Sand dunes discovered on Titan
Recent images of Saturn's moon Titan, taken by Nasa's Cassini spacecraft, appear to show vast regions of sand dunes stretching across Titan's surface. When astronomers first noticed the dark equatorial (close to the equator) regions on Titan, they thought they could be looking at oceans of liquid methane.
Radar images of Titan's surface, showing evidence of sand dunes. © NASA
However, new radar evidence shows they are seas - but seas of sand dunes like those in the Arabian or Namibian Deserts (see image below) that are created by winds blowing in different directions. It is thought that Saturn's powerful gravity causes gentle winds on Titan, which transport sand from across the moon and deposit it around the equator.
Space shuttle image, showing desert sand dunes in Namibia. © NASA
Radar detectors work by bouncing radio waves off the surface of moons and planets, and use the time difference between the radio wave leaving and returning to the detector to make an image of the planet's surface. Because the spacecraft is above the surface, radio waves which arrive back quickly must have bounced off high objects and those which take longer bounce off lower objects. In other words we can generate a picture of mountains and valleys, or in this case .... sand dunes.
The radar images were taken when Cassini flew past Titan in October 2005 and show dunes over 100 metres (330 feet) high that run parallel to each other for hundreds of miles around Titan's equator. One dune field runs for more than 1500 km (930 miles). At present, it is not known how the sand formed or whether it is made from rock, methane ice or water ice.
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