Molecules on Saturn’s moon Titan are breaking a fundamental rule of chemistry, research reveals
Frigid conditions on the surface of Saturn’s largest moon, Titan, allow simple molecules in its atmosphere to break one of the most fundamental rules of chemistry, a new study suggests.
According to this principle, known as “like dissolves,” mixtures containing both polar and nonpolar components, such as oil and water, generally do not mix and instead form separate layers.
“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,” said the lead author of the study. Martin Rahmassociate professor of chemistry, biochemistry and chemical engineering at Chalmers University of Technology, said in a statement statement.
The new study, published July 23 in the journal PNASchallenges a long-standing pillar of chemistry and could open the door to the discovery of more exotic solid structures across the solar system.
Recreating the surface of Titan
Conditions on Titan’s surface bear a striking resemblance to those on early Earth, research suggests. Its atmosphere contains high levels of nitrogen and the simple hydrocarbon compounds methane and ethane, which evolve in a localized weather system, much like the water cycle on Earth.
However, until now, researchers were unsure of the fate of the hydrogen cyanide produced by reactions in this atmosphere. Is it deposited on the surface in solid form? Does it react with its environment? Or could it be transformed into the first molecules of life?
To study these questions, the NASA team replicated conditions on Titan’s surface by combining mixtures of methane, ethane and hydrogen cyanide at temperatures of around 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 contrasting compounds interacted much more closely than ever before.
It appeared that non-polar methane and ethane molecules had inserted themselves into gaps in the solid crystal structure of hydrogen cyanide – a process known as intercalation – to create an unusual co-crystal containing both sets of molecules.
Usually, polar and nonpolar molecules do not mix. Polar compounds, such as water and hydrogen cyanide, have an uneven distribution of their charge in the molecule, creating some areas that are slightly positive and others that are slightly negative. These oppositely charged regions are attracted to each other, forming strong intermolecular interactions between different polar molecules and largely ignoring nonpolar components.
Meanwhile, nonpolar oils and hydrocarbons have a fully symmetrical charge arrangement and interact very weakly with neighboring nonpolar molecules and not at all with polar particles. As a result, mixtures containing polar and nonpolar components, such as oil and water, generally form distinct layers.
To explain their bizarre observations, the NASA team joined forces with researchers at Chalmers University of Technology to model hundreds of potential cocrystal structures, evaluating each for its likely stability under Titan’s conditions.
“Our calculations predict not only that the unexpected mixtures are stable under Titan conditions, but also light spectra that coincide well with NASA measurements,” Rahm explained.
Their theoretical analysis identified several possible stable crystal forms, which they believe are stabilized by a surprising increase in the strength of intermolecular forces in the hydrogen cyanide solid triggered by this mixture.
Their rigorous combination of theory and experiment impressed Athena Coustenisplanetologist at the Paris-Meudon Observatory in France. She’s eager to see how future data, including from NASA’s Dragonfly probe (which is expected to arrive at Titan in 2034), will complement the study’s findings.
“Comparing laboratory spectra with data from the upcoming Dragonfly mission could reveal signatures of these solids on Titan’s surface, providing insight into their geological roles and potential importance as low-temperature prebiotic reaction environments,” Coustenis told Live Science in an email. Further work could even extend this approach to other molecules likely generated by Titan’s atmosphere, notably cyanoacetylene (HC3N), acetylene (C2H2), hydrogen isocyanide (HNC) and nitrogen (N2), she said. “[This] will test whether such mixing is a general feature of Titan’s organic chemistry. »
