Cassini probe scans and explores Titan’s methane lakes

With data collected since 2004, the NASA/ESA Cassini-Huygens mission has shown that Saturn’s largest moon, Titan, has three large methane seas and many smaller lakes that together cover around 2 percent of its surface.

Titan is the only one of our solar system’s hundreds of moons that has a dense atmosphere and significant bodies of stable liquid on its surface. Like Earth, it has a nitrogen-dominated atmosphere, however it has very little oxygen and the remainder of its atmosphere is mostly methane, as well as trace amounts of other gases that including ethane.

Because Saturn is so far from the sun, the cold temperatures mean that methane and ethane can exist on Titan’s surface in liquid form. Cassini has revealed more than 620,000 square miles (1.6 million square kilometers) of surface liquid, and the second largest of the three seas, Ligeia Mare, is said to be similar in size to Lake Huron and Lake Michigan combined. All of the three are close to the moon’s north pole, with just one large lake located in the southern hemisphere.

A new study published in the Journal of Geophysical Research: Planets has confirmed that Ligeia Mare is methane rich. Although Cassini has explored Saturn’s system for over a decade and reservoirs on Titan’s surface have been theorized long before that, the composition of the liquid was not known until 2014.

“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 study, which is based on data collected with Cassini’s radar instrument during flybys of Titan between 2007 and 2015.

Possible explanations for the sea’s methane composition are still numerous, Le Gall explained.

“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,” she said.

The research used radar observations of heat given off by Ligeia Mare, together with data from a 2013 experiment that bounced radio signals off the sea. In 2013, its depth was ascertained by detecting echoes from the sea floor – the first ever detection of the bottom of an extraterrestrial sea. It was found to be as great as 525 feet (160 meters). Le Gall and her colleagues used the depth-sounding information to separate the contributions made to the sea’s observed temperature by the liquid sea and the seabed.

A layer of sludge 

A wide variety of organic materials are produced in Titan’s atmosphere as a result of the reaction of nitrogen and methane. It’s believed that once these materials reach the sea they are dissolved in the liquid methane, and that the insoluble compounds, such as nitriles and benzene, sink to the sea floor. This leaves what Le Gall calls “a sludge layer of organic-rich compounds.

“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.”

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Image credit: NASA

Source: Phys.org

http://phys.org/news/2016-04-cassini-explores-methane-sea-titan.html#jCp

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