U.S. scientists have found lakes on the largest moon on the second largest planet in our solar system, Saturn. But these are not lakes of water on the moon Titan. Instead, they are beds of methane, a hydrocarbon that apparently rains on Titan as water does on Earth. VOA's David McAlary reports.
Titan is the solar system's second largest moon and is of interest to scientists because they think its exotic atmosphere - rich in methane - is like that of the early Earth before oxygen became abundant.
Because methane dominates the moon's atmosphere, scientists have long wondered whether it has lakes or oceans of methane, but the dense methane haze surrounding it has hidden it from view.
Now, a team of investigators led by Ellen Stofan of Proxemy Research in Virginia and University College, London has an answer. It is based on radar images of Titan's northern hemisphere taken last July by the U.S. space agency's Cassini satellite.
"There were just lakes everywhere," she said. "We detected a whole lot of lakes, in fact over 75 lakes, and some of them quite substantial in size."
Stofan says Titan's lakes range in diameter from three to 70 kilometers.
How can she be sure they are lakes? Part of the answer comes from the way Cassini's radar beams reflected back from them. The researchers report in the journal Nature that the low reflectivity from these patches indicates a smooth surface, suggesting they are made of liquid, rock or ice. But Stofan says additional features favor the notion that they are liquid.
"They look very similar to lakes on Earth," she said. "They have channels feeding into them, just like you have rivers feeding into lakes on the Earth. The shapes of them, their shorelines, all of those geologic aspects of the lakes are actually very familiar."
Methane is a liquid on Titan because the moon is so frigid that what is a gas here on Earth condenses so far away from the Sun.
In a commentary accompanying the study, astrophysicist Christophe Sotin of the University of Nantes in France writes that the findings provide further strong evidence that methane plays the same role on Saturn's moon as water does on Earth.
Ellen Stofan points out that Titan is the only other body in the solar system known to have an active liquid cycle like Earth's. The liquid evaporates from the surface, the vapor eventually condenses, and rainfall replenishes the bodies of water.
"Obviously at some point in the past, Mars had that, but on Titan, it is happening right now, and that is extremely exciting from a scientific point of view because our ability to study climate and our ability to understand how these cycles work - if we only had the Earth to study, we can only have a limited understanding," she said. "So I think in the long run understanding how climates evolve, how bodies evolve, Titan has an awful lot to tell us."
Christophe Sotin at the University of Nantes writes that Earth is the only planetary body known to be more dynamic than Titan. He says similar processes have shaped them.