Imaging titan's organic haze at atomic scale

Abstract

Titan, Saturn's largest moon, has its atmosphere filled with a thick organic photochemical haze. These suspended solid nanoparticles are one of the most complex organic materials in the Solar System. In situ measurements from the successful Cassini space mission gave first clues on the aerosol's chemical composition: Pyrolysis coupled to mass spectrometry revealed a nitrogen-rich core, whereas infrared measurements highlighted poly-aromatichydrocarbon (PAH) signatures. The combination of these observations supports a general model of nitrogenatedpolycyclic aromatic hydrocarbon (N-PAH). To constrain the generic picture and understand the formation of such macromolecules in Titan's atmosphere, we simulated the haze synthesis in the laboratory. Small (3-10 rings) N-PAH molecules composing the material were extracted, focusing on the prime aromatization and growth processes. By high-resolution atomic force microscopy (AFM), we imaged key chemical structures with atomic resolution. We resolved N-rich elongated molecules involving five-membered aromatic rings, consistent with a repetitive cata-condensation pattern via addition of C3N units. These atomic-scale observations bridge the gap between gas phase atmospheric reactants and the macroscopic structure of Titan's haze.