Methane lakes and ice pebbles
28 July 2006
Saturn’s moon Titan is one of the largest and most fascinating satellites in the Solar System. Unfortunately, its surface is hidden beneath a thick blanket of orange cloud. Until recently, no one knew what Titan’s surface is like.
One way to pierce the cloud layer is to use the radar instrument on NASA’s Cassini spacecraft. Radio signals from Cassini go through the cloud and bounce back from the surface. By studying the echoes, scientists have recently been able to discover many dark patches. Some of the patches are linked to narrow, winding channels – possibly rivers.
It seems likely that the smooth, dark areas in the radar images are lakes. However, Titan’s surface temperature is -180C, so they cannot contain water. Instead, they seem to be made of liquid methane or ethane. (On Earth, methane gas is burnt in boilers and cookers.)
Many people hoped that ESA’s Huygens probe would splash down in a methane lake during its descent to the surface on 14 January 2005. However, its camera showed a dry plain covered in ice pebbles.
Since this spectacular success, scientists have found an unusual way to learn more about Huygens’ surroundings. The key was an unusual up and down pattern in the radio signal from Huygens to Cassini. Some of the radio beam was reflecting from the surface, not going straight to Cassini. Analysis of the signal showed that the surface is fairly flat up to 2 km from the probe. This dry lake bed is covered mostly in ice pebbles about 5-10 cm across.
