Challenging conventional wisdom, study suggests the dunes formed through centuries of slow irradiation.
Sprawling fields of dark-coloured sand dunes on Saturn’s giant moon Titan may have been produced by eons of irradiation by galactic cosmic rays, scientists say.
The dunes, discovered by NASA’s Cassini spacecraft and the ESA’s Huygens lander, sprawl over 10 million square kilometres of Titan’s surface, an area about the size of the US, including Alaska. They reach heights of nearly 100 metres.
But their grains are not like Earth’s sands. Most probably they are made of a mix of water ice and complex organics, such polycyclic aromatic hydrocarbons (PAHs), which are composed of multiple carbon rings linked together.
Historically, scientists have believed that the dunes’ organic compounds were formed by the action of sunlight on methane and nitrogen in Titan’s thick atmosphere, via a photochemical process akin to that which creates smog in polluted cities. Over time, they thought, these smog-like particles gradually settled on Titan’s surface.
In a paper published the journal Science Advances, however, scientists from the University of Hawaii at Manoa discovered that similar materials could be formed via irradiation from galactic cosmic rays.
Galactic cosmic rays are an extremely powerful form of radiation that enters our solar system from interstellar space. Earth is largely protected by its magnetic field, says principle investigator Ralf Kaiser from the University of Hawaii at Manoa in Honolulu.
Titan has no such magnetic shielding, although its dense atmosphere – much denser than Earth’s – does block most of the radiation.
But if enough gets through that, over time, it has a major effect, Kaiser says.
In laboratory experiments, his team bombarded acetylene ice – a material known to exist on Titan – with high-energy electrons, which are a good stand-in for actual cosmic rays. They continued until the acetylene had received the equivalent of 100 years worth of space radiation falling on Titan’s surface.
They then cataloged the reaction products, discovering PAHs with up to three or four rings.
“This is against conventional wisdom,” Kaiser says, “because scientists think that to form aromatic structures you need high temperatures like combustion.”
But the energy from the cosmic rays was so intense that these compounds formed at temperatures far below that of Titan’s surface of -179 degrees Celcius.
The process, he adds, works very quickly, especially compared to geological time scales. “Lots of organic material could accumulate,” he says.
More importantly, he says, it also works in a vacuum. That means that other acetylene-containing bodies in the outer solar system could also have PAHs, a possible explanation for why some of them have mysterious dark patches of organic compounds on their surfaces.
Kaiser’s team hasn’t proven that cosmic rays are the only ways by which PAHs can be formed on Titan, says Ralph Lorenz, a planetary scientist at Johns Hopkins University's Applied Physics Laboratory in Laurel, Maryland, who was not part of the study team. In fact, he says, there is evidence that these chemicals also exist in Titan’s atmosphere.
“[But] they’ve shown that the chemistry story doesn’t (completely) stop when material settles out of the atmosphere,” he says. “It is interesting that future processing on the surface by the (small) flux of cosmic rays is possible.”
Happily, Lorenz says, NASA recently green-lighted a return mission to Titan, called Dragonfly, scheduled for launch in 2026. “Dragonfly will initially land among sand dunes,” Lorenz says. “So it should find out.”