Titan Breaks a Chemistry Rule: Polar HCN and Oil-like Methane/Ethane Form Exotic Co-crystals

Cold Chemistry on Titan: Polar and Nonpolar Molecules That Shouldn't Mix, Do

Scientists from NASA's Jet Propulsion Laboratory and the Chalmers University of Technology report that Saturn's largest moon, Titan, hosts conditions in which molecules that normally refuse to combine on Earth can form stable solids together. The new study, published July 23 in PNAS, shows that the polar molecule hydrogen cyanide (HCN) can form stable co-crystals with highly nonpolar hydrocarbons such as methane (CH4) and ethane (C2H6) at Titan-like temperatures.

Why this is surprising

Chemists often summarize solubility behavior with the maxim "like dissolves like": polar compounds (like water) interact strongly with other polar compounds, and nonpolar compounds (like oils and many hydrocarbons) tend to interact only weakly and separate. On Earth, this prevents stable mixed solids of polar and nonpolar species. But on Titan—where surface temperatures fall near −297°F (−183°C)—the team found a different outcome.

How the discovery was made

In the lab, researchers recreated Titan-surface conditions by mixing methane, ethane and hydrogen cyanide at cryogenic temperatures and probing the samples with spectroscopy to observe how they absorb and scatter light. Spectral signatures indicated an intimate structural relationship between HCN and the hydrocarbons rather than simple phase separation.

"It should not be possible to combine these polar and nonpolar substances," said lead author Martin Rahm, Associate Professor at Chalmers University of Technology, summarizing why the result was unexpected.

To explain the observations, the team used computational modeling to screen hundreds of possible co-crystal structures and assess their stability under Titan-like conditions. The combined experimental and theoretical evidence suggests that methane and ethane molecules can intercalate—that is, lodge into gaps within an HCN crystal lattice—producing stable co-crystals. The models indicate that the presence of hydrocarbons may actually strengthen intermolecular forces within the HCN solid, stabilizing these unusual mixed crystals.

Implications

This discovery expands our view of what solids can form in extremely cold environments and has several implications: it could change how we interpret surface spectroscopy of Titan, inform models of surface and atmospheric chemistry, and point to new low-temperature environments where prebiotic chemistry might occur. Athena Coustenis, a planetary scientist at the Paris–Meudon Observatory, noted that future data from NASA's Dragonfly mission (planned to arrive at Titan in 2034) could test for the spectral fingerprints of these co-crystals on Titan's surface.

Coustenis and others also suggest expanding laboratory and modeling efforts to other nitrogen-containing and hydrocarbon species found on Titan—such as cyanoacetylene (HC3N), acetylene (C2H2), hydrogen isocyanide (HNC) and molecular nitrogen (N2)—to determine whether similar mixing is a general feature of Titan's organic chemistry.

Bottom line: Under Titan's extreme cold, chemistry can behave in unexpected ways: polar and nonpolar molecules that don't mix on Earth can form stable, mixed solids, opening new questions about surface processes and the potential pathways for prebiotic chemistry in the outer solar system.