Sand dunes are common on Earth, Mars, Venus and - unexpectedly - on Saturn's giant moon, Titan. Now detailed analysis of radar observations gathered during the Cassini spacecraft's flybys of cloud-shrouded Titan is enabling scientists to understand the distribution, shape and dimension of its exotic dunes.

Most people are familiar with the piles of loose, granular material which make up sand dunes in coastal areas and deserts on Earth. Derived from the weathering and erosion of rocks over many millennia, these wind-blown, shifting dunes are capable of transporting huge amounts of material.

While Titan's dunes resemble in many ways the features found on Earth, they are made of tiny particles of organic (carbon-rich) material which have fallen to the surface as a never-ending "drizzle" from the dense, orange clouds above. As such, they are the largest known reservoir of organics on Titan, playing a key role in the moon's carbon and methane cycles.

With the exception of seemingly featureless plains, these dune fields are the most widespread landform on Titan. However, although they cover about 13 per cent of the surface, the dunes are confined to the tropical regions, between latitudes 30 degrees north and south.

The majority of Titan's dunes are linear in morphology, but their morphometry (width, length, spacing, thickness of the sand cover in the interdune area) seems to vary with location. Shaped by east-west zonal winds, they are typically 1-2 km wide, 1-4 km apart and perhaps 100 m high.

As on Earth, several conditions must be met in order to develop dunes: a supply of sand-sized sediments, winds strong enough to transport these sediments from their source areas, the absence of sediment removal or a trapping system (such as expanses of liquid, rough surfaces or collection basins) and climatic and topographic conditions favourable to sand deposition.

Furthermore, dune dimensions and shapes reflect the meteorological and geological boundary conditions under which they have formed and evolved.

Now, detailed studies by an international team, led by Alice Le Gall from the Laboratoire Atmospheres, Milieux, Observations Spatiales (LATMOS-UVSQ), Paris, have attempted to explain the regional variations amongst these dunes. (Some of her research for the paper took place when Le Gall held a postdoctoral post at NASA's Jet Propulsion Laboratory in California.)

Radar studies of Titan's dunes
In a recent paper in the journal Icarus, the authors discuss correlations in dune morphometry with altitude and latitude. The regional variations are investigated by analysing the microwave electromagnetic signatures of dune fields obtained during passive and active observations by the radar instrument on Cassini.

The instrument, which operates at a wavelength of 2.2 cm, can be used as a synthetic aperture radar (SAR), a scatterometer, an altimeter or, in a passive mode, as a radiometer. Information can also be inferred from the radar data through direct altimetry and techniques such as SAR-derived topography.

By studying the radar backscatter and emissivity of Titan's dune terrains, up to and including Cassini's Titan 55 flyby (21 May 2009), the team discovered that, with the exception of its polar lakes, dune fields are the most emissive and least reflective features on Titan. However, towards the north they become less emissive and brighter at radar wavelengths.

"We believe that this increase in brightness (and concurrent decrease in emissivity) is caused by a larger proportion of interdune area in the radar footprint," explained Le Gall. "The corridors between the dunes are generally more radar-bright than the dunes themselves. As we go north, the dunes become thinner or/and wider apart."

The data suggest that the quantity of windblown sand tends to decrease towards the north. This could result from a gradual increase in surface moisture with latitude, possibly caused by the asymmetrical seasons associated with Titan's current orbital configuration.

"The eccentric orbit of Titan at the present time results in the southern summer being shorter, but warmer, than the northern summer," said Le Gall. "The less intense northern summers reduce evaporation relative to precipitation, probably resulting in increased soil moisture. This explanation was first advanced to explain why most of the lakes on Titan are at high northern latitudes.

"If the surface contains more moisture, the sand grains are more likely to stick together and the dunes are starved of material."

The team also discovered that the morphometry of the dunes varies with altitude. Using SAR-derived topography, they found that Titan's main dune fields (Shangri-La, Fensal, Belet and Aztlan) tend to occupy the areas of lowest elevation in equatorial regions, occurring at mean elevations between -400 and 0 metres (relative to the geoid, or mean altitude of Titan).

"Titan's dune fields tend to occupy the lowest regions in the equatorial belt and none of them are located in the most elevated areas," noted Le Gall. "On Earth, dune fields commonly form within topographic basins, where air entering the basin expands and decelerates, resulting in the deposition of sediment. It seems that this same aerodynamic condition also controls the location of dune field development on Titan. Seasonal streams may also carry sediment into the basins, which then dry out.

"The main exception is the large basin known as Xanadu, which may receive no sand or be swept clear of sand by wind currents. We also found that dunes do not occur in the lowest terrains on Titan, where the surface may be moist due to interaction with a potential subsurface reservoir of liquid hydrocarbons."

In elevated dune terrains, the data show a trend towards thinner dunes or/and a wider separation of the dunes, and possibly thinner sand cover in the interdune areas. This is consistent with the idea that sediment sources occur in lowlands, whereas the supply of sand is reduced in elevated regions. Increased rates of sediment erosion by the wind, rivers or rain at higher altitudes may also contribute.

How old are the dunes on Titan?
Although Titan's wind speeds are thought to be low - 0.5-2 metres per second - they are probably capable of transporting the light sand particles. However, the massive dunes observed by Cassini may have formed under the other climatic conditions.

Earth experiences dramatic climate changes due to changes in its orbit. Many linear dunes on Earth were formed during the Last Glacial Maximum, about 20,000 years ago.

In much the same way, the orbital cycle on Titan is expected to reverse over a period of 42,000 years, so it is possible that the distribution and morphometry of its dune fields were modified during the latest switch to longer northern summers. Alternatively, they could all be stabilised, representing fossil dunes that evolved in a very different climate.

"Radar images show that dunes cut across most of the other geological features suggesting that they are among the youngest geological features on Titan," said Le Gall.

"For example, when dunes encroach upon an impact crater, we know that they are younger than the crater."

"On the other hand, even though there is an altitudinal / latitudinal control of the dunes, processes which limit the sand supply, transport capacity or sand availability do not appear to vary that much from one location to another, suggesting that the dune fields were all built at the same time."

"Understanding how the dunes form, as well as explaining their shape, size and distribution on Titan, is of great importance in improving our knowledge of its climate and geology," said Nicolas Altobelli, ESA's Cassini-Huygens Project Scientist.

"In particular, as their material is made out of frozen atmospheric hydrocarbons, 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."

A. Le Gall, et al., "Latitudinal and altitudinal controls of Titan's dune field morphometry", 2012, Icarus, 217, 231-242. DOI: 10.1016/j.icarus.2011.10.024. ;A. Le Gall, et al., "Cassini SAR, radiometry, scatterometry and altimetry observations of Titan's dune fields", 2011, Icarus, 213, 608-624. DOI: 10.1016/j.icarus.2011.03.026.