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Chemists find possible explanation for Titan’s haze

Lab experiments suggest low-temperature photochemistry produces the moon’s hazy PAHs

by Sam Lemonick
October 16, 2018 | A version of this story appeared in Volume 96, Issue 42


Photo of Titan
Credit: NASA/JPL-Caltech/Space Science Institute
Polycyclic aromatic hydrocarbons could contribute to Titan's characteristic yellow haze.

The thick, hazy atmosphere of Saturn’s moon Titan has provided a curious mystery for scientists. In 2005, the European Space Agency’s Huygens lander detected atmospheric benzene on Titan via mass spectrometry, and evidence from other sources has suggested the presence of polycyclic aromatic hydrocarbons (PAHs)—possible sources of Titan’s haze. PAHs create haze here on Earth, and chemists have long understood how benzene forms the multiringed molecules at high temperature, but conditions on Titan are downright frigid, ranging from about -70 °C to -200 °C. Now, scientists using laboratory experiments and theoretical calculations have shown how these reactions could happen at low temperatures (Nat. Astron. 2018, DOI: 10.1038/s41550-018-0585-y).

Some aromatic chemistry in Titan’s atmosphere is already understood. Solar ultraviolet radiation penetrates hundreds of kilometers into Titan’s atmosphere, possibly photolyzing benzene to a phenyl radical, which reacts with vinyl acetylene to make naphthalene. To figure out how that process could produce more complex PAHs, a team led by Ralf I. Kaiser of the University of Hawaii, Manoa, and Musahid Ahmed of Lawrence Berkeley National Laboratory reacted naphthyl radical with vinyl acetylene under conditions that simulate Titan’s low temperature. Through radical intermediates, those reactions produced three-ringed anthracene and phenanthrene molecules—identified by mass spectrometry.

Reaction scheme showing benzene to anthracene via radical intermediates and addition of vinyl acetate
Solar photons create radical intermediates that can react with vinyl acetylene to build polycyclic aromatic hydrocarbons at low temperatures.

To explain how these products could form at low temperatures, Kaiser and Ahmed collaborated with Alexander M. Mebel of Samara University and Florida International University and colleagues to calculate energetically feasible reaction pathways from naphthalene and vinyl acetylene to the three-ringed PAHs. Mebel says he was surprised to find four pathways with surmountable energy barriers that led from each of the possible naphthyl radicals to both anthracene and phenanthrene.

“This is a new route towards PAH formation,” says Xander Tielens of Leiden University, who studies PAHs in interstellar space. He explains that the results show how solar UV photons can create radicals at low temperatures, avoiding the high energy barrier to PAH formation seen in combustion and other high-temperature processes. And Tielens says this mechanism could explain PAH formation elsewhere in space, like in molecular clouds.

The group is now adapting their experiments to explore formation of larger PAHs, including molecular cages and other three-dimensional structures, as well as aromatic molecules containing nitrogen, the major component of Titan’s atmosphere.


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