Cassini finds liquid ethane on Titan

Saturn’s largest moon has a hydrocarbon lake

Anyone diving into the extraterrestrial lake known as Ontario Lacus would find an oil baron’s dream. The chilly reservoir, located on the south pole of Saturn’s largest moon, Titan, is composed of a key component of crude oil — liquid ethane.

ETHANE ONTARIO The dark feature in this composite infrared view of Titan’s south pole depicts a lakelike feature called Ontario Lacus. The region has now been found to contain liquid ethane. JPL/NASA, Space Science Institute

After years of speculation, scientists have now confirmed that Titan, shrouded in hydrocarbons, has at least one ethane lake, Robert Brown of the University of Arizona in Tucson and his colleagues report in the July 31 Nature.

Researchers identified liquid ethane by analyzing infrared data gathered by a spectrometer on the Cassini spacecraft. Ontario Lacus resembles a lake slightly larger than LakeOntario, and models have long suggested that hydrocarbon aerosols in Titan’s thick atmosphere rain down on the moon’s surface. But no one knew if Ontario Lacus or any other lakelike feature on Titan actually contained liquid.

Two lines of evidence support the ethane detection, Brown notes. The spectrometer showed a dip at the exact infrared wavelengths at which liquid ethane absorbs light. In addition, Ontario Lacus reflects virtually no light at an infrared wavelength of 5 micrometers. For the lake to be that dark, its surface has to have a smooth, liquid surface.

Methane is the predominant hydrocarbon in Titan’s atmosphere and ethane is produced when methane reacts with sunlight. It’s likely that the lake contains liquid methane and other light hydrocarbons, along with dissolved nitrogen, Brown’s team suggests. Other lakelike features on Titan probably have a similar composition, Brown says.

Despite the lake’s low temperature, about –180° Celsius, high-energy cosmic rays bombarding the region may produce other organic compounds, creating a brew that may produce some of the chemicals necessary for life, notes François Raulin of the University of Paris in Créteil.

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