The massive and towering dunes that cover about one-eighth of the surface of Saturn's largest moon, Titan, pose a conundrum to scientists.

Data gathered by the Cassini spacecraft, which was sent to observe the Saturn system, show Titan has prevailing east-to-west winds. However, the moon's dunes, which could reach 100 meters tall and span a kilometer or more at the base, appear to form in the opposite direction.

Using a refurbished NASA wind tunnel, a group of researchers has found what is behind the formation of these structures, which could explain the seemingly bizarre direction of the dunes' formation.

Planetary scientist Devon Burr from the University of Tennessee and colleagues used a wind tunnel, which was built in the 1980s and initially intended for studying the physics of wind-blown sand on planet Venus, to recreate the conditions on the surface of Titan that can affect the formation of its dunes such as having a lower gravity and an atmosphere that is thicker than the Earth's.

The study "Higher-than-predicted saltation threshold wind speeds on Titan," published in the journal Nature on Dec. 8, found that the minimum wind speed needed for transporting Titan's sand, which is made up of a more viscous material compared with those found in the sands of the Earth's Sahara desert, is higher compared than the speed of the prevailing winds on Saturn's largest natural satellite.

Burr and colleagues discovered the regular east-to-west winds do not have sufficient strength to shape the viscous material into the large dune shapes. This leads them to believe that the dunes were instead shaped by the less occurring but more powerful westerly winds.

Atmospheric models suggest that the winds on Titan reverse twice in one Saturn year, or the equivalent of thirty Earth years, and this occurs when the sun crosses over the equator and causes the shifting of the atmosphere and the winds. Burr proposed that it is during this short period of fast westerly winds that the dunes are shaped.

"If the predominant winds are light and blow east to west, then they are not strong enough to move sand but a rare event may cause the winds to reverse momentarily and strengthen," Burr said. "The high wind speed might have gone undetected by Cassini because it happens so infrequently."

In another study published in Nature Geoscience on Dec. 8, Ryan Ewing from Texas A&M University and colleagues used data gathered by the Cassini spacecraft to study the sand dune patterns in Titan and found an indication that it may take as long as 3,000 Saturn years for these dunes to form.