A giant, glassy lake larger than North America's Lake Ontario graces the south pole of Saturn's largest moon Titan, new research confirms.
"This is the first observation that really pins down that Titan has a surface lake filled with liquid," said lead researcher Robert Brown of the University of Arizona's Lunar and Planetary Laboratory in Tucson.
Called Ontario Lacus, the lake extends 150 miles (235 kilometers) and covers an area of about 7,800 square miles (20,000 square kilometers). The lake structure is filled mostly with methane and ethane, hydrocarbons that are gases on Earth but liquid on the bone-chilling surface of Titan.
The finding, detailed in the July 31 issue of the journal Nature, is just another gold star for Titan, which has become one of the most fascinating bodies in the solar system for exploring environments that may give rise to life. Boasting a size that's one-and-a-half times that of Earth's moon and bigger than Mercury, Titan is the only solar system moon known to support a planet-like atmosphere.
The idea of liquid features on Titan is not new. Before the Cassini mission to Saturn, astronomers thought Titan was covered in a global ocean, though the spacecraft's flybys of the moon sucked the life out of that wet notion. Even still, evidence for features similar to Earth's lakes and seas, along with telltale signs of erosion from flowing water, have since come to light.
Possible evidence for lake-like features came from radar images, but this method can't distinguish between liquid and very fine gravel or other tiny solids, Brown explained.
"Detection of liquid ethane in Ontario Lacus confirms a long-held idea that lakes and seas filled with methane and ethane exist on Titan," said researcher Larry Soderblom of the U.S. Geological Survey in Flagstaff, Ariz.
Soderblom, Brown and their colleagues used the infrared abilities of Cassini's visual and infrared mapping spectrometer, or VIMS, to peer through Titan's cotton-ball-like veil of hydrocarbons that extends more than 620 miles (1,000 km) above the moon's surface.
The measurements showed three features of the lake structure: the shoreline, or the outer edge of the lake; the so-called beach just inside the shoreline, which could be a sort of "bathtub ring" of material left behind as the ethane mixture evaporated; and the lake's interior, which appears dark, the researchers say.
"It seems to have partially evaporated, and that makes sense because the south pole has just gone through summer," Brown told SPACE.com. "The maximum evaporation will have already occurred or is in the process of occurring."
He added, "There's still a lot of liquid left in that lake, and we don't think it's going to evaporate much further."
Plus, the lake feature could be replenished by Titan's dreary drizzle of methane, which when hit by sunlight breaks down into ethane.
As for how the liquid lake feature would feel if one were to step in, "Liquid ethane-methane mix would be less viscous than water," Brown said. "If riffling your fingers through it you wouldn't feel as much resistance."
The Cassini mission is an international cooperative effort of NASA, the European Space Agency (ESA), Italian Space Agency (ASI) and several separate European academic and industrial contributors.