Tholinomics--chemical analysis of nitrogen-rich polymers

The polymeric composition of Titan's tholins--laboratory analogues of Titan's aerosols--is elucidated using high-resolution mass spectrometry. This complex organic matter is produced by plasma discharge in a gaseous nitrogen-methane mixture and analyzed with a hybrid linear trap/orbitrap mass-spectrometer. The highly structured mass spectra are treated with tools developed for petroleomics (Kendrick and van Krevelen diagrams), with original adaptations for nitrogen-rich compounds. Our goal is to find the best chemical basis set to describe the compositional space that these polymers occupy, to shed light onto the chemical structure of tholins. We succeeded in assigning the molecules identified in the mass spectra of tholins to a small number of regularly distributed X-(CH(2))(m)(HCN)(n) families, where the balanced copolymer (m = n) is determined to play a central role. Within each family, the polymer lengths n and m present Poisson-type distributions. We also identify the smallest species of a subset of families as linear and cyclic amino nitrile compounds of great astrobiological interest: biguanide, guanidin, acetamidine, aminoacetonitrile, and methylimidazole.