Saturn’s largest moon may hold key to cleaner engines

The atmospheric haze of Titan, Saturn’s largest moon (pictured here along Saturn’s midsection), is captured in this natural-color image (box at left). (Credits: NASA Jet Propulsion Laboratory, Space Science Institute, Caltech)

A new discovery on Titan’s haze is revealing new information about burning fuels on Earth.

FIU chemist Alexander Mebel and a team of international researchers have been studying Saturn’s largest moon, trying to unlock a mystery brewing beneath Titan’s thick, hazy atmosphere — How is it that dunes of hydrocarbons exist on the moon’s frozen surface?

On Earth, the kinds of hydrocarbons that cause soot are only known to occur during the combustion process under very high temperatures. They are the kinds of byproducts that engineers usually try to eliminate when engines burn fuel.

By examining data from NASA’s Cassini-Huygens probe, the researchers determined hydrocarbons can form the type of complex chains that create Titan’s orange-brown haze layers at temperatures as low as 90 degrees Kelvin, which is about -298 degrees Fahrenheit — that’s nearly 330 degrees below freezing on Earth.

According to the researchers, this provides evidence that a low-temperature reaction pathway not previously considered could provide a missing link in Titan’s chemistry and yield clues to the development of complex chemistry on other moons and planets including Earth.

“Our calculations revealed the reaction mechanism,” Mebel said. “We showed that you don’t need any energy to drive the reaction of naphthyland vinylacetylene, so the reaction should be efficient even in the low-temperature and low-pressure atmospheric conditions on Titan.”

Scientists have explored the chemistry at work when combining two gases: one composed of two-ring molecular structure known as naphthyl radicals (upper left), and the other composed of a hydrocarbon called vinylacetylene (lower left). The white spheres represent hydrogen atoms and the dark spheres represent carbon atoms. Behind these 3-D molecular representations is an image of Saturn’s moon Titan, taken by NASA’s Cassini spacecraft. (Credits: Wikimedia Commons, NASA Jet Propulsion Laboratory, Caltech, Space Science Institute, John Hopkins University Applied Physics Laboratory, University of Arizona)

Benzene, a simple hydrocarbon with a six-carbon single-ring molecular structure, has been detected on Titan and is believed to be a building block for larger hydrocarbon molecules with two- and three-ring structures. These multiple-ring hydrocarbon molecules are known as polycyclic aromatic hydrocarbons, or PAHs for short. PAHs are something engineers want to avoid producing in combustion here on Earth.

The study shows PAHs are more widely spread than anticipated and can be produced at low temperatures. Mebel’s research can inform engine design by updating models using this new finding.

Next steps are investigating how chemical reaction rates in these models are calculated. They want to see how far they need to drill down and stop calculating the rates that so far have been assumed.

Mebel studies theoretical and quantum chemistry. His research looks at how molecules behave using computer simulations. These calculations help explain how certain molecules interact with each other in a chemical reaction and eventually form different molecules.

The study, a collaboration of scientists from Florida International University, the Department of Energy’s Lawrence Berkley National Laboratory, the University of Hawaii at Manoa and Samara University in Russia, was published today in the scientific journal Nature Astronomy.

The work at FIU, Berkley Lab and the University of Hawaii was supported by the U.S. Department of Energy’s Office of Basic Energy Sciences, including the Division of Chemical Sciences, Geosciences and Biosciences. Modeling studies in Samara were sponsored by the Ministry of Education and Science of the Russian Federation.

— Chrystian Tejedor contributed to this story.