Polar and nonpolar molecules – long thought incompatible – can intermingle on Saturn’s moon Titan, researchers have discovered. The finding breaks the time-honored “like dissolves like” rule and could reshape how scientists understand chemical interactions in extreme environments.
Using a combination of cryogenic spectroscopy and quantum mechanical modeling, researchers from NASA’s Jet Propulsion Laboratory and Chalmers University of Technology, Sweden, observed that hydrogen cyanide (HCN), a strongly polar molecule, can form stable crystalline structures with nonpolar hydrocarbons like methane and ethane – two of Titan’s most abundant surface liquids. These so-called cocrystals or solid solutions are held together not by traditional bonding rules, but by weak van der Waals interactions and hydrogen bond strengthening at frigid temperatures around 90 Kelvin (-183°C).
“Our calculations predicted not only that the unexpected mixtures are stable under Titan's conditions, but also spectra of light that coincide well with NASA's measurements,” said Martin Rahm, lead author and Associate Professor at Chalmers, in a press release. “I see it as a nice example of when boundaries are moved in chemistry and a universally accepted rule does not always apply,” he says.
The team simulated Titan-like conditions in the lab by layering liquified methane or ethane over preformed HCN crystals. Raman spectroscopy revealed small but consistent shifts in vibrational modes – most notably a red shift in the HCN C-H stretch – indicative of stronger hydrogen bonding and lattice rearrangement. Thousands of structure prediction calculations confirmed that ethane can nestle between hydrogen-bonded HCN chains, forming stable low-energy crystalline phases.
Beyond theoretical novelty, the results carry geochemical implications. Titan’s surface features – including lakes, dunes, and the mysterious “magic islands” that appear and disappear in radar images – could be shaped in part by such exotic cocrystal chemistry. The presence of HCN is especially intriguing, as it’s a known precursor to prebiotic molecules like amino acids and nucleobases.
“These are very exciting findings,” Rahm noted, “that can help us understand something on a very large scale, a moon as big as the planet Mercury.”
NASA’s Dragonfly mission is scheduled to arrive on Titan in 2034 to investigate prebiotic chemistry and surface composition in detail.
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