Ethane is a simple hydrocarbon produced when ultraviolet light from the sun breaks up ethane's parent molecule. The ethane in the lake is mixed with methane, nitrogen, and other hydrocarbons, which are also dominant in the atmosphere.
The lake is about 150 miles (235 kilometers) long—just a bit larger than Lake Ontario.
And it looks weird, even for researchers who knew what to expect.
"It was hard for us to accept the fact that the feature was so black when we first saw it," study leader Brown said. "More than 99.9 percent of the light that reaches the lake never gets out again."
Jonathan Lunine, a University of Arizona planetary scientist, proposed back in 1983 that Titan would be awash in global oceans of ethane and other light hydrocarbons.
That was the only way to account for all the methane and ethane that would have been produced over the 4.5 billion years of solar system history, he suggested.
But 40 close flybys of Titan by the Cassini spacecraft have shown that no such oceans exist.
"The beautiful work described by Brown et al actually strengthens the basic premise that methane is being converted to ethane, which in turn is deposited on Titan's surface," said Lunine, who was not involved in the new study.
"But the lakes do not hold enough ethane to account for what can be produced over the age of the solar system. So we still have a mystery here."
Study co-author Larry Soderblom said indications have pointed to an underground ocean, though that has not been confirmed.
"Everybody can be right at once," he said.
Retired University of Arizona scientist Martin Tomasko dedicated much of his career to the Huygens probe.
He called the new results "quite spectacular," adding that they "provide crucial information about the methane-ethane 'hydrological' cycle on Titan."
Study co-author Larry Soderblom says even more will be revealed in the coming years. Titan tilts like the Earth, and it has seasons like Earth. But a year there equals 16 Earth years, he said.
"Over the next several years, those lakes will move into the sunlight," Soderblom said, adding that the Cassini mission was originally designed to end this year but now has a good chance of an extension.
"We're now looking at the possibility of Cassini operations going well into the next decade," he said. "That would allow us to study these lakes in a time-lapse sort of way."
Soderblom expects the lakes region could become even weirder—revealing seasonal changes in composition, storms, and even seasonal climate cycles that transfer liquids among different regions on Titan—or even between the surface and subsurface.
Already, the VIMS observations suggest Ontario Lacus is evaporating, and ringed with a dark beach.
"There may be a whole new chapter," he said.
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