Saturn's moon Titan has vast stretches of bizarrely electric sand dunes made of organic sand, and researchers found out where they come from.
Johns Hopkins University experts teaming up with research firm Nanomechanics have found that the sand dunes spread out through Titan's equatorial regions were formed right where they were found. This is in contrast to previous theories that they may have been carried by polar winds or liquid streams coming from the moon's polar regions, where methane lakes abound.
By studying the mechanical properties of Earth analogs for extraterrestrial sand, the team was able to conclude that the sands on Titan are too soft to be transported by wind or water ice.
Organic Sand On Titan
NASA's Cassini orbiter, which spent 13 years in orbit around Titan, first found sand dunes on the icy moon in 2006. Data gathered from the spacecraft showed long, dark streaks of sand akin to the windswept dunes on Earth.
Scientists first suggested that the dunes were made of sand formed near the methane lakes in the moon's north pole, which were later picked up by the winds and deposited in the equator.
However, subsequent data showed that the grains of sand are made of particles of organic material. These particles are produced via a photochemistry reaction, where methane molecules in Titan's atmosphere absorb ultraviolet light.
Each grain of sand is at least 100 microns in size, while the particles that form one grain are minuscule, never measuring bigger than 1 micron, which is about 1/50 of the width of a human hair. Scientists believe each grain consists of at least 1 million organic particles.
Where these particles are processed and how they formed the grains of sand on Titan's equator have remained a mystery for more than a decade.
Mechanical Properties Of Titan's Sand
In a new paper submitted to the Journal of Geophysical Research: Planets, the team of researchers explain how they used Earth analogs to study the mechanical properties of the sand on Titan.
Led by graduate student Xinting Yu of JHU's Department of Earth and Planetary Sciences, the team used a variety of materials, including various natural Earth sands ranging from silicate sand, which is the sand found on most beaches on Earth, white gypsum sand, and carbonate sand.
They also used laboratory-made tholins, a complex organic material made from methane molecules irradiated with ultraviolet light. These particles simulate the conditions that led to the rise of organics in Titan's atmosphere.
Other simulated sands included materials typically used for wind tunnels, such as ground walnut shells, activated charcoal, and instant coffee.
The researchers used a method called nanoindentation to study various properties of the analog materials, including their stiffness, hardness, and brittleness.
The method involves placing the sands in a wind tunnel to look at how they moved and if they could be scattered in the same way as the sand dunes on Titan. The movement of the grains will create very tiny indents that can be analyzed to determine how stiff, hard, and brittle they are.
The findings from the study show the sands of Titan are much softer than the sands on Earth. In other words, they are not strong enough to withstand being carried by wind or water over the 1,200-mile distance between the poles and the equator. The most likely conclusion is that the grains of sand formed close to where they are found now.
"The soft and brittle organic particles would be ground to dust before they reach the equator," Yu explains.
Sticky Electric Sand
The research builds on earlier research done by Yu's team showing evidence supporting the electro-static property of Titan's sand.
In another study published in JGR: Planets, they found that the tholins on Titan displayed strong inter-particle forces, allowing them to "stick" to each other more easily than silicate sand.
The grains cling to each other through electricity, in the same way as styrofoam packing peanuts cling to a cat's fur. This could mean that the sand on Titan formed from organic particles by simply "sticking" to each other.