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Cassini confirms methane sea on Titan

Sunlight glints off Titan's northern seas

Sunlight glints off Titan’s northern seas this near-infrared, color mosaic from NASA’s Cassini spacecraft. Image Credit: NASA/JPL/Univ. Arizona/Univ. Idaho

Saturn’s moon Titan is the only moon in the Solar System that has a dense atmosphere and large liquid reserves on its surface. A new study using data gathered by NASA’s Cassini mission finds that Ligeia Mare, the second largest of Titan’s seas, is composed mostly of pure liquid methane. The findings provide independent confirmation of an earlier study. 

The Cassini spacecraft, which arrived at the Saturnian system in 2004, has discovered that over 620,000 square miles (1.6 million square kilometers) of Titan’s surface are covered in liquid. There are three large seas near Titan’s north pole which are surrounded by several smaller lakes in the northern hemisphere. Only one large lake has been discovered in the southern hemisphere.

Ligeia Mare on Titan

Ligeia Mare is the second largest body of liquid on Saturn’s moon Titan. (Click to enlarge) Image Credit: NASA/JPL-Caltech/ASI/Cornell

The composition of these liquid reservoirs was unknown until 2014 when Cassini’s radar instrument was first used to show that Ligeia Mare, similar in size to Lake Huron and Lake Michigan combined, is rich in methane. The new study, published in the Journal of Geophysical Research: Planets, confirmed the result by using the radar instrument in a different mode.

“Before Cassini, we expected to find that Ligeia Mare would be mostly made up of ethane, which is produced in abundance in the atmosphere when sunlight breaks methane molecules apart. Instead, this sea is predominantly made of pure methane,” said Alice Le Gall, a Cassini radar team associate at the French research laboratory LATMOS, Paris, and lead author of the new study.

There are a number of possible explanations that could account for the sea’s methane composition.

“Either Ligeia Mare is replenished by fresh methane rainfall, or something is removing ethane from it. It is possible that the ethane ends up in the undersea crust, or that it somehow flows into the adjacent sea, Kraken Mare, but that will require further investigation,” said Le Gall.

The new study is based on several radar observations of heat given off by Ligeia Mare. Researchers also used data from a 2013 experiment that bounced radio signals off Ligeia.

During the 2013 experiment, the radar instrument detected echoes from the seafloor resulting in the first-ever detection of the bottom of an extraterrestrial sea. Researchers were surprised to find depths in the sea as great as 525 feet (160 meters) at the deepest point along the radar track.

Le Gall and her fellow researchers used the depth-sounding data to separate the contributions made to the sea’s observed temperatures by the liquid sea and the seabed, which provided insights about their respective compositions.

“We found that the seabed of Ligeia Mare is likely covered by a sludge layer of organic-rich compounds,” adds Le Gall.

The study also found that Ligeia Mare’s shoreline may be porous and flooded with liquid hydrocarbons. The data span a period from local winter to spring, and the researchers expected that the surrounding solid terrains would warm more quickly than the sea.

Cassini’s observations did not show any significant difference between the temperature of the sea and the shore during this period. This lack of temperature variation suggests that the shoreline surrounding the lakes and seas may be wet with liquid hydrocarbons, which would cause them to warm up and cool much like the sea itself.

“It’s a marvelous feat of exploration that we’re doing extraterrestrial oceanography on an alien moon,” said Steve Wall, deputy lead of the Cassini radar team at NASA’s Jet Propulsion Laboratory in Pasadena, California. “Titan just won’t stop surprising us.”

Video courtesy of NASA


Jim Sharkey is a lab assistant, writer and general science enthusiast who grew up in Enid, Oklahoma, the hometown of Skylab and Shuttle astronaut Owen K. Garriott. As a young Star Trek fan he participated in the letter-writing campaign which resulted in the space shuttle prototype being named Enterprise. While his academic studies have ranged from psychology and archaeology to biology, he has never lost his passion for space exploration. Jim began blogging about science, science fiction and futurism in 2004. Jim resides in the San Francisco Bay area and has attended NASA Socials for the Mars Science Laboratory Curiosity rover landing and the NASA LADEE lunar orbiter launch.

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