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When astronomers first discovered huge sand dunes on Titan a decade ago, they were baffled by their discovery - because Titan's winds were thought to be too timid to create them.

But now scientists, using a wind tunnel on Earth, have discovered a remarkable process may be taking place that explains how they formed.

Based on their research, the team discovered that on rare occasions the winds suddenly change direction and increase their speed by 50 per cent - but why they do this is, in itself, a new mystery.

Scientists in Arizona have found an explanation for sand dunes on Titan. They say the moon's winds occasionally reverse and increase in speed. In this image the dark lines are dunes crawling across the surface of Titan, Saturn's largest moon

Of the more than 150 moons in the solar system, Titan is the only one known to have an atmosphere, and in turn winds, on its surface.

It also has windswept dunes on its surface that are more than a mile (1.6km) wide, hundreds of miles long and up to 300 feet (90 metres) high - about the size of Big Ben.

But the appearance of the dunes on the surface was a mystery, as not only were the winds thought to be too weak, but they also formed in the opposite direction to Titan's steady east-west wind.

The latest research could finally explain this puzzling mystery of Titan - and it may explain winds on other bodies as well.

The experiment was carried out by a team of scientists using a high-pressure wind tunnel at Arizona State University's Planetary Aeolian Laboratory.

After two years of many models and recalibrations, the team discovered that the minimum wind on Titan had to reach speeds of 3.2mph (5.1kph) to move the sand.

But this is 50 per cent faster than the average speed of wind on Titan's surface - suggesting their must be a hidden process taking place.  

'Our models started with previous wind speed models, but we had to keep tweaking them to match the wind tunnel data,' said Dr Devon Burr from the University of Tennessee.

'We discovered that movement of sand on Titan's surface needed a wind speed that was higher than what previous models suggested.'

From the experiment, and other atmospheric models, it has been deduced that the wind must reverse twice during a Saturn year - which is about 30 Earth years.

The reversal happens when the sun crosses over Titan's equator, causing its atmosphere - and the winds - to shift.

This not only causes the wind to blow in the opposite direction - west to east - but also increases its speed by the required 50 per cent, for reasons unknown. 

And the shift must occur very briefly, according to Dr Burr, which may explain why the Cassini spacecraft currently in orbit around the Saturnian system has not been able to detect this atmospheric phenomenon.

Of the more than 150 moons in the solar system, Titan is the only one known to have an atmosphere, and, in turn winds, on its surface. It is seen here, covered in thick clouds, approaching the limb of Saturn (upper left), by the Cassini probe as it approached the Saturn system

The latest experiment was carried out by a team of scientists using a high-pressure wind tunnel, shown, at Arizona State University's Planetary Aeolian Laboratory. They used 23 different types of sand to come to the conclusion that wind speeds on the moon must occasionally increase in order to create the dunes

Dunes begin to form when the wind picks up loose particles from the ground and drives them downwind.

A key part of understanding dunes is to identify the threshold wind speed that causes dune particles to start to move.

Geologists have found threshold speeds for sand and dust under various conditions on Earth, Mars and Venus. But for Titan, with its bizarre conditions, this had remained unknown - until now.

The scientists led by Dr Burr began their study with carefully designed wind tunnel experiments.

'We refurbished the high-pressure wind tunnel previously used to study conditions on Venus,' Dr Smith of Arizona State University explained.

Titan's windswept dunes on its surface are more than a mile (1.6km) wide, hundreds of miles long and up to 300 feet (90 metres) high - about the size of Big Ben (stock image shown)

The high-pressure wind tunnel at Arizona State University's Planetary Aeolian Laboratory, shown, was originally built to simulate winds at the surface of Venus, where atmospheric pressures are about 90 times greater than on Earth. After modifications, it became the Titan Wind Tunnel

To recreate in the tunnel on Earth the wind conditions on Titan, the scientists had to increase the air pressure in the wind tunnel to about 12 times the surface pressure of Earth.

Dr Burr dedicated six years to refurbishing the defunct Nasa high-pressure wind tunnel to recreate Titan's surface conditions.

She and her team then turned up the tunnel's pressure to simulate Titan's dense atmosphere, turned on the wind tunnel fan, and studied how the experimental sand behaved.

Because of uncertainties in the properties of sand on Titan, they used 23 different varieties in the wind tunnel to capture the possible sand behaviour on Titan.

And they compensated for the low density of Titan 'sand' and the moon's reduced gravity through numerical modeling.

For simplicity, the wind-tunnel modeling ignored some factors, among them whether Titan dune particles are sticky.

If they are, the paper's scientists note, then it will take yet-stronger winds to get the particles moving, and the contrasts will be even greater between the normal east wind pattern and the stronger west winds that shape the dunes.

The new results should help in understanding atmospheric forces on other icy moons and planets with very thin or thick atmospheres, such as Neptune's moon Triton, Pluto or on comets.

They can also help us better understand movement of particles in fluids in general. Particle flows are important in a wide range of situations, including coal-mine or grain-elevator dust explosions, environmental pollution and lubricants.

Cassini radar has previously seen sand dunes on Saturn's giant moon Titan (upper photo) that are sculpted like Namibian sand dunes on Earth (lower photo). The bright features in the upper radar photo are not clouds but topographic features among the dunes