The Dunes of Titan

NASA's Cassini spacecraft has unexpectedly found an extensive system of dunes girding the equator of Saturn's moon Titan. Titan is thus the fourth planetary body known to have dunes, after Earth, Mars, and Venus. The dunes are longitudinal in nature, implying a surface wind regime that varies, possibly on seasonal timescales. Fully 20% of the moon's surface area is covered by dunefields or sand seas, corresponding to a total areal coverage similar to that of dunes on the Earth. The dunes are confined to a belt within 30 degrees of the moon's equator -- the mechanism that drives this latitudinal distribution is not yet understood but may be climatic in nature. The dunes' size (50-150 meters in height) and spacing (1-3 km) proves to be very similar to the dimensions of linear dunes in terrestrial sand seas, in particular the Namib. Despite Titan's crust being composed primarily of water ice, the near-infrared spectrum of the dunes is not consistent with that of icy sand grains. Instead, the dunes seem to be composed of organic grains. The proximal source of sand is uncertain. However, the ultimate source of organic compounds is presumably atmospheric haze that is produced by methane photo- and radio-chemistry in the upper atmosphere. I will discuss both what we know about the dunes so far and what remains to be studied on this new planetary geology topic.