Under pressure: revealing the chemistry of Titan’s atmosphere

Carbon, hydrogen and nitrogen are among the most abundant atoms in the solar system. Exploring their chemical reactions is crucial to understanding the origin of the life. The chemistry of simple molecules under extreme conditions offers key insights into planetary processes and prebiotic chemistry.

Prior to this study, methane (CH₄) and nitrogen (N₂), common in giant planet and Titan-like atmospheres, were considered remarkably inert under pressure. However, understanding how these stable molecules interact under compression could illuminate pathways to complex organic chemistry. An international team of researchers have now demonstrated that high pressure initiates the formation of novel molecular compounds. Their work is published in Angewandte Chemie and includes experiments performed on beamline I15 at Diamond Light Source. 

Methane and nitrogen dominate many planetary atmospheres, including that of Titan, the largest moon of Saturn. On this satellite, these atoms form photochemically driven “tholins” which are complex hydrocarbons and nitriles. Yet, at high static pressures, their interactivity remained largely unexplored. This study used precise pressure control to explore whether dense CH₄–N₂ mixtures yield new chemistry beyond the well-known photochemical pathways. By compressing equimolar N2 and CH₄-rich mixtures, the scientists aimed to discover whether simple molecular solids emerge and remain stable under conditions akin to planetary interiors. Improvements in high-pressure crystallography and spectroscopy were necessary to unambiguously capture these subtle molecular transformations. 

The team conducted diamond anvil cell (DAC) experiments combined with synchrotron X-ray single-crystal and powder diffraction alongside Raman spectroscopy and density functional theory (DFT) modelling at the I15 beamline and other synchrotrons including the ESRF, Petra III and Spring-8. Diamond anvil cells are compact devices that allow the compression of samples between two opposing diamonds.

Diamonds are used as it’s a very hard and incompressible material, and transparent to visible light and X-rays, allowing to optically view samples as well as the usage of many spectroscopic and diffraction techniques to characterise samples.

By combining the diamond-anvil cell with a cryostat, resistive heaters and a laser-heating system, researchers can independently vary the pressure and temperature of samples, from a few gigapascals up to a few 100 GPa, and from 4 K to 6,000 K, reproducing the extreme conditions found deep inside planets and moons, all within the laboratory. 100 GPa is 1 million times the pressure at the surface of the Earth, far exceeding everyday experience. 

Researchers compressed CH₄–N₂ mixtures and they were able to show that above 7 GPa, two new van der Waals molecular compounds appear: (CH₄)₇(N₂)₈ (stable at ~7 GPa) and (CH₄)₅N₂ (stable at ~13 GPa). Interestingly, either further compression at room temperature or an increase of the temperature is able to break the N2 molecules and new molecules are formed with covalent bonds between C, H or N atoms. 

Annette Kleppe, principal beamline scientist at I15 commented: “It’s truly exciting to see how experiments on I15 have helped unlock new pathways to organic complexity in the deep interiors of planets and moons. Even more compelling is the potential for these laboratory findings to align with in situ observations from upcoming planetary missions such as the planned Dragonfly mission to Titan." 

Studies at the I15 beamline were crucial as it enabled diffraction under extreme conditions, allowing crystal structures to be resolved despite the weak scattering from light elements and small sample volumes. The combined experimental and theoretical approach confirmed the existence and stability of these novel compounds, underscoring that even the simplest molecular systems can form unexpectedly complex structures under pressure.

This study demonstrates that pressure alone can drive the emergence of previously unknown molecular compounds from simple gases, offering a new mechanism for organic complexity in planetary interiors and possibly exoplanet atmospheres. Future research could probe the chemical pathways at higher pressures or include additional gas mixtures, helping to model chemistries relevant to Titan, gas giants, or exoplanets.  

To find out more about the I15 beamline, please contact the Principal Beamline Scientist Annette Kleppe: annette.kleppe@diamond.ac.uk 

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