of their pronounced no-planarity.…”

“…Global analytical techniques gave an overview of the overall structure of the material. Solidphase NMR showed that tholins' structure is mainly based on unsaturated C = N bonds such as imines or triazine units (Derenne et al 2012) and ion mobility highlighted a global structure with a slightly higher double-bond equivalent (DBE) than PAH, more in agreement with a nitrogenated PAH (N-PAH) global structure (Derenne et al 2012;Maillard et al 2020).…”