JPL: Cassini Sees the Two Faces of Titan's Dunes

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JPL: Cassini Sees the Two Faces of Titan's Dunes

Post by bystander » Mon Jan 23, 2012 9:41 pm

Dune Patterns
NASA JPL-Caltech | Cassini Solstice Mission | 2012 Jan 23

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 altitude effect suggests that the "sand" (likely composed of hydrocarbons) needed to build the dunes is mostly in the lowlands of Titan. Saturn's elliptical orbit may explain why dunes tend to be thinner, more widely separated and less sand-covered in the areas in between dunes as one moves northward. Summers in the southern hemisphere are shorter and warmer than in the northern hemisphere, possibly leaving the soil in the south less moist because northern areas experience more evaporation and condensation. When soil is moist, it is more difficult to move sand particles because they are sticky and heavier. As a result, it is more difficult to build dunes..

The images of Belet and Fensal were obtained by Cassini's radar instrument on Oct. 28, 2005, and April 10, 2007. The images have been processed to show the same spatial scale and stretch. In these images, Titan's dunes are the dark streaks that are 0.6 to 1.2 miles (1 to 2 kilometers) wide and the areas between dunes (bright streaks) are 0.6 miles to 2.5 miles (1 to 4 kilometers) wide. Fensal appears much brighter in these radar images than Belet because there is a thinner sand cover in the areas between the dunes. These interdune areas are also wider than Belet's.

The image of the Oman dunes, also known as dunes in the Rub' al Khali or Empty Quarter, was obtained by the Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER), an instrument aboard NASA's Terra satellite. For more information, see ASTER Dunes.

The image of the Kalahari dunes, in the Namib Desert, was also obtained by ASTER. For more information on this image, visit Eastern Namibia Sand Dunes.

Credit: NASA/JPL-Caltech, and NASA/GSFC/METI/ERSDAC/JAROS and U.S./Japan ASTER Science Team

Cassini Sees the Two Faces of Titan's Dunes
NASA JPL-Caltech | Cassini Solstice Mission | 2012 Jan 23
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.

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.

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."

The two faces of Titan's dunes
ESA Space Science | 2012 Jan 23
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