Titan passing in front of Saturn and its rings in an image released by NASA.

The dark and massive fields of sand dunes on the surface of Saturn's largest moon have long puzzled scientists. The dunes, near the equator of Titan, are the colour of tar. They stand hundreds of  metres tall and extend for hundreds of kilometres.

The sand that makes up the dunes is not made of silicates like the sand on Earth. Instead, scientists believe it is made of hydrocarbons and may include particles of water ice. Keep in mind that water ice is as hard as rock on Titan, where the surface temperature is minus 183 degrees.

Because of their different compositions, the sands of Titan are much less dense and more powdery than the sand on Earth: as light as freeze-dried coffee grains or talcum powder. Combine that with Titan's low gravity, which is just one-seventh the gravity of Earth, and scientists have an indication of how the dunes were made.

A processed image of Titan from the European Space Agency's 2005 probe, showing rock-like lumps on the surface.

Two scientific studies looking at  how the dunes were formed were published last week.

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A team of scientists have tried to recreate the conditions of Titan in a wind tunnel in Arizona State University's Planetary Aeolian Laboratory. The wind tunnel was originally built in the 1980s to recreate the surface of Venus, but has been now been refurbished as a Titan simulator. The results, published on Monday in the journal Nature, revealed that it would take winds of at least 5.15km/h to lift the sand and make it move across the moon's surface. 

"It still seems really slow, but it turns out that the winds on Titan are really slow because there is no real temperature difference or pressure difference to drive fast winds", said Joshua Emery, an assistant professor at the University of Tennessee, Knoxville and a co-author of the paper.

This radar image, released by NASA in 2007, shows what scientists believe to be sea-size bodies of liquid, shown in blue, on the surface of Saturn's largest moon Titan. Photo: AP

Another conundrum was that the patterns of the dunes suggest they were created by westerly winds, even though the prevailing winds on Titan blow in an easterly direction. But it turns out that about twice a year, when the sun crosses Titan's equator, the atmosphere becomes turbulent enough  to make the winds switch direction and get significantly stronger.

Devon Burr, a planetary scientist at the University of Tennessee, Knoxville, said: "This work highlights the fact that the winds that blow 95 per cent of the time might have no effect on what we see."

In a separate paper published in the journal Nature Geoscience, another group of researchers studied images of Titan's sand dunes taken by NASA's Cassini probe. They discovered that it would take at least 3000 Saturn years (or 88,000 Earth years) for dunes on Titan to form. These findings suggest that the dunes of Titan, like the dune fields on our planet, are shaped by long-term climate cycles rather than seasonal cycles.

Titan is the only moon in the solar system with sand dunes, although planets Venus, Mars and Earth have dunes. It is the only other body besides Earth that has standing reservoirs of liquid on its surface, but the lakes and rivers of Titan are filled with methane and ethane, rather than water.

TItan has a thick atmosphere, about 1.5 times as dense as Earth's. A person standing on the surface of Titan would encounter the same pressure as if they were standing at the bottom of a swimming pool on our planet.

Los Angeles Times