A Search for Cyclic Molecules on Titan with ALMA

Titan, largest moon of Saturn, is the only satellite in the solar system to have a substantial atmosphere. Its nitrogen-dominated but methane-rich composition provides a rich environment for photochemistry, giving rise to a wide variety of organic molecules. Laboratory experiments and modeling predictions have fueled speculation about the level of molecular complexity reached, and how far the atmospheric chemistry proceeds towards production of key biomolecules. In 2016 we observed Titan with ALMA in Band 6 (~250 GHz) in an attempt to detect new cyclic molecules in its atmosphere, beyond the single molecule (benzene) already known from infrared spectroscopy. In particular, our search focused on the single ring N-heterocycles pyridine (c-C₅H₅N) and pyrimidine (c-C₄H₄N₂) that could be important precursors of nucleobases. Our search for these molecules was unsuccessful, but we were able to determine the first numerical upper limits for their abundance. We did however make the first detection of a smaller cyclic molecule (cyclopropenylidene, c-C₃H₂) on Titan, which has previously only been seen in the interstellar mediumand carbon-rich AGB star outflows. The implications of this finding for the photochemical understanding of Titan’s atmosphere will be discussed, along with the potential for future new molecular detections.