2014 Annual Science Report
NASA Jet Propulsion Laboratory - Titan Reporting | SEP 2013 – DEC 2014
Structural Investigation of Titan Tholin
Using analytical methods including nuclear magnetic resonance spectroscopy, mass spectrometry and non-aqueous microchip gel electrophoresis, we investigate the properties and structure of the major components of laboratory analogues of Titan tholins.
In our previous work, we have investigated the chemical composition of Titan aerosols analogs (tholin) and identified several nitrogenous organic molecules. Here we continue our structural investigation and identified four more molecules in Titan tholin by using NMR and GC-MS and standard sample comparison, including aminoacetonitrile, succinonitrile, acetoguanamine and adenine. On the basis of their structural characteristics, we suggested their formation pathways via simple precursors like methanimine (CH2NH), HCN, NH3 and C2H2 in laboratory N2-CH4 plasma or potentially Titan’s organic aerosols. Among these molecules, aminoacetonitrile is a potential precursor of amino acid and peptide, and adenine is a necessary ingredient for DNA and RNA. The identification of these molecules in Titan’s organic aerosol analogs increases our knowledge of Titan’s happening organic chemistry and its prebiotic implications. Using non-aqueous microchip gel electrophoresis, we separate and detect several specific long chain amines from the tholin, demonstrating capability for moderate sized molecule separation of tholin components, a goal that had eluded investigators to date, These methods further open the complex tableau of tholin inventory to molecular separation and definitive structural identification, which is critical to the further understanding of the nature of complex prebiotic organic environments.
We have also investigated the solubility and thermal stability of Titan aerosol analogs (tholins) to understand the basic property of organics on Titan and the potential for chemical modification upon in situ sampling. The tholin generated by AC discharge in CH4/N2 (5/95) mixture preferentially dissolves in polar solvent to non-polar solvent, which is indicated by the solubility (mg/mL) by evaporation method and proved by the solubility ratio from the integration of quantitative 1H NMR spectra. The 1H and 13C NMR spectra of respective deuterated solutions indicate the structural information of soluble tholin components in each solvent and confirm the large percentage of polar species in tholins, including amine, nitrile and N-heteroaromatics. The solubility study implies that most of Titan aerosols should be scantily in low temperature non-polar hydrocarbon lakes or oceans on Titan surface, and also helps us select solvents and develop methods for the liquid-based in situ analysis. Thermal stability studies indicate significant structural changes of tholins when heated beyond 150 oC for even short time periods in inert atmospheres. The 300 oC-pyrolysis products of one major component (2-cyanoguanidine) in the tholin are identified. This study demonstrates the tholins’ thermal lability and the requirement of nondestructive 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.
This work has benefitted from the collaborations with all members of team, but in particular those of Peter Willis, JPL. This effort helped spur our efforts to investigate NMR as an analytical method for Titan tholins in parallel with our mass spectrometric studies over the past decade. The microanalytical results of Willis and coworkers suggest a strong aliphatic amine component to these materials not suggested in the mass spectrometry or other groups’ chromatographic efforts. Although not dominant, this component does appear in the NMR but is dominated by unsaturated functionality.