An important discovery on the formation of complex molecules on Saturn’s big moon Titan

Saturn and Titan with the haze in the moon's atmosphere in the inset (Image NASA Jet Propulsion Laboratory, Space Science Institute, Caltech)
Saturn and Titan with the haze in the moon’s atmosphere in the inset (Image NASA Jet Propulsion Laboratory, Space Science Institute, Caltech)

An article published in the journal “Nature Astronomy” describes a possible way for the molecules known as polycyclic aromatic hydrocarbons (PAHs) to form at the very low temperatures existing on Titan, Saturn’s big moon. A team of researchers carried out experiments and simulations to understand how certain complex molecules could form in the haze layers in Titan’s atmosphere when in theory they required much higher temperatures. The result is the discovery that the presence of two gases can produce that type of molecules even at very low temperatures.

Titan’s atmosphere is dense and full of chemical compounds including hydrocarbons, so much so that there’s a methane cycle similar to water cycle on Earth. In simple words, there are seas, lakes and rivers of methane that partly evaporate, form clouds in the atmosphere and condense in rain. The presence of other compounds forms a rich mixture that until now wasn’t fully explained because there were no known chemical reactions that could create some complex molecules at temperatures typical of Titan, which on the surface are around -180° C (-292° F).

Benzene is a compound that’s part of the PAH group detected on Titan which according to scientists is one of the building blocks that make up more complex hydrocarbons existing in that moon’s atmosphere. To solve the mystery, the authors of this research carried out experiments at the US Department of Energy’s Berkeley Lab. They mixed two gases that probably exist in Titan’s atmosphere, also part of the PAH group: the naphthyl radical and a hydrocarbon called vinylacetylene. They reacted producing more complex PAHs at the temperatures typical of Titan.

A problem in the study of PAHs such as the ones studied at Berkeley Lab is that they have properties that make them very difficult to identify in deep space. So far, no PAH has been detected in interstellar space, does that mean that there are none or is it too difficult to identify them even for our more sophisticated instruments? If these compounds can react chemically even at very low temperatures it’s possible that in space they’re more abundant than previously thought.

This type of research will also be useful to understand how certain chemical reactions occur under different conditions, for example in an atmosphere such as Titan’s and in one such as the Earth’s. This is especially important concerning complex organic compounds that can lead to the formation of life’s building blocks.

Titan and the molecular structure of naphthyl radical and vinylacetylene (Image NASA Jet Propulsion Laboratory, Caltech, Space Science Institute, John Hopkins University Applied Physics Laboratory, University of Arizona)
Titan and the molecular structure of naphthyl radical and vinylacetylene (Image NASA Jet Propulsion Laboratory, Caltech, Space Science Institute, John Hopkins University Applied Physics Laboratory, University of Arizona)

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