A decade ago, radar images taken by the Cassini spacecraft first revealed that 20% of the surface of Saturn's largest moon is covered by wind-formed dunes hundreds of yards high, more than a mile wide and hundreds of miles long. But how is this possible, when data suggests that Titan only has light breezes?
What's more, the Cassini data showed that the predominant winds that shaped the dunes — which are likely made of tiny sand-sized particles of frozen hydrocarbons — blew from east to west. However, the streamlined appearance of the dunes around obstacles like mountains and craters (below) indicate they were created by winds moving in exactly the opposite direction.
A team of researchers led by planetary scientist Devon Burr spent eight years studying the mystery by refurbishing a defunct NASA high-pressure wind tunnel to recreate Titan's surface conditions. Since they were uncertain about the precise properties of the frozen particles that form the dunes, they used 23 different varieties of sand.
Their experiments and computer models confirmed that the minimum wind speed on Titan would have to be 50% faster than previously thought to create the dunes. They've concluded that this is likely the result of long-term climate cycles on Titan:
"If the predominant winds are light and blow east to west, then they are not strong enough to move sand," says Burr. "But a rare event may cause the winds to reverse momentarily and strengthen."
According to atmospheric models, the wind reverses twice during a Saturn year, which is equal to about thirty Earth years. This reversal happens when the sun crosses over the equator, causing the atmosphere—and subsequently the winds—to shift. Burr theorizes that it is only during this brief time of fast winds blowing from the west that the dunes are shaped.
"The high wind speed might have gone undetected by Cassini because it happens so infrequently."
The computer simulations suggest that the dunes formed over a period of 3,000 Saturn years — that's 88,200 years on Earth.