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Beneath a Methane Sea

Back when Cassini was approaching Saturn and we all anticipated the arrival of the Huygens payload on the surface, speculation grew that rather than finding a solid surface, Huygens might ‘splash down’ in a hydrocarbon sea. I can remember art to that effect in various Internet venues of the time. In the event, Huygens came down on hard terrain, but since then Cassini’s continuing surveys have shown that seas and lakes do exist on the moon. Over 1.6 million square kilometers (about two percent of the surface of Titan) are covered in liquid.


Image: Ligeia Mare, shown here in a false-colour image from the international Cassini mission, is the second largest known body of liquid on Saturn’s moon Titan. It measures roughly 420 km x 350 km and its shorelines extend for over 3,000 km. It is filled with liquid methane. The mosaic shown here is composed from synthetic aperture radar images from flybys between February 2006 and April 2007. Credit: NASA/JPL-Caltech/ASI/Cornell.

The liquid, of course, is not water but methane and ethane, existing in an atmosphere that is almost 95 percent nitrogen (with methane, small amounts of hydrogen and ethane making up the rest). Cassini has shown us three large seas near the north pole that are surrounded by numerous smaller lakes, while only a single lake has thus far been found in the southern hemisphere. New work on Cassini flyby data between 2007 and 2015 now confirms that Ligeia Mare, one of Titan’s largest seas, is made up primarily of liquid methane.


Image: A radar image of Titan’s north polar regions (centre), with close ups of numerous lakes (left) and a large sea (right). The sea, Ligeia Mare, measures roughly 420 x 350 km and is the second largest known body of liquid hydrocarbons on Titan. Its shorelines extend for some 2000 km and many rivers can be seen draining into the sea. By contrast, the numerous lakes are typically less than 100 km across and have more rounded shapes with steep sides. Credit: NASA/JPL-Caltech/ASI/USGS; left and right: NASA/ESA. Acknowledgement: T. Cornet, ESA.

The finding is a bit surprising given that ethane is produced when sunlight breaks methane molecules apart. Thus expectations for Ligeia Mare involved primarily ethane. Alice Le Gall (Laboratoire Atmosphères, Milieux, Observations Spatiales and Université Versailles Saint-Quentin, France), who led the new study, comments on the finding:

“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.”

As this work progressed, Le Gall and team relied on a radio sounding experiment performed in 2013, described in this ESA news release. The radio sounding, led by Marco Mastrogiuseppe, detected seafloor echoes and was able to derive the depth of Ligeia Mare along Cassini’s track, which marked the first time we have ever detected the bottom of an off-Earth sea. The deepest depth recorded was 160 meters. Le Gall used the sounding data along with observations of thermal emissions from Ligeia Mare at microwave wavelengths in her work.

The result: The new paper reports that the researchers were able to separate the thermal emissions from the seafloor from those of the liquid sea. The seabed is found to be covered by what Le Gall calls “a sludge layer of organic-rich compounds.”


Image: How different organic compounds make their way to the seas and lakes on Titan, the largest moon of Saturn. A recent study revealed that Ligeia Mare, one of Titan’s three seas, consists of pure methane and has a seabed covered by sludge of organic-rich material. Credit: ESA.

You can see the process at work in the image above. Nitrogen and methane in Titan’s atmosphere produce organic molecules, the heaviest of which fall to the surface. Reaching the sea through rain or one of Titan’s rivers, some are dissolved, while others sink to the ocean floor. We also find that the surface areas surrounding the lakes and seas are likely flooded with liquid hydrocarbons, based on the lack of temperature change between the sea and the shore.

The paper is Le Gall et al., “Composition, seasonal change, and bathymetry of Ligeia Mare, Titan, derived from its microwave thermal emission,” Journal of Geophysical Research: Planets, published online 25 February 2016 (abstract). Marco Mastrogiuseppe’s work on the depth of Ligeia Mare is described in “The bathymetry of a Titan sea,” Geophysical Research Letters, published online 4 March 2014 (abstract).


Comments on this entry are closed.

  • Alex Tolley April 27, 2016, 15:32

    Is it possible that some form of pre-living chemistry/life is breaking up the ethane to extract energy?

    It seems more likely though that the greater density of liquid ethane compared to methane results in it sinking to the bottom of the lake, perhaps also being held as a clathrate with the water ice.

  • DJ Kaplan April 27, 2016, 16:34

    Thank you! I do love Titan so! Fascinating place.

    Would this be a good time to do some Titan-oriented speculation?

    Reflections from the surface of Ontario Lacus suggest that the surface is smoother than what would be assumed, considering the power of the wind and the calculated viscosity of the methane lake’s surface. Based on Titan’s gravity, atmospheric density, temperature, observed wind speeds, composition of the lake in question, the temperature of that liquid, its presumed depth, it is surprising that the surface of this lake is nearly mirror-smooth. Of course, we can only glimpse this phenomenon through a glass darkly, as it were, but the mind runs riot with explanations that include extraterrestrial lifeforms.


    • Robert G April 28, 2016, 12:30

      Link goes to page not found. Typo?

