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Ice reactions could have helped form Titan’s dunes

Lab experiments demonstrate how cosmic radiation converts acetylene to polycyclic aromatic hydrocarbons

by Sam Lemonick
October 23, 2019 | A version of this story appeared in Volume 97, Issue 42

Credit: C&EN/ACS Productions
The Cassini spacecraft taught us about Titan's atmospheric chemistry.

On Saturn’s moon Titan, the lakes, seas, and rain are made from hydrocarbons, not water. The moon even has hydrocarbon dunes, standing 100 m high. Until now, scientists have proposed that reactions in Titan’s atmosphere produced the dunes’ particles, but new laboratory experiments suggest that cosmic radiation interacting with acetylene ice on the moon’s surface could provide another explanation (Sci. Adv. 2019, DOI: 10.1126/sciadv.aaw5841).

Image of black wavy dunes on the surface of Saturn's moon Titan.
Credit: NASA/JPL-Caltech/ASI
Titan's black dunes are made of hydrocarbons.

One clue that atmospheric reactions might not be the only source of the dune hydrocarbons is that observations suggest the dunes’ particles are more than 100 times as large as what atmospheric chemistry can produce. Ralf Kaiser and colleagues at the University of Hawai‘i at Mānoa decided to test an alternative origin with a lab experiment that simulates the effect of galactic cosmic radiation (GCR) on acetylene ices, which are thought to exist on Titan’s surface in the same areas as the dunes.

GCR itself doesn’t have enough energy to initiate ring-forming reactions from acetylene, but electrons that GCR knocks out of molecules in the ices could. The researchers bombarded acetylene ices at 5 K and very low pressure with electrons, and spectroscopically observed the formation of benzene, three-ringed phenanthrene, and other polycyclic aromatic hydrocarbons. Kaiser says the dunes’ color indicates that they contain larger PAHs than those made in the lab, but the team plans to test whether a longer reaction process could build up molecules with more rings.

Ralph Lorenz of Johns Hopkins University doesn’t question the researchers’ chemistry, but points out that Titan’s atmospheric chemistry is so rich that we might not need another explanation for the dunes’ source. He suggests that some process on the moon’s surface or seas might aggregate organic particles to the size of those found in the dunes. Lorenz is part of a NASA project to land a robot near Titan’s dunes, which he and Kaiser agree could help answer these questions.


This story was updated on Oct. 25, 2019, to clarify the photograph caption. Titan's dunes are made of a range of hydrocarbons, not just polycyclic aromatic hydrocarbons.



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