Titan's Giant Dunes

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Data from NASA’s Cassini spacecraft show that the sizes and patterns of dunes on Saturn’s moon Titan vary as a function of altitude and latitude. The dunes in areas that are more elevated or are higher in latitude, such as in the Fensal region pictured at bottom left, tend to be thinner and more widely separated, with gaps that have a thinner covering of sand. Dunes in the Belet region, pictured at top left, are at a lower altitude and latitude. The dunes in Belet are wider, with thicker blankets of sand between them. The Kalahari dunes in South Africa and Namibia, located in a region with limited sediment available and pictured at bottom right, show effects similar to the Fensal dunes. The Belet dunes on Titan resemble Earth’s Oman dunes in Yemen and Saudi Arabia, where there is abundant sediment available. The Oman dunes are shown at top right. The images of Belet and Fensal were obtained by Cassini’s radar instrument on Oct. 28, 2005, and April 10, 2007. The images of Belet and Fensal were obtained by Cassini’s radar instrument on Oct. 28, 2005, and April 10, 2007. The image of the Kalahari dunes, in the Namib Desert, was also obtained by ASTER. Credit: NASA

A new analysis of radar data from NASA’s Cassini mission, in partnership with the European Space Agency and the Italian Space Agency, has revealed regional variations among sand dunes on Saturn’s moon Titan. The result gives new clues about the moon’s climatic and geological history.

Dune fields are the second most dominant landform on Titan, after the seemingly uniform plains, so they offer a large-scale insight into the moon’s peculiar environment. The dunes cover about 13 percent of the surface, stretching over an area of 4 million square miles (10 million square kilometers). For Earthly comparison, that’s about the surface area of the United States.

Though similar in shape to the linear dunes found on Earth in Namibia or the Arabian Peninsula, Titan’s dunes are gigantic by our standards. They are on average 0.6 to 1.2 miles (1 to 2 kilometers) wide, hundreds of miles (kilometers) long and around 300 feet (100 meters) high. However, their size and spacing vary across the surface, betraying the environment in which they have formed and evolved.

Using radar data from the Cassini spacecraft, Alice Le Gall, a former postdoctoral fellow at NASA’s Jet Propulsion Laboratory, Pasadena, Calif., who is currently at the French research laboratory LATMOS, Paris, and collaborators have discovered that the size of Titan’s dunes is controlled by at least two factors: altitude and latitude.

In terms of altitude, the more elevated dunes tend to be thinner and more widely separated. The gaps between the dunes seem to appear to Cassini’s radar, indicating a thinner covering of sand. This suggests that the sand needed to build the dunes is mostly found in the lowlands of Titan.

Image of Namibian sand dunes on Earth to compare with Cassini radar images of sand dunes on Saturn’s giant moon. Credit: NASA/JSC

Scientists think the sand on Titan is not made of silicates as on Earth, but of solid hydrocarbons, precipitated out of the atmosphere. These have then aggregated into grains 0.04 inch in size by a still unknown process.

In terms of latitude, the sand dunes on Titan are confined to its equatorial region, in a band between 30 degrees south latitude and 30 degrees north latitude. However, the dunes tend to be less voluminous toward the north. Le Gall and colleagues think that this may be due to Saturn’s elliptical orbit.

Titan is in orbit around Saturn, and so the moon’s seasons are controlled by Saturn’s path around the sun. Because Saturn takes about 30 years to complete an orbit, each season on Titan lasts for about seven years. The slightly elliptical nature of Saturn’s orbit means that the southern hemisphere of Titan has shorter but more intense summers. So the southern regions are probably drier, which implies they have less ground moisture. The drier the sand grains, the more easily they can be transported by the winds to make dunes.

"As one goes to the north, we believe the soil moisture probably increases, making the sand particles less mobile and, as a consequence, the development of dunes more difficult." says Le Gall.

Cassini is one of the most scientifically capable spacecraft ever launched. Credit: NASA

Backing this hypothesis is the fact that Titan’s lakes and seas are not distributed symmetrically by latitude. These reserves of liquid ethane and methane are predominantly found in the northern hemisphere, suggesting again that the soil is moister toward the north and so, again, the sand grains are less easy to transport by the wind.

"Understanding how the dunes form as well as explaining their shape, size and distribution on Titan’s surface is of great importance to understanding Titan’s climate and geology because the dunes are a significant atmosphere-surface exchange interface", says Nicolas Altobelli, ESA’s Cassini-Huygens project scientist. "In particular, as their material is made out of frozen atmospheric hydrocarbon, the dunes might provide us with important clues on the still puzzling methane/ethane cycle on Titan, comparable in many aspects with the water cycle on Earth."

Titan is the only moon in our solar system with a thick atmosphere, and the only world other than Earth that is known to have liquid at its surface. Astrobiologists are interested in Titan for numerous reasons. Some scientists have hypothesized about the potential for life on Titan – life that is vastly different than organisms found on Earth. Comparing environmental process on Titan can also help us understand the modern and ancient climate of planet Earth.