In a recent study, a team led by scientists from the NASA Ames Research Center simulated the complex chemistry that may be occurring in the hazy atmosphere of Saturn’s largest moon, Titan. The work provides new insights into what future Titan probes may encounter upon arrival and what the atmosphere of Earth may have been like eons ago.
“Titan is an interesting body to study because it has a dense atmosphere, mostly made of nitrogen, like Earth, and it has a methane cycle resembling Earth’s water cycle,” said Ella Sciamma-O’Brien, a researcher at NASA Ames and co-author of the study. “So there’s an interest in understanding the very complex chemistry that is occurring between nitrogen and methane in Titan’s atmosphere and how that compares to what happened on Earth before life appeared.”
The COsmic SImulation Chamber (COSmIC) at NASA Ames uses a pulsed plasma discharge to simulate the bombardment of molecules by electrons in Titan’s atmosphere. Researchers can explore different chemical pathways and reaction steps by “tuning” the composition of the initial gas mixture. Thus, to focus on later steps in the reaction, the researchers added acetylene—a larger hydrocarbon—to an initial gas mixture of nitrogen and methane. Acetylene is a known precursor of benzene, which has been detected in Titan’s atmosphere.
COSmIC’s reaction products are tiny grains (“tholins”) analogous to aerosols in Titan’s atmosphere. Here, the tholins were produced in nitrogen/methane and nitrogen/methane/acetylene gas mixtures. The grains were collected on substrates and analyzed using x-ray absorption near-edge structure (XANES) spectroscopy at Beamline 188.8.131.52 of the Advanced Light Source. “We found that just adding a little bit of acetylene to the mixture changes the chemistry a lot,” said co-author Michel Nuevo, also a researcher at NASA Ames. “Just adding 0.5% acetylene doubled the ratio between the carbon and nitrogen detected in the reaction products, demonstrating the impact that initial precursors have on the nitrogen chemistry.”
The results should help in the interpretation of observations and the development of models of Titan’s atmosphere. Such observations and models are essential to support future activities, ranging from telescopic measurements to NASA’s Dragonfly mission to land on Titan, launching in 2027.M. Nuevo, E. Sciamma-O’Brien, S.A. Sandford, F. Salama, C.K. Materese, and A.L.D. Kilcoyne, “The Titan Haze Simulation (THS) experiment on COSmIC. Part III. XANES study of laboratory analogs of Titan tholins,” Icarus 376, 114841 (2022), doi:10.1016/j.icarus.2021.114841.