Titan

Dark terrain

In the first images of Titan's surface taken by Earth-based telescopes in the early 2000s, large regions of dark terrain were revealed straddling Titan's equator.[45] Prior to the arrival of Cassini, these regions were thought to be seas of organic matter like tar or seas of liquid hydrocarbons.[46] However, radar images captured by the Cassini spacecraft has instead revealed some of these regions to be extensive plains covered in longitudinal sand dunes. The sand dunes are believed to be formed by wind generated as a result of tidal forces from Saturn on Titan's atmosphere, which are 400 times stronger than the tidal forces of the Moon on Earth. The tidal winds cause dunes to build up in long parallel lines, with Titan's zonal winds aligning the dunes west-to-east. The dunes break this pattern around mountains, where the wind direction is shifted.

Widespread evidence has also recently been found to support the claim that lakes of hydrocarbons do in fact exist on Titan's North pole during the second of several planned Cassini spacecraft flybys.[47]

The sand on Titan might have formed when liquid methane rained and eroded the ice bedrock, possibly in the form of flash floods. Alternatively, the sand could also have come from organic solids produced by photochemical reactions in Titan's atmosphere.