There are dark and massive fields of sand dunes on the surface of Saturn's moon Titan, and this week two scientific papers look at how they came to be.
Titan is one of the most intriguing moons in our solar system. 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.
It has a thick atmosphere, about 1.5 times as dense as what we have on Earth. 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.
And near the equator, scientists have discovered another mysterious feature - dark sand dunes the color of tar that stand hundreds of feet tall and stretch hundreds of miles in length.
Sand dunes have been found in just a few places throughout the solar system - on Venus, Mars, Earth and Titan - but Titan is the only moon where dunes have been discovered.
The sand that makes up Titan's dunes is not made of silicates like the sands we find on Earth, however. 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 negative 297 degrees Fahrenheit).
Because of their different compositions, the sands of Titan are much less dense and more powdery than the sand on Earth. Combine that with Titan's low gravity, which is just one-seventh of the gravity of Earth, and you get "sand" that is as light as freeze-dried coffee grains.
To understand how a 300-foot-tall dune of this material might form on this distant moon, a team of scientists 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 of their experiment, published Monday in a Nature study, revealed that it would take winds of at least 3.2 mph to lift the sand and cause it to move across the moon's surface.
That may not sound very strong, but it is 40% to 50% stronger than previous estimates, the scientists said.
"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 and a coauthor of the paper.
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 that the wind switches direction and gets significantly stronger.
"This work highlights the fact that the winds that blow 95% of the time might have no effect on what we see," said Devon Burr, a planetary scientists at the University of Tennessee, Knoxville, in a statement. "
In a separate paper published in the journal Nature Geoscience, another set of researchers studied images of Titan's sand dunes taken by NASA's Cassini probe. They discovered that it would take at least 3,000 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.
So that's a few mysteries solved from the surface of Titan, but there are plenty more to explore. For example, Emery said they may want to look into how much the grains of Titan sand stick together, which would also impact the forces needed for dune construction.