Imagine a world where the average daytime temperature is -179°C, and torrential rains of liquid methane fall from the skies, forming vast but shallow pools that cover an area larger than the Great Lakes. Although the scene may smack of science fiction, it's actually Saturn's biggest moon, Titan, according to the latest results from the Cassini spacecraft. The work suggests that frigid Titan actually resembles Earth in some key ways, such as seasonal weather, and it also hints at the possibility that the moon holds vast underground reservoirs of liquid hydrocarbons--a first for any other body in the solar system.
Despite its 1.2-billion-kilometer distance from the sun, Titan is one of the most active bodies in the solar system. Its atmosphere is thicker than Earth's and is home to clouds and wind. What's more, scientists have been seeing evidence of seasonal weather--in the form of dark zones that may be lakes filling with methane rain--ever since Cassini arrived in 2004.
Now it turns out that those lakes are real. New images sent back from Cassini show the dark spots expanding over time, confirming that they are indeed bodies of methane. And visible, overhanging clouds seem to be filling them, a team led by Elizabeth Turtle of the Johns Hopkins University Applied Physics Laboratory in Laurel, Maryland, reports this week in Geophysical Research Letters.
The findings have generated a new mystery, however. "UV radiation is continually destroying the methane," says planetary scientist and co-author Anthony Del Genio of NASA's Goddard Institute for Space Studies in New York City, so all of the methane should have disappeared long ago, even accounting for rainfall. Then where is the methane coming from? According to the researchers, the source most likely lies beneath the surface of the moon. There may be volcanoes releasing plumes of methane, instead of lava, from the interior. That methane, a leftover from the primordial gas cloud that formed Titan, could be plentiful enough to sustain the rainy weather.
It's "beautiful work," says planetary scientist Jonathan Lunine of the University of Arizona, Tucson. The next step, he says, will be to extend the Cassini mission until 2016 or 2017, to "allow us to observe the lakes through the complete northern springtime." By then, scientists can observe the lakes filling up in closer to real time, something that could provide more clues to the methane's source.