Scientists claim to have spotted a lake of liquid methane and several ponds amid the tropical dunes at the equator of Titan,the biggest moon of Saturn.
The lake in the otherwise dry tropics of Titan’s equator hints that subterranean channels of liquid methane might feed it from below,the scientists said.
Titan has clouds,rain and lakes,like Earth,but these are composed of methane rather than water. However,methane lakes were seen only at Titan’s poles until now.
Now near-infrared pictures of Titan from NASA’s Cassini spacecraft currently orbiting Saturn since 2004 suggest a vast methane lake exists on the surface in the moon’s tropics,one about 2,400 square km large and at least three feet deep.
Titan’s tropical lake is roughly the size of the Great Salt Lake in Utah during its lowest recorded level, study lead author Caitlin Griffith,a planetary scientist at the University of Arizona at Tucson,told Space.com.
Our work also suggests the existence of a handful of smaller and shallower ponds similar to marshes on Earth with knee- to ankle-level depths.
A number of models of methane’s behaviour on Titan show that lakes are not stable at the moon’s tropical latitudes.
Any liquid deposited in the tropical surface evaporates quickly and eventually is transported by Titan’s circulation to the poles,where the large polar lakes appear, Griffith said,noting that the discovery was absolutely not expected.
Lakes at the poles are easy to explain,but lakes in the tropics are not, she added.
Scientists argue the lake’s presence during the moon’s dry season several years before the arrival of seasonal tropical clouds and its long lifetime are evidence against it being a rain puddle,which would evaporate quickly.
Instead,Griffith and her colleagues,who detailed their work in the journal Nature,suggest this tropical lake is fed by subterranean channels,essentially making it an oasis in the desert.
Past hints of subterranean methane breaking through to Titan’s surface had also been suggested by data gathered by the Huygens probe that landed on the moon in 2005.
The landing site,although surrounded by a vast dune field,reveals a landscape carved almost entirely by three different liquid erosion events – rainfall,flooding and seepage, Griffith said.
While rainfall may have carved the downhill drainage features,the flood plain,in which the probe landed,was caused by liquid flowing from a different source of methane. In addition,a puff of methane was detected upon landing,which suggests that the site was damp.
The question of how a subterranean aquifer might form on Titan is still unclear,but Griffith said Titan’s orbit about Saturn might give us a clue. “It is not circular. Some scientists suggest that Titan’s non-circular orbit was once even more elliptical and only recently decayed to its current slightly non-circular orbit.
During this decay,the gravitational pull of Saturn on Titan would have led to powerful tides,releasing heat that could have caused subsurface reservoirs of methane to form and outgas,the researchers added.
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