The rules of chemistry on Saturn’s largest moon, Titan, may have to be rewritten thanks to a new discovery that shows how frozen crystals of hydrogen cyanide can mix with liquid hydrocarbons, in a combination that had not been thought possible until now.
Experiments at NASA’s Jet Propulsion Laboratory (JPL) in Southern California, coupled with computer simulations performed by researchers at the Chalmers University of Technology in Sweden, have shown how molecules of liquid ethane and methane, which fill the seas and lakes on Titan, can mix with crystals of hydrogen cyanide, which is frozen in the moon’s frigid minus 179 degrees Celsius temperature.
Hydrogen cyanide is what’s described as a polar molecule, in the sense that it has one side with a positive electric charge and another side that is negative. This means that it prefers to link up with other polar molecules, with opposite charges attracting.
On the other hand, methane and ethane, which are both hydrocarbon compounds (i.e. they are formed of atoms of hydrogen and carbon) are non-polar molecules, meaning that their electric charge is symmetrical, with both positive and negative charges on each side of their molecular structure.
Ordinarily, polar and non-polar substances don’t mix. It’s a little like oil remaining separate from water.
Hydrogen cyanide is formed in Titan’s atmosphere via reactions with ultraviolet light from the sun, which breaks down hydrocarbons and reforms them as other molecules. Given that non-polar hydrocarbons are common throughout Titan’s atmosphere and surface, scientists at JPL wanted to know what happens to the hydrogen cyanide after its creation. Yet their laboratory experiments mixing hydrogen cyanide with methane and ethane, performed at a temperature of minus 292 degrees Fahrenheit (minus 180 degrees Celsius), produced some surprising results that they didn’t understand. So they approached chemist Martin Rahm and his group at Chalmers, who had prior expertise with hydrogen cyanide at cold temperatures, in their search for answers.
“This led to an exciting theoretical and experimental collaboration between Chalmers and NASA,” said Rahm in a statement. “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 non-polar substances.”
Rahm’s computer simulations found that methane and ethane can penetrate into frozen hydrogen cyanide’s crystal lattice, forming a new and stable structure called a “co-crystal.”
“This can happen at very low temperatures, like those on Titan,” said Rahm. “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.”
Titan is the only moon in the solar system to possess a thick atmosphere, and its hydrocarbon chemistry is similar to the prebiotic soup that scientists think existed on Earth before life began. Although the cold temperatures on Titan seem to preclude the kinds of chemical reactions that could lead to life as we know it, Titan’s astrobiological worth is as a starting point, presenting what the molecular inventory might have been like on early Earth. Despite its toxicity to life now, hydrogen cyanide in particular is one of the building blocks of amino acids, which are used to construct proteins, and nucleobases in RNA and DNA.
“Hydrogen cyanide is found in many places in the universe, for example in large dust clouds, in planetary atmospheres and in comets,” said Rahm. “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 non-polar molecules can also enter the hydrogen cyanide crystals and, if so, what this might mean for the chemistry preceding the emergence of life.”
Either way, the findings suggest even closer interactions between Titan’s atmosphere, its frozen surface of ice dunes, and its lakes and seas of methane and ethane, than anyone had anticipated. When it arrives at Titan in 2034, NASA’s new rotorcraft, called Dragonfly, will be making stops on the surface and sampling materials, including hydrogen cyanide ice, where it will be able to verify the new results and look for even more complex and unexpected chemistry.
The findings were published in July in the journal PNAS.
