On a Saturn moon, chemicals can do things they can't on Earth

Some substances that don't mix on Earth can combine in ultra freezing conditions like those on Saturn's largest moon, breaking one of the best-known rules of chemistry, new research shows.

Titan, a hazy orange world about 880 million miles away in space, shares similarities with early Earth and is the only other place in the solar system with a thick atmosphere, as well as lakes and dunes on its surface. A key reason scientists study the moon is to try to figure out how the first chemical steps toward creating life may have unfolded on our own planet. 

Now a team of researchers from NASA and Chalmers University in Sweden has found that on Titan, the cold environment allows molecules to combine in ways scientists hadn't thought were possible. Its main chemicals — methane, ethane, and hydrogen cyanide — can form crystals together. Normally, hydrogen cyanide doesn't mix with methane and ethane, similar to how oil and water separate. 

That discovery opens up new possibilities for how the basic ingredients of life might begin in worlds once thought too hostile for complex chemistry, said Martin Rahm, a chemistry professor at Chalmers. The team's paper, for which Rahm is a coauthor, appears in the journal Proceedings of the National Academy of Sciences.

"Hydrogen cyanide is found in many places in the universe," he said in a statement. "The findings of our study may help us understand what happens in other cold environments in space." 

Titan, a moon that rivals Mercury in size, apparently breaks a basic chemistry rule related to the distribution of electrical charges in molecules. This property is known as polarity. A polar molecule has one end that is slightly negative and another that is slightly positive, whereas a nonpolar molecule has balanced charges. 

In short, nonpolar substances on Earth tend to dissolve nonpolar substances, and polar substances tend to dissolve polar substances — or, as Rahm puts it, "like dissolves like." 

NASA's Cassini spacecraft captured Ligeia Mare, the second-largest known body of liquid on Saturn's moon Titan. Credit: NASA / JPL-Caltech / ASI / Cornell

The study began when NASA scientists tried to mix hydrogen cyanide, which is highly polar, with nonpolar molecules methane and ethane at nearly -300 degrees Fahrenheit, matching the temperature of Titan. When they looked at the mixtures using laser spectroscopy, a method for studying a material based on how light interacts with it, they saw that the chemicals hadn’t changed, but something else unusual had happened.

A group of scientists at Chalmers who are experts in hydrogen cyanide helped analyze the data, revealing that liquid methane and ethane actually merged with hydrogen cyanide inside a new kind of crystal. This finding is exciting for astrobiologists because hydrogen cyanide is thought to have played a key role in the creation of amino acids and nucleobases, the building blocks of proteins and DNA, Rahm said.

"So our work also contributes insights into chemistry before the emergency of life, and how it might proceed in extreme, inhospitable environments," he said.

Titan is a special moon because its cold atmosphere — filled with nitrogen and methane — may mimic what young Earth was like billions of years ago. It's also the only other world around the sun known to have active weather, though its precipitation isn't water. Titan's lakes and seas are filled instead with methane and ethane, which cycle through Titan's atmosphere, forming clouds and oily rain that reshape the landscape, much like Earth’s water cycle. 

For years, scientists have wondered if these liquids might also allow the crucial chemistry necessary for life to occur — or at least the kind familiar to us. That question drives NASA's $3.35 billion Dragonfly mission, set to launch in July 2028 and reach Titan six years later. Scientists think a deep underground ocean of liquid water may also exist within Titan, possibly capable of supporting life.

The Chalmers team used computer models to simulate thousands of possible molecular combinations. They found that organic compounds could sneak into hydrogen cyanide’s crystal structure and form "co-crystals." Because these could exist at Titan's frigid temperatures and they match NASA's lab results, there's a good chance these chemical mixtures actually occur on Titan, the scientists say. 

"I see it as a nice example of when boundaries are moved in chemistry and a universally accepted rule does not always apply," Rahm said.