Cassini explores a methane sea on Titan

April 26, 2016 by Preston Dyches, NASA
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

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 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.

This colorized flyover movie from NASA's Cassini mission takes viewers over the two largest seas on Saturn's moon Titan and nearby lakes.
"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."

Ligiea Mare is the second largest body of liquid on Saturn's moon Titan. Credit: NASA/JPL-Caltech/ASI/Cornell

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.

This labeled graphic illustrates how different organic compounds make their way to the seas and lakes on Titan, the largest moon of Saturn. Credit: ESA
"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 . 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 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.

Explore further: Cassini sheds light on Titan's second largest lake, Ligeia Mare

More information: A. Le Gall et al. Composition, seasonal change, and bathymetry of Ligeia Mare, Titan, derived from its microwave thermal emission, Journal of Geophysical Research: Planets (2016). DOI: 10.1002/2015JE004920 , dx.doi.org/10.1002/2015JE004920

Marco Mastrogiuseppe et al. The bathymetry of a Titan sea, Geophysical Research Letters (2014). DOI: 10.1002/2013GL058618 , dx.doi.org/10.1002/2013GL058618

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wduckss
1 / 5 (1) Apr 27, 2016
"There are three large seas, all located near the moon's north pole, surrounded by numerous of smaller lakes in the northern hemisphere."

Here there is a suspicion.

The average temperature on Titan is -179.5 ° C.

At the polesin generally have a 20 ° C lower temperature, it is in favor of liquid nitrogen. The atmosphere could be a confirmation.
jonesdave
3 / 5 (6) Apr 27, 2016
Where is the link to the poles being 20C colder? And don't forget that Titan has a higher atmospheric pressure than Earth, therefore the temperature at which liquid nitrogen forms will be lower than here.
Might have been true in the past, however: http://www.exocli...y-titan/
wduckss
1 / 5 (1) Apr 28, 2016
Where is the link to the poles being 20C colder? And don't forget that Titan has a higher atmospheric pressure than Earth, therefore the temperature at which liquid nitrogen forms will be lower than here.
Might have been true in the past, however: http://www.exocli...y-titan/


"..nitrogen Ice does not float on liquid nitrogen, so there would be no iceberg or calving glaciers and sea and nitrogen, and nitrogen ice has a face- .." your link
Why consider a non-existent condition? Temperature should be -210 ° C.
All bodies (Mars, Mercury, etc.) have a cold polar caps, why link?
jonesdave
3 / 5 (6) Apr 28, 2016
"....why link?"

Because you are proposing something that nobody within the scientific community has proposed. So one assumes you have some facts to back up your speculation. Or are you just one more internet genius, who has never bothered studying such things, and expects everybody to just accept your assertions as fact?
jonesdave
2.6 / 5 (5) Apr 28, 2016
^^^^Also might be worth reading this, if you have access: http://www.nature...100.html
"The identification of liquid ethane in Titan's Ontario Lacus."
Brown, et al.
wduckss
1 / 5 (3) Apr 28, 2016
"....why link?"

Because you are proposing something that nobody within the scientific community has proposed. So one assumes you have some facts to back up your speculation. Or are you just one more internet genius, who has never bothered studying such things, and expects everybody to just accept your assertions as fact?


You need to link for the obvious facts? Should the confirmation link or some authority when the day and when night?
CH4 becomes solid on -182.5 ° C. It is only 3 degrees lower than the average temperature on Titan (-179.5 ° C).
It is completely obvious that your authority is not right.
In the future, for link, use the comments Weitter Duckssa on article http://phys.org/n...tan.html
Sory, because I'm not an authority for you.

jonesdave

Ethane on -182.8 ° C, is also solid. Maybe some higher power keeps them in liquid state?
jonesdave
3 / 5 (6) May 01, 2016
^^^So stop going on about it on here, and email the authors of the paper I linked, and tell them why they are wrong. I'm sure they will retract the paper, on your say so, in double quick time. Either that or write up a rebuttal and get it published, otherwise you are just wasting pixels.
Have you factored in the air pressure on Titan? Are you aware that the average temperature of the planet we call Earth is 15C? Are you aware that H2O exists as a solid, liquid and gas, despite that average temperature? What is the triple point of ethane and methane at ~ 1.5 bars?
Answer those questions and than use them as proof in the paper you will never write, because you'd rather spout nonsense on here, where nobody of any note will see it.
Like a good proportion of the wannabes on here.
jonesdave
2 / 5 (4) May 01, 2016
"Surface Temperatures on Titan during Northern Winter and Spring"
http://adsabs.har...16L..17J
"Meridional brightness temperatures were measured on the surface of Titan during the 2004-2014 portion of the Cassini mission by the Composite Infrared Spectrometer. Temperatures mapped from pole to pole during five two-year periods show a marked seasonal dependence. The surface temperature near the south pole over this time decreased by 2 K from 91.7 ± 0.3 to 89.7 ± 0.5 K while at the north pole the temperature increased by 1 K from 90.7 ± 0.5 to 91.5 ± 0.2 K. The latitude of maximum temperature moved from 19 S to 16 N, tracking the sub-solar latitude. As the latitude changed, the maximum temperature remained constant at 93.65 ± 0.15 K."
So much for a 20K difference between poles and equator!
jonesdave
1 / 5 (3) May 01, 2016
"Does ice float in Titan's lakes and seas?"
http://www.scienc...12004824
" Methane and ethane both have triple points near to the temperatures and pressures observed at the surface of Titan and may exist in all three phases at the surface."

So, there you go; I've done the research for you. Took all of 15 minutes. Now use the data to show why these well qualified scientists are wrong.

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