Solubility and stability investigation of Titan aerosol analogs: New insight from NMR analysis

Abstract

We investigated the solubility and thermal stability of Titan aerosol analogs (tholins) to understand the potential properties of organics on Titan and susceptibility to chemical modification upon in situ sampling. The tholin generated by AC discharge in CH4/N2 (5/95) mixture preferentially dissolves in polar solvents over non-polar solvents, as determined by solute isolation and independently by solubility from the integration of quantitative 1H NMR spectra. The 1H and 13C NMR spectra of respective deuterated solutions determine the structural identification of soluble tholin components in each solvent and confirm the large percentage of polar species in tholins, including amines, nitriles and N-heteroaromatics. The solubility study implies that most of Titan aerosols should be rarely present in low temperature non-polar hydrocarbon lakes or oceans on the Titan surface, and also helps to select solvents and develop methods for potential liquid-based in situ analysis. Thermal stability studies indicate significant structural changes of the tholin when heated beyond 150°C for only 5min in inert atmospheres. Several molecules are identified as 300°C-pyrolysis products of the tholin. This study demonstrates tholin thermal lability and the requirement of non-destructive instruments and methods for structural analysis, such as NMR and LC/MS. This study is critical to ongoing discussion regarding the development of in situ, low bias analysis methods and instruments for Titan missions and other outer planet exploration.