Titan’s Frozen Lakes Just Broke a Fundamental Law of Chemistry

An international team of researchers searched for clues about molecules essential to life on Saturn’s largest moon, Titan, and discovered surprising details about its chemistry. They found that hydrogen cyanide can form stable crystals with methane and ethane, challenging basic rules of chemistry. 

Scientists believe that studying Titan could reveal how life began on Earth. This is because the icy moon has a thick atmosphere abundant in nitrogen and methane, similar to the conditions thought to have existed on Earth in the past. Therefore, for decades, they have wondered if the conditions on Titan can reveal how Earth became habitable. 

When in Rome

In the work published in the journal Proceedings of the National Academy of Sciences, researchers at Chalmers University of Technology in Sweden and the US space agency NASA revealed that methane, ethane, and hydrogen cyanide, present in large quantities in Titan’s atmosphere, can interact in a manner that was not previously considered possible. Because hydrogen cyanide is a polar molecule and methane and ethane are nonpolar substances, they should behave like oil and water when mixed. 

Polar substances are made up of molecules that have an uneven distribution of electrical charges, meaning they possess a positive side and a negative side. In contrast, nonpolar substances have a symmetrical charge distribution, where the charges are evenly balanced. Because of this difference, polar and nonpolar molecules tend not to mix, as polar molecules preferentially attract one another through electrostatic interactions.

However, surprisingly, their study indicated that hydrogen cyanide crystallizes with methane and ethane, challenging a fundamental rule of chemistry. The discovery of the interaction between these substances under Titan’s extreme cold conditions is remarkable, as it can redirect the scope of research on the moon. 

“These are very exciting findings that can help us understand something on a very large scale, a moon as big as the planet Mercury,” Martin Rahm, Associate Professor at Chalmers' Department of Chemistry and Chemical Engineering, said in a statement. “The discovery of the unexpected interaction between these substances could affect how we understand the Titan’s geology and its strange landscapes of lakes, seas, and sand dunes,” he added.

He also explained that hydrogen cyanide likely helped form some of the basic chemicals needed for life, such as amino acids (which make up proteins) and nucleobases (which form DNA and RNA). By studying this compound, the researchers are learning more about how life’s first building blocks might have formed naturally, even in harsh environments before life began.

How did it all begin?

A group of scientists at NASA’s Jet Propulsion Laboratory (JPL) in California was curious about what happens to hydrogen cyanide after it is created in Titan’s atmosphere. To seek an answer, they began an experiment in which they mixed hydrogen cyanide with methane and ethane under very cold conditions, around –180°C (degrees Celsius). In this state, hydrogen cyanide becomes a crystal (solid) while methane and ethane stay liquid.

When they studied this mixture using lasers to study how the molecules behaved, they found that the molecules were intact, yet something unusual was happening. To understand what was going on, they contacted Martin Rahm’s research group at Chalmers, and together, they explored a bold idea: maybe methane or ethane could actually mix into hydrogen cyanide crystals, which would be chemically impossible.

To understand how this was possible, researchers used advanced computer simulations to test thousands of different ways the molecules could arrange themselves in solid form. They discovered that the hydrocarbon molecules had actually entered the hydrogen cyanide crystal structure and created stable new combinations called co-crystals.

“This can happen at very low temperatures, like those on Titan. 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,” Rahm, one of the leads of the study, said, adding, “I see it as a nice example of when boundaries are moved in chemistry and a universally accepted rule does not always apply.” However, he does not think it is time to rewrite the chemistry books. 

As hydrogen cyanide is found in many regions in space, such as large dust clouds, planetary atmospheres, and comets, Rahm is hopeful that their findings will enhance the understanding of similar events occurring in freezing conditions in space. The researchers may also be able to explore if other nonpolar substances can interact with hydrogen cyanide, like methane and ethane.

NASA’s Dragonfly probe is set to arrive at Titan in 2034 to study its surface. Until then, Martin Rahm and his team will keep researching hydrogen cyanide chemistry, working in collaboration with NASA.