Cassini Sees the Two Faces of Titan's Dunes

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Jia-Rui Cook 818-354-0850
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Markus Bauer 011-31-71-565-6799
European Space Agency, Noordwijk, the Netherlands
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Feature: 2012-021 Jan. 23, 2012

Cassini Sees the Two Faces of Titan's Dunes

The full version of this story with accompanying images is at:
http://www.jpl.nasa.gov/news/news.cfm?release=2012-021&cid=release_2012-021

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 Cassini-Huygens mission is a cooperative project of NASA, the European Space Agency and
the Italian Space Agency. NASA's Jet Propulsion Laboratory, a division of the California Institute
of Technology in Pasadena, manages the mission for NASA's Science Mission Directorate,
Washington, D.C. The Cassini orbiter was designed, developed and assembled at JPL. The radar
instrument was built by JPL and the Italian Space Agency, working with team members from the
U.S. and several European countries.

For more information about the Cassini mission, visit: http://www.nasa.gov/cassini and
http://saturn.jpl.nasa.gov .

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