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Cassini Explores a Methane Sea on Titan
Jet Propulsiion Laboratory
April 26, 2016

Fast Facts:

* 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 percent 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 possible existence of hydrocarbon lakes and seas on Titan, and data from the NASA/ESA Cassini-Huygens
mission does not disappoint. Since arriving in the Saturn system in 2004,
the Cassini spacecraft has revealed that more than 620,000 square miles
(1.6 million square kilometers) of Titan's surface -- almost two percent
of the total -- are covered in liquid.

There are three 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 published in
the Journal of Geophysical Research: Planets, which used the radar instrument in a different mode, independently confirms this result.

"It's a marvelous feat of exploration that we're doing extraterrestrial oceanography on an alien moon," said Steve Wall, Cassini radar team deputy lead.

"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.

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 results of that experiment
were presented in a 2014 paper led by radar team associate Marco Mastrogiuseppe

at Cornell University, Ithaca, New York, who also contributed to the current study.

During the 2013 experiment, 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 (160 meters) at the deepest point along the radar track.

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, 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. Le Gall and colleagues think that 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 running 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.

"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."


The Cassini-Huygens mission is a cooperative project of NASA, ESA (European Space Agency) and the Italian Space Agency. JPL, a division of the California Institute of Technology in Pasadena, manages the mission for NASA's Science Mission Directorate in Washington. The radar instrument was built by JPL
and the Italian Space Agency, working with team members from the US and several European countries.


News Media Contact

Preston Dyches
Jet Propulsion Laboratory, Pasadena, Calif.
818-354-7013
preston.dyches@jpl.nasa.gov

Markus Bauer
European Space Agency, Noordwijk, Netherlands
011-31-71-565-6799
markus.bauer@esa.int

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