|Saturn's Moon Titan a World of Rivers and Lakes
By Ker Than
posted: 03 January 2007
01:01 pm ET
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Images shot last summer by NASA's Cassini spacecraft provide the strongest evidence yet that Titan, a saturnian moon and one of the most Earth-like celestial bodies in the solar system, is dotted with a multitude of liquid lakes.
"At the time we first announced it, we were like, 'Well, we think these are probably lakes,’ but that was about our level of confidence," said study team member Ellen Stofan of University College London and Caltech. "I would say at this point, we've analyzed the data to the extent that we feel very confident that they are liquid-filled lakes."
"It's going to behave like water," Stofan said about liquid methane. "It's transparent just the way water is. So if you were standing by the shoreline, you would be able to see down to whatever pebbles or gunk that was on the bottom."
Titan is the only moon in the solar system to have a dense atmosphere with thin layers of methane and nitrogen clouds—a setup similar to that of early Earth. Atmospheric methane is destroyed by sunlight over time and must constantly be renewed. Scientists thus speculated that lakes or even oceans of methane might exist on, or just beneath, the moon's icy surface and that evaporation from these liquid bodies was replenishing the atmosphere. The first confirmation of this thinking came last July when Cassini's radar spotted more than 75 large, dark patches around the surface of the moon's northern pole.
"The lakes are basically black in the [radar] data, which is how a liquid would behave," Stofan said. Radar data alone wasn't enough, however. A very smooth deposit of fine soil would also appear black on radar, Stofan explained.
The clincher that the patches were liquid lakes came from looking at the surrounding terrain. Some of the patches appeared to be fed by sinuous channels, or "rivers," some more than 62 miles (100 kilometers) long. Others appeared to be contained within rimmed circular depressions, similar to crater lakes or volcanic calderas on Earth.