Sunlight glints off of Titan’s northern seas this near-infrared, color mosaic from NASA’s Cassini spacecraft. Credit: NASA/JPL/Univ. Arizona/Univ. Idaho
A new study finds that a large sea on Saturn’s moon Titan is composed mostly of pure liquid methane, independently confirming an earlier result. The seabed may be covered in a sludge of carbon- and nitrogen-rich material, and its shores may be surrounded by wetlands. Of the hundreds of moons in our solar system, Titan is the only one with a dense atmosphere and large liquid reservoirs on its surface, making it in some ways more like a terrestrial planet.
Both Earth and Titan have nitrogen-dominated atmospheres — over 95% nitrogen in Titan’s case. However, unlike Earth, Titan has very little oxygen; the rest of the atmosphere is mostly methane and trace amounts of other gases, including ethane. And at the frigid temperatures found at Saturn’s great distance from the sun, the methane and ethane can exist on the surface in liquid form. For this reason, scientists had long speculated about the existence of hydrocarbon lakes and seas on Titan, and data from the Cassini-Huygens mission does not disappoint. Since arriving in the Saturn system in 2004, the Cassini spacecraft has revealed that >620,000 square miles of Titan’s surface – almost 2% of the total – are covered in liquid.
There are 3 large seas, all located close to the moon’s north pole, surrounded by numerous of smaller lakes in the northern hemisphere. Just one large lake has been found in the southern hemisphere. The exact composition of these liquid reservoirs remained elusive until 2014, when the Cassini radar instrument was first used to show that Ligeia Mare, the second largest sea on Titan and similar in size to Lake Huron and Lake Michigan combined, is methane-rich. A new study which used the radar instrument in a different mode, independently confirms this result.
“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.
The new study is based on data collected with Cassini’s radar instrument during flybys of Titan between 2007 and 2015. A number of possible explanations could account for the sea’s methane composition, according to Le Gall. “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.”
In their research, the scientists combined several radar observations of heat given off by Ligeia Mare. They also used data from a 2013 experiment that bounced radio signals off Ligeia. The radar instrument detected echoes from the seafloor and inferred the depth of Ligeia Mare along Cassini’s track over Ligeia Mare – the first-ever detection of the bottom of an extraterrestrial sea. The scientists were surprised to find depths in the sea as great as 525 feet at the deepest point along the radar track. They used the depth-sounding information to separate the contributions made to the sea’s observed temperature by the liquid sea and the seabed, which provided insights into 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.
In the atmosphere of Titan, nitrogen and methane react to produce a wide variety of organic materials. Scientists believe the heaviest materials fall to the surface. They think when these compounds reach the sea, either by directly falling from the air, via rain or through Titan’s rivers, some are dissolved in the liquid methane. The insoluble compounds, such as nitriles and benzene sink to the sea floor.
The study also found that the shoreline around Ligeia Mare may be porous and flooded with liquid hydrocarbons. The data span a period from local winter to spring, and the scientists expected that – like the seaside on Earth – the surrounding solid terrains would warm more rapidly than the sea. However, Cassini’s measurements did not show any significant difference between the sea’s temperature and that of the shore over this period. This suggests that the terrains surrounding the lakes and seas are wet with liquid hydrocarbons, which would make them warm up and cool down much like the sea itself.
http://www.jpl.nasa.gov/news/news.php?release=2016-114
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