Titan: mile-wide dunes on Earth's frozen twin intrigue scientists

Some dunes on Titan – a Saturn moon seen as a chillier version of early Earth – are huge. Scientists now have suspicions about what's going on, but they don't yet know how Titan's version of sand formed. 

The discovery of intriguing differences among vast fields of sand dunes on Saturn's moon Titan is opening a window on the haze-shrouded satellite's geology and climate, researchers say.

Radar images from NASA's Cassini orbiter reveal that the size and spacing of the dunes change depending on the latitude of the dune fields and the elevation of the land on which they sit. The findings may help uncover the distribution of winds on the moon and yield clues to help resolve a long-standing debate over how and where the sand itself formed, according to the team reporting the results in the January issue of the journal Icarus.

"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," notes Nicolas Altobelli, Cassini project scientist with the European Space Agency, one of NASA's partners in the mission, in a statement.

Titan, Saturn's largest moon and the second largest in the solar system, has been a prime target for Cassini's instruments since the craft began touring Saturn and its moons in June 2004.

Formed mainly of water ice and rock, Titan hosts lakes of liquid hydrocarbons. When it rains on Titan, it drizzles liquid methane, rather than water. 

With its vast array of different hydrocarbons, astrobiologists have long viewed Titan as a chillier version of Earth prior to the emergence of life. Some even suggest that the moon may hide potential habitats for life below its surface today.

Titan's dunes cover an area about the size of the continental United States, or roughly 13 percent of the moon's surface. The grains of sand making up Titan's dunes are thought to be clumps of solid hydrocarbons, rather than tiny grains of silicates, as on Earth.

From orbit, the dunes – which appear within Titan's tropics – display the same shapes and patterns seen in large dune fields on Earth. But Titan's dunes are larger, with dunes up to a mile wide, 300 feet tall, and hundreds of miles long.

Using data gathered by Cassini's radar, a team of US and French scientists determined that dune size appears be linked to the altitude of the surface the dune covers.

The largest dune fields appear in areas of lowest elevation near Titan's equator and tend to get smaller, less tightly packed, and show less sand between dunes at higher elevations. Dunes follow a similar trend the farther north they appear from the equator.

The team suggests that changes with altitude may mean that at higher elevations, wetter weather is eroding the sand faster than it is replaced.

Thinned-out dune fields in the northern tropics may be influenced by Titan's hydrocarbon lakes. Increased moisture around the lakes may dampen sand in source areas, keeping it from blowing into regions where the dunes form. In addition, winds may be weaker at high latitudes.

Periodic changes in wind patterns around an Australia-size plateau dubbed Xanadu also may affect the accumulation of Titan's tropical dunes, the team posits.

And where does the sand come from? The subject is a topic of intense debate, says team leader Alice Le Gall of the Laboratory for Atmosphere, Environments, and Spatial Observations in Paris, in an e-mail. 

Some measurements "suggest that the dune material is mainly made up of solid organics, which points to an atmospheric origin," she writes. Essentially, hydrocarbon aerosols form from reactions between gases in the atmosphere energized by sunlight. The grains then fall to Titan's surface. Such reactions have appeared in laboratory experiments on Earth aimed at simulating conditions in Titan's atmosphere.

But these aerosols are tiny, less that 1/100th the width of a human hair. How they gather into grain-size clumps remains a mystery.

Scientists have posited several potential answers to the "clump" mystery. They include:

• The sand could be formed by erosion as liquid methane flows along the surface, much like the action of occasional floods in usually dry river beds in deserts on Earth.
• The larger grains may form through chemical reactions as Titan's northern lakes periodically dry out – although its unclear how enough material could find its way south to build the large dunes seen in the regions along Titan's equator.
• Perhaps prolonged long wet periods provided the moisture needed erode Titan's surface, forming the sand.
• Or perhaps the erosive moisture welled up from beneath the surface in key locations.