A ‘tropical’ lake has been found near the equator on Saturn’s moon Titan. The observation was made by the Cassini spacecraft, which spotted the apparently hydrocarbon-composed lake among dunes in the equatorial region.
The finding suggests that oases of liquid methane might lie beneath Titan’s surface. This liquid methane could possibly serve the same function for life on Titan as water does for life on Earth. This is all according to a new study just published in the journal Nature.
Other than the Earth, Titan is the only object in the solar system with a ‘water’ cycle involving rain and evaporation. On Titan though, the rain isn’t water, it’s methane.
Finding liquid bodies near Titan’s equator was unexpected for the researchers, as the area is very low humidity and doesn’t receive much rain, being primarily dunes.
“The equatorial belt is like a desert on Earth, where evaporation trumps precipitation,” says astrobiologist Jonathan Lunine of Cornell University in Ithaca, New York.
Anything evaporating there should only rain down when it hits cooler air, which on Titan is near the poles. That’s how the evaporation and rain cycle works with water on Earth, and theoretically should work the same on Titan.
“Lakes at the poles are easy to explain, but lakes in the tropics are not,” says Caitlin Griffith, a planetary scientist at the University of Arizona in Tucson. Cassini has already found hundreds of lakes and three large seas near Titan’s polar regions.
The new lake was found by researchers examining data taken from Cassini’s observations between 2004-2008. The lake measures around 60 kilometres long and 40 kilometres wide, and at least 1 metre deep.
“It appears as a black splotch at seven near-infrared wavelengths that can travel relatively unimpeded through the moon’s thick atmosphere, which blocks visible light.”
The researchers also spotted four smaller splotches that may be “shallower ponds similar to marshes on Earth, with knee-to-ankle-level depths,” according to Griffith.
Because evaporation rates should be so high near the equator, the researchers think that the lake might be replenished by an underground “spring” of liquid methane.
If it is, it will expand the potential points for life to have originated on Titan. Methane is the source of other, more complicated organic molecules that are also found on Titan.
“There may be organic chemical processes that occur in liquid hydrocarbons that could lead to compounds analogous to proteins and information-carrying molecules,” says Lunine, who was not involved in the work. “There might be a kind of life that works in liquid hydrocarbons.”
“Lunine and Griffith are members of a proposed NASA mission to look for such complex chemistry, called the Titan Mare Explorer (TiME). The TiME probe would spend three months bobbing around Ligeia Mare, a sea near Titan’s north polar region, measuring its chemistry with a mass spectrometer.”
Even though the lake finding seems pretty solid, the TiME mission would focus on the seas near the poles because of how much more evidence we already have for those. Speaking about what the lake could possibly be if not liquid methane, Lunine said, “Something else that just happens to be dark at those wavelengths,” such as a solid organic compound, which might mimic a lake.
So far, only 17% of Titan’s equatorial region has been analyzed at a high enough resolution to spot things like this new lake, but the lower resolution maps previously analyzed suggest they are rare.
The idea of deep tropical oases on Titan appeals to Lunine though. “There’s a place on Titan named Xanadu, and if you go back to the Coleridge poem on Xanadu, he talks about ‘caverns measureless to man’,” he’s quoted as saying, adding that he would love to find caverns like that on Titan.
Image Credits: NASA/JPL/University of Arizona