Frozen crystals of hydrogen cyanide, a highly poisonous gas, could nonetheless play an essential role in giving rise to life on frigid alien worlds, including Saturn’s moon Titan, research reveals.
Despite the substance’s lethality, frozen hydrogen cyanide crystals are highly reactive, enabling certain chemical reactions to occur at temperatures below normal. This discovery was developed through computer models and reported in a recent paper published in ACS Central Science, with the hope that the breakthrough could explain the chemistry of how life started on Titan.
Life on Titan
“We may never know precisely how life began, but understanding how some of its ingredients take shape is within reach,” said co-author Martin Rahm of the Chalmers University of Technology in Gothenburg, Sweden. “Hydrogen cyanide is likely one source of this chemical complexity, and we show that it can react surprisingly quickly in cold places.”
Hydrogen cyanide (HCN) is widespread throughout the universe, appearing on comets, moons, and planets. That includes one of the most promising locations for life elsewhere in our solar system: Saturn’s moon Titan.
While hydrogen cyanide is toxic to life in its pure form, when combined with water, it can help form key building blocks of life, including polymers, nucleobases, and amino acids.
“This article is but the latest, not final, piece of a puzzle that our team have worked on for several years,” Rahm told The Debrief. “We are trying to understand how HCN, likely so important for the abiotic making of some of life’s building blocks, might behave in cold environments such as that of Saturn’s moon Titan.”
“There HCN is not a gas nor a liquid, as in warmer environments, but exists as special kinds of crystals at lower temperatures,” Rahm added. “I had the idea for the HCN-to-HNC transformation mechanisms on such crystals many years ago, but it necessitated a lot of work to build realistic enough computer models to test them.”
Modeling Hydrogen Cyanide
For their study, the researchers created a three-dimensional computer model of a hydrogen cyanide crystal measuring about 450 nanometers in length. The model featured a rounded base at one end and a multifaceted top at the other, resembling a cut gemstone. The design was based on real-world observations of crystallized hydrogen cyanide “cobwebs,” which branch outward from a single point and connect at multifaceted ends.
“The challenge in quantum chemistry is always to build a model large enough to be realistic, but small enough to be computationally feasible,” Rahm explained. “One thing that is special about HCN is its large dipole moment.”
“When many such molecules are lined up head to tail, they generate a large electric field,” Dr. Rahm continued. “To describe this field, we had to make models large enough such that the field strength calculated at the surfaces had converged, which was tricky.
“In real experiments, those fields are large enough such that crystals of HCN are known to behave rather strange, on occasion glowing, jumping even fracturing,” Rahm said.
The model showed that the crystals’ unique structure could enable chemical reactions that normally would not occur under extreme cold. In particular, the chemical properties of the crystal surface could convert hydrogen cyanide into hydrogen isocyanide, a more reactive variant. Depending on temperature, the process could occur on timescales ranging from minutes to days.
Testing Hydrogen Cyanide
The researchers suggest this may be only the beginning, as more complex prebiotic precursors could also form on the crystal surface. Now that these properties have been demonstrated theoretically, the next step is laboratory testing.
“We hope that those skilled in handling HCN would be interested in doing studies in which HCN crystals are crushed at low temperature, so to expose the reactive surface,” Rahm said. “If this is done in the presence of suitable chemistry or radiation that can add or subtract protons, it should be possible to detect the formation of HNC with surface-sensitive spectroscopy. Then more complicated molecules are likely to follow, depending on conditions.”
If such laboratory work proves that these chemical reactions are possible at extremely low temperatures in the presence of hydrogen cyanide crystals, it will inform scientists about where they may expect to find life beyond Earth. Including Titan.
“Chemistry moves faster when it is warm, and while reactivity in liquids at warmer conditions are likely to be crucial for prebiotic chemistry, there is an awful lot of real estate out there that is cold,” Rahm concluded.
“Discovering how far chemical complexity can progress in such conditions [is] important for understanding the environmental limits of life, and what came before it.”
The paper, “Electric Fields Can Assist Prebiotic Reactivity on Hydrogen Cyanide Surfaces,” appeared in ACS Central Science on January 14, 2025.
Ryan Whalen covers science and technology for The Debrief. He holds an MA in History and a Master of Library and Information Science with a certificate in Data Science. He can be contacted at ryan@thedebrief.org, and follow him on Twitter @mdntwvlf.