The discovery of Saturn’s moon Titan challenged what scientists thought was a fundamental rule of chemistry.
There, in the extreme cold, some seemingly fundamentally incompatible molecules can combine to form solids never before seen in the Solar System, new research suggests.
This foreign material, according to a team led by chemist Fernando Izquierdo-Ruiz of Sweden’s Chalmers University of Technology, is likely to be abundant on Titan.
“These are very exciting discoveries that could help us understand something on a very large scale, the moon.” [Titan] the size of the planet Mercury,” says chemist Martin Rahm of Chalmers University of Technology.
Related: Titan may have an alien biosphere – but it might be the size of a dog
Titan is a fascinating little corner of the solar system. Its lakes of methane and hydrocarbons contain a complex chemistry that is remarkably close to the prebiotic chemistry needed to give rise to life. This doesn’t mean that life is possible there, but it does provide insight into the conditions under which life can occur.
A special cornerstone of prebiotic chemistry is hydrogen cyanide, which under the right conditions forms compounds that can become the building blocks of life, such as nucleobases and amino acids. Titanium is rich in hydrogen cyanide.
YouTube thumbnail
It is also a strongly polar molecule with an uneven distribution of electrons, giving it an unequal charge.
Normally, polar and nonpolar molecules, such as methane and ethane on Titan, tend to repel each other. It takes more energy to connect them than to separate them. This is a precise mechanism that prevents (polar) water from mixing with (non-polar) oil.
Researchers have been studying the likely behavior of hydrogen cyanide on Titan as scientists at NASA’s Jet Propulsion Laboratory tried to figure out what happens after the molecule forms in Titan’s atmosphere.
They conducted the experiments at a temperature of about -180 degrees Celsius (-292 Fahrenheit), which corresponds to the surface temperature of Titan. At extreme cold, hydrogen cyanide is a crystal, while methane and ethane are in liquid form.
Mid-article ad Astro
When the experiment was done and they analyzed the resulting mixtures, the NASA scientists could tell that something had changed, but they weren’t sure what, so they recruited Chalmers chemists.
“This led to an interesting theoretical and experimental collaboration between Chalmers and NASA,” says Rahm. “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 goes against the chemical rule of like dissolves like, which basically means that it shouldn’t be possible to combine these polar and non-polar substances.”
The experimental setup was similar: a chamber adjusted to about -180 degrees Celsius in which the researchers grew hydrogen cyanide crystals. They introduced methane, ethane, propane and butane into this environment, using Raman spectroscopy to record how the molecules vibrated.
They recorded small but distinct shifts in hydrogen cyanide fluctuations after exposure to methane and ethane, suggesting that these incompatible materials were not just hanging out next to each other, but were also interacting.
The directions of these shifts suggested that the hydrogen bonds in hydrogen cyanide were subtly strengthened, bent, and stretched in methane and ethane.
The team then turned to computer simulations to confirm their suspicions: methane and ethane slipped between the gaps of the hydrogen cyanide crystal lattice, joining together to form structures known as co-crystals, which remain stable at titanium’s temperature.
YouTube thumbnail
The researchers concluded that under titanium-like conditions the molecules are not thermally excited the way they are at higher temperatures, which allowed methane and ethane to penetrate the hydrocyanide, showing how molecules that normally hate each other can interact and bond.
“The discovery of unexpected interactions between these materials could have implications for how we understand Titan’s geology and its strange landscapes of lakes, seas and sand dunes,” says Rahm.
Unfortunately, we may have to wait a few years for the significance of this strange chemistry to be confirmed, as the planned Dragonfly probe won’t reach the shores of Saturn’s odd moon until 2034.
“Until then, these structures are a humbling reminder of how surprising basic chemistry can be,” the researchers write.
In future work, the researchers hope to discover what other non-polar materials might play nicely with hydrogen cyanide if the conditions are right.
The study was published Proceedings of the National Academy of Sciences.