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Titan is the only other place in the solar system with liquid oceans on its surface, but it also has huge swathes of desert covered in sand dunes. The material that makes up these dunes is commonly assumed to have fallen from the sky, but a new study suggests it may have been made on the ground instead.

The dunes on Titan, Saturn’s largest moon, stretch across its equatorial region and reach heights of up to about 100 metres in some places. Images from NASA’s Cassini spacecraft have shown that the dunes contain some dark-coloured organic molecules, which are built around long chains of carbon atoms.

We can also see organic molecules in Titan’s thick, hazy atmosphere, leading many researchers to infer that they form there and then fall to the ground as rain. But Ralf Kaiser at the University of Hawai’i at Mānoa and his colleagues have found that they may also form on the ground in a process that could also take place on other worlds, even those without atmospheres.

Cassini found the signature of acetylene ice in the same regions as the dunes on Titan, so Kaiser and his team performed an experiment to see if this ice could be chemically converted into complex organic molecules. They bombarded acetylene ice in a laboratory with high-energy radiation similar to the cosmic rays that propagate through the galaxy, then heated the ice up until it sublimated so they could determine its final makeup.

They found that radiation hitting the ice did cause it to react chemically to create the organic molecules we see in Titan’s dunes. These molecules are also likely created in the atmosphere, so they could come from there as well, says Ralph Lorenz at Johns Hopkins University in Maryland.

The process is probably even more efficient on airless worlds such as the dwarf planets Pluto and Makemake, which also show signs of organic ice, says Kaiser.

“Bottom line, we don’t know exactly what the sand is and we don’t know how it got there,” says Lorenz. We know very little about Titan’s surface, which is why NASA is sending the Dragonfly mission there in 2026. Dragonfly will land near the dunes in 2034 and is designed to answer many of our questions about Titan’s chemistry.