The frigid conditions on the surface of Saturn's largest moon, Titan, allow simple molecules in its atmosphere to violate one of the most fundamental rules of chemistry, according to a new study.
See also: Saturn: 128 new moons discovered in orbit around it
According to this principle, known as "like dissolves like," mixtures containing both polar and nonpolar components, such as oil and water, usually do not mix and form separate layers.
But scientists from NASA's Jet Propulsion Laboratory and Chalmers University of Technology in Sweden were surprised to discover that the polar molecule hydrogen cyanide forms stable co-crystals with the highly nonpolar hydrocarbons methane and ethane on Titan's cold surface — molecules that are normally completely incompatible on Earth.
The new study, published July 23 in the journal PNAS, challenges a long-held pillar of chemistry and could open the door to the discovery of more exotic solid structures throughout the solar system.
See also: Uranus – Neptune: Maybe they're not as "frozen" as we think!
Conditions on Titan's surface bear a striking resemblance to those of early Earth, according to research. Its atmosphere contains high levels of nitrogen and the simple hydrocarbons methane and ethane, which are recycled in a local weather system, much like Earth's water cycle. However, until now, researchers were unsure of the fate of hydrogen cyanide produced by reactions in this atmosphere.
To explore these questions, the NASA team replicated conditions on Titan's surface by combining mixtures of methane, ethane and hydrogen cyanide at temperatures of about minus 297 degrees Fahrenheit (minus 183 degrees Celsius). A spectroscopic analysis — a way of studying chemicals through their interactions with different wavelengths of light — yielded unexpected results, suggesting that these opposing compounds interacted much more closely than had ever been observed before.
See also: James Webb: New moon around Uranus!
To explain their strange observations, the NASA team joined forces with researchers at Chalmers University of Technology to model hundreds of possible co-crystal structures, assessing each for its potential stability under Titan's conditions. Their theoretical analysis identified several possible stable crystal forms, which they propose are stabilized by a surprising enhancement in the strength of intermolecular forces in solid hydrogen cyanide triggered by this mixing.
