Saturn Moon's 'Lake Ontario': Shallow and Virtually Wave-free
This image, obtained using Cassini's Visual and Infrared Mapping Spectrometer (VIMS), shows the first observed flash of sunlight reflected off a lake on Saturn's moon Titan.
Credit: NASA/JPL/University of Arizona/DLR.

SAN FRANCISCO ? A giant hydrocarbon lake on Saturn's moon Titan is mirror-flat and surprisingly shallow, with average depths comparable to that of a backyard swimming pool, according to a new study.

Ontario Lacus, the largest lake in Titan's southern hemisphere, covers about 6,000 square miles (15,000 square kilometers). While big, it's not exactly fierce; Ontario's waves are less than a dime's-width high, and the lake can't be any deeper than 24 feet (7.4 meters) in any one spot, researchers found.

"The volume of hydrocarbons is actually quite small," said lead author Lauren Wye of Stanford University, who presented her team's results here Wednesday (Dec. 15) at the fall meeting of the American Geophysical Union. "The shallowness was a surprise."

The lakes of Titan

Titan, Saturn's largest moon, is a frigid place, with average surface temperatures around minus 290 degrees Fahrenheit (minus 179 degrees Celsius). It has a thick, nitrogen-rich atmosphere and a weather cycle based on methane. Scientists recently discovered that lakes filled with liquid hydrocarbons ? stuff like methane, ethane and propane ? dot its surface.

Ontario Lacus is the biggest of these lakes south of Titan's equator. It spans a huge area ? nearly as much as its Earth namesake, North America's Lake Ontario.

Wye and her team used radar measurements made by NASA's Cassini spacecraft during two Titan flybys ? one in July 2009 and the other in January 2010 ? to figure out how high Ontario's waves get, and how deep the lake goes.

Researchers had previously mapped some of Ontario's depths close to its shoreline. But the new results represent some of the first solid depth measurements of a Titan lake across its entire length and breadth, Wye said.

In a confirmation of previous findings, the researchers determined that Ontario's surface is flat as flat can be, with maximum wave heights of less than 1 millimeter (0.04 inches). No liquid body on Earth's surface can come close to matching this degree of smoothness, researchers have said.

These measurements suggest that Titan's winds may have been virtually nonexistent at the time measurements were made, Wye said. It's also possible that the hydrocarbons filling Ontario Lacus are extraordinarily viscous.

"We have no idea what these material properties are," she told

The researchers also found that Ontario is surprisingly shallow for such an enormous body. Its average depth is somewhere between 1.3 and 10.5 feet (0.4 ? 3.2 meters), and the maximum depth is in the neighborhood of 9.6 to 24.4 feet (2.9 ? 7.4 meters). (The team derived lower and upper bounds for these values from Cassini's measurements.)

These numbers put Ontario Lacus' volume somewhere between 1.7 and 12 cubic miles (7 to 50 cubic km), researchers said. Earth's Lake Ontario, by contrast, holds about 393 cubic miles (1,640 cubic km) of water.

Northern lakes different

The team also looked at a few lakes in Titan's northern hemisphere, which has considerably more lakes than the south. One of them, the huge Ligeia Mare, was deeper ? though the researchers couldn't put a hard number on how deep it goes, since Cassini's radar signal lessens in strength beyond depths of 26 feet (8 meters) or so.

"All we can say is it's likely deeper than eight meters in much of the center of the lake," Wye said.

The depth differences likely reflect a difference in formation mechanisms. Ontario Lacus appears to be a sedimentary basin, according to Wye.

"It may be a relatively flat basin, like Racetrack Playa in Death Valley," California, she said.

Ligeia Mare, on the other hand, probably arose from some different process. The new study, and others like it, could help researchers tease out just what these various formation mechanisms are, according to Wye.

"Depth maps like these can be used for that purpose," she said.