What are Titan's dunes made of?

Solid hydrocarbons. Titan has vast dune fields around its equator - dark, parallel ridges hundreds of kilometres long, covering an area comparable to the United States - and the sand is organic grains that formed in the atmosphere and settled out, not silica.

Titan's atmosphere is nitrogen with methane, and sunlight breaking those molecules apart drives a chemistry that builds up heavier compounds, which fall as a continuous organic drizzle. Over geological time that has built a sand supply. At minus 180 degrees those grains are hard, and water ice is bedrock.

The dunes also point the wrong way. Their shape indicates winds blowing east, while models of Titan's circulation predicted westerlies at the surface - which was unresolved until researchers proposed that the rare, violent storms of the equinox do the work rather than the prevailing wind. It is a reminder that a landscape records the extremes rather than the average, which is true of deserts on Earth too.