Where Does Titan Sand Come From: Insight From Mechanical Properties of Titan Sand Candidates

Extensive equatorial linear dunes exist on Titan, but the origin of the sand, which appears to be organic, is unknown. We used nanoindentation to study the mechanical properties of a few Titan sand candidates, several natural sands on Earth, and common materials used in the Titan Wind Tunnel, to understand the mobility of Titan sand. We measured the elastic modulus (E), hardness (H), and fracture toughness (Kc) of these materials. Tholin's elastic modulus (10.4 ± 0.5 GPa) and hardness (0.53 ± 0.03 GPa) are both an order of magnitude smaller than silicate sand, and it is also smaller than the mechanically weak white gypsum sand. With a magnitude smaller fracture toughness (Kc = 0.036 ± 0.007 MPa·m1/2), tholin is also much more brittle than silicate sand. This indicates that Titan sand should be derived close to the equatorial regions where the current dunes are located, because tholin is too soft and brittle to be transported for long distances. Plain Language Summary Sand dunes, which are probably made of organic materials, are observed on Titan in the equatorial region, but the origin of the organic sand is a mystery. We measured mechanical properties of several Titan sand analogs, so that we can estimate their ability to transport on Titan's surface and help us constrain the source region of Titan sand. We found out that most of the possible candidates of Titan sand, including tholin (Titan aerosol analog), water ice, and some simple organics, are all less stiff, softer, and more brittle than the silicate sand being transported on Earth's surface. This suggests that sand on Titan may be too weak mechanically to transport long distances on Titan. Thus, it is unlikely for Titan sand to originate from the polar regions of Titan, where the methane lakes and seas are located and have been suggested as one possible formation location. Key Points Tholin has a high elastic modulus and hardness but low brittleness compared to common polymers due to its complex cross‐linked structure With a magnitude lower modulus, hardness, and fracture toughness than silicate sand, tholin may be hard to transport over long distances on Titan Under Titan conditions, water ice and simple organics are also mechanically weak and thus may be even more difficult to transport on Titan