Saturn's moon Titan just broke one of chemistry’s oldest rules

Saturn's largest moon has long fascinated researchers because its evolution could shed light on the early chemical processes that once shaped our own planet. Titan's frigid surface and its dense atmosphere, rich in nitrogen and methane, are thought to resemble the conditions that existed on the young Earth billions of years ago. By exploring Titan, scientists hope to uncover new clues about the origins of life itself.

Martin Rahm, Associate Professor at Chalmers' Department of Chemistry and Chemical Engineering, has spent years investigating Titan's chemistry. He and his colleagues now believe their latest finding -- that certain polar and nonpolar substances* can combine under extreme cold -- could guide future research into the moon's surface and atmosphere.

"These are very exciting findings that can help us understand something on a very large scale, a moon as big as the planet Mercury," he says.

New insights into the building blocks of life in extreme environments

The study, published in PNAS, reveals that methane, ethane, and hydrogen cyanide -- compounds abundant on Titan's surface and in its atmosphere -- can interact in ways once thought impossible. The fact that hydrogen cyanide, a strongly polar molecule, can form crystals together with nonpolar substances like methane and ethane is remarkable, since these types of molecules usually stay separate, much like oil and water.

"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. In addition, hydrogen cyanide is likely to play an important role in the abiotic creation of several of life's building blocks, for example amino acids, which are used for the construction of proteins, and nucleobases, which are needed for the genetic code. So our work also contributes insights into chemistry before the emergence of life, and how it might proceed in extreme, inhospitable environments," says Martin Rahm, who led the study.

An unanswered question led to NASA collaboration

The Chalmers research began with a simple but unresolved question about Titan: What happens to hydrogen cyanide after it forms in the moon's atmosphere? Does it accumulate in thick layers on the surface, or does it react with its surroundings in some way? To investigate, scientists at NASA's Jet Propulsion Laboratory (JPL) in California performed experiments mixing hydrogen cyanide with methane and ethane at extremely low temperatures of about 90 Kelvin (around -180 degrees Celsius). At these temperatures, hydrogen cyanide becomes a crystal, while methane and ethane remain liquid.

When the team analyzed the mixtures using laser spectroscopy, which examines materials and molecules at the atomic level, they found that although the molecules stayed intact, something unusual had occurred. To understand it, they reached out to Rahm's group at Chalmers, known for its deep expertise in hydrogen cyanide chemistry.

"This led to an exciting theoretical and experimental collaboration between Chalmers and NASA. The question we asked ourselves was a bit crazy: Can the measurements be explained by a crystal structure in which methane or ethane is mixed with hydrogen cyanide? This contradicts a rule in chemistry, 'like dissolves like', which basically means that it should not be possible to combine these polar and nonpolar substances," says Martin Rahm.

Expanding the boundaries of chemistry

The Chalmers researchers used large scale computer simulations to test thousands of different ways of organizing the molecules in the solid state, in search of answers. In their analysis, they found that hydrocarbons had penetrated the crystal lattice of hydrogen cyanide and formed stable new structures known as 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," he says.

The discovery challenges one of the best-known rules of chemistry, but Martin Rahm does not think it is time to rewrite the chemistry books.

"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.

In 2034, NASA's space probe Dragonfly is expected to reach Titan, with the aim of investigating what is on its surface. Until then, Martin Rahm and his colleagues plan to continue exploring hydrogen cyanide chemistry, partly in collaboration with NASA.

"Hydrogen cyanide is found in many places in the Universe, for example in large dust clouds, in planetary atmospheres and in comets. The findings of our study may help us understand what happens in other cold environments in space. And we may be able to find out if other nonpolar molecules can also enter the hydrogen cyanide crystals and, if so, what this might mean for the chemistry preceding the emergence of life," he says.

More about the research

The scientific article Hydrogen cyanide and hydrocarbons mix on Titan has been published in the journal PNAS. It was written by Fernando Izquierdo Ruiz, Morgan L. Cable, Robert Hodyss, Tuan H. Vu, Hilda Sandström, Alvaro Lobato Fernandez and Martin Rahm. The researchers are based at Chalmers University of Technology, Sweden, NASA's Jet Propulsion Laboratory (JPL) at the California Institute of Technology (Caltech), USA, and Universidad Complutense de Madrid, Spain.

The research at Chalmers was funded by the Swedish Research Council.

More on Titan and Dragonfly Saturn's largest moon, Titan, is among the Solar System's most unusual worlds -- and it may share features with Earth's early evolution. Titan is surrounded by a thick atmosphere composed mostly of nitrogen and methane, a composition that could resemble the atmosphere on Earth billions of years ago, before life emerged. Sunlight and other radiation from space cause these molecules to react with each other, which is why the moon is shrouded in a chemically complex, orange-coloured haze of organic (i.e. carbon-rich) compounds. One of the main substances created in this way is hydrogen cyanide.

Titan's extremely cold surface is home to lakes and rivers of liquid methane and ethane. It is the only other known place in our solar system, apart from Earth, where liquids form lakes on the surface. Titan has weather and seasons. There is wind, clouds form and it rains, albeit in the form of methane instead of water. Measurements also show that there is likely a large sea of liquid water many kilometres below the cold surface which, in principle, might harbour life.

In 2028, the US space agency NASA plans to launch the Dragonfly space probe, which is expected to reach Titan in 2034. The aim is to study prebiotic chemistry, the chemistry that precedes life, and to look for signs of life.

Notes

* About polar and nonpolar substances: Polar substances consist of molecules with an asymmetrical charge distribution (a positive side and a negative side), while nonpolar materials have a symmetrical charge distribution. Polar and nonpolar molecules rarely mix, because polar molecules preferentially attract one another via electrostatic interactions.