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Titan's lakes

Geoscientist 17.3 March 2007

Emily Baldwin explores the environment of Saturn's moon, where lakes of liquid methane nestle amid mountains and cryovolcanoes…

Titan, Saturn’s largest moon, is the only moon in the Solar System to boast a dense atmosphere, which produces a surface pressure 1.5 times that at the Earth’s surface. It also shares with the Earth a predominantly nitrogen atmosphere, and appears to have numerous lakes located in high northern latitudes according to recent research published in Nature. Of course, Titan’s lakes are not formed of water like their terrestrial counterparts, but of the second most abundant component of the atmosphere - methane. It is the prevalence of methane and aerosols on Titan that has, until the arrival of the Cassini-Huygens mission in the Saturnian system in July 2004, shrouded this enigmatic moon in a seemingly impenetrable cloak.

Examples of the lakes from the T16 radar swath.

Caption: Examples of the lakes from the T16 radar swath. Arrows mark (a) sinuous radar-dark channels leading into two lakes, (b) irregular shaped lakes connected by a channel, (c) a large lake with an apparent topographic edge suggesting the liquid may have once filled to a higher level, and to the right, a small circular depression filled with liquid, and (d) a dry lakebed, with features similar to the filled lakes.

The initial flybys of Titan revealed a terrain of mountains, cryovolcanoes and river beds. The discovery is based on a July 2006 radar swath, which revealed around 75 radar-dark patches ranging from three to 70km in size. “Such dark areas are characteristic of very smooth surfaces, such as the surface of a lake” reports Dr Ellen Stofan, lead author, who works on the Cassini Radar Imaging Team. “The additional evidence that these are lake beds comes in the form of river channels leading to these lakes.”

Although the composition of the liquid cannot be confirmed by radar, methane is the strongest candidate, playing the role that water does on the Earth. The methane could be supplied in a manner similar to Earth's water cycle, evaporating, condensing and falling as methane rain. Alternatively the lakes could be filled from below, by a liquid-methane "water" table filling the natural topographic lows of the surface. These depressions are likely to be impact craters, volcanic calderas or sinkholes characteristic of karst landscapes formed on Earth by the dissolution of carbonate rocks by rainwater.

The lakes have so far only been seen in high northern latitudes, probably because of seasonal variations - expanding during the winter and shrinking in the summer as Saturn orbits the Sun. Some 20 future flybys of Titan are planned, which will result in a radar coverage of just 15% of its surface. An extended mission is currently under discussion. This will be imperative if we are to gain a better understanding of the processes that have dominated not only Titan’s evolution, but that of any Earth-like planet.
  • Ref: Nature 445, 4 January 2007, 61-63