    • Alex Tolley April 29, 2016, 10:14

      If it turns out that the surface is too smooth, that implies a surface film that prevents wind picking up the liquid. Might this film be a biofilm of some kind?

      I’m reminded of Barlowe’s “Expedition” and its “amoebic sea”.

      • Michael T April 29, 2016, 22:50

        It’s also reminiscent of Stanislav Lem’s thoughtful and vivid novel “Solaris” (since made into 2 movies) , with its intelligent acellular living ocean covering the planet. One of the themes was science’s issues with how to classify it, whether it WAS conscious/intelligent or even alive at all.

        Incidentally Solaris sounds somewhat similar to Titan in terms of atmosphere etc.

        “Expedition” sounds like a wonderful book.

  • RobFlores April 27, 2016, 18:58

    Interesting that the expected Ethane is not there.
    This unexpected finding is in addition to the Cassini mission
    not finding much Acetylene and Hydrogen. One hypothesis was that
    living organism were consuming them for energy. Chemical energy
    is probably the at the heart of any life on Titan.
    Now the finding that Ethane is not part of the surface of liquid lakes as expected brings another curve ball. Could organisms be consuming Ethane as a source material, using the equivalent of catalysts to convert the substance into more useful compounds. This would probably be happening at the bottom of these lakes, since the density of ethane is greater than methane one would expect the ethane to percolate to the bottom.
    All of this begs the question, if life on Titan uses Acetylene and Hydrogen
    and Ethane, does this not mean that any organism must be motile, being
    able to control it’s buoyancy at least. Rising to the near surface to grab the H and Acetylene for an energy boost and diving to near the bottom to aquire cellular building material. Yet that would not mean they are multi-celled
    animals since on Earth, single celled organism can and do control their
    position within their ecological neighborhood

  • Tom April 28, 2016, 1:25

    It would be exciting to find alien life inside our solar system.
    I wouldn’t dismiss it out of hand; but historically there is precedence.
    Mars & Venus were speculated as life supporting planets.
    Even today we have some very concise speculation for microbial and even ‘fossils’ for some fringe researchers.
    Ultracold temps with exotic compounds and throw in what you want… but reason and sobriety tells me to wait.
    If we could detect thermal vents in these ‘lakes’… I’d be salivating.

    I’m more convinced Titan should be exploited for ‘petroleum’ than anything else.
    All the internal combustion engines ‘smoggin’ up Mars would be a better use of resources than developing a new biology model for Titan?

    I’m willing to be wrong… but we definitely need more study coupled with discovery to make a really cool leap either way.

    • Alex Tolley April 28, 2016, 9:14

      I suspect it would be cheaper to make methane feedstock from CO2 and the hydrogen from water on Mars, rather than shipping hydrocarbons down the gravity well from Titan.

  • Frank Smith April 28, 2016, 8:35

    There are two places in the solar system I’d really visit.

    I’d like to sit on the rim of Olympus Mons caldera on Mars with the sun coming up and watch sunlight fill that 80 km caldera.

    I’d like to stand at the edge of Ligeia Mare and see Saturn and its rings reflected in a methane sea.

    • Paul Gilster April 28, 2016, 12:19

      I love the second one especially. Imagine what Chesley Bonestell could have done with it!

    • Alex Tolley April 29, 2016, 10:06

      I’d like to stand at the edge of Ligeia Mare and see Saturn and its rings reflected in a methane sea.

      It is a lovely thought, but Titan’s atmosphere is too murky to see through. What you might see is something more akin to the brown skies of “Blade Runner” Los Angeles.

  • Infinite123Lifer April 28, 2016, 12:27

    I wonder … deeply wondering just how Life got kickstarted here. Seeing that we are the analog and we have considerably a water world if the depths or pressures of that global ocean contributed to Life taking root. Of course I like the idea of the soup left by the drasticitity of tidal forces playing into the mixing and standing still process but I wonder sometimes … perhaps Life started in the depths and began a trek, perhaps it began on the shore, perhaps we were seeded, perhaps it was a miracle beyond recognition … or perhaps all of these this gas are possible somewhere given the right conditions. It would truly be extraordinary to find Life elsewhere, especially in such lowly ponds that are found on Titan.

    • EricSECT April 28, 2016, 15:42

      If we find a second, separate genesis of life on Titan? And let us assume it is single-celled prokaryotic. That discovery would mean that life is ubiquitous. We should expect to find it everywhere. The Fermi Paradox becomes more urgent. Now the question becomes: OK, so besides 2 billion years of waiting….. what leads to multi-cellular life? And thus….. what special conditions lead to intelligent life?

      • Michael T April 28, 2016, 23:09

        This is especially so, as it would be life using vastly different biochemistry in a vastly different environment to Earth’s (much more so than life on Mars, even though that would be wonderful). It would broaden the definition of the “Habitable Zone” and increase the number of prospective biospheres. Goldilocks could eat very cold porridge, so to speak :-)

        Yes, finding any extraterrestrial (but non-intelligent/non-communicative) life would actually make the Fermi paradox that much more difficult to explain.

      • Infinite123Lifer April 29, 2016, 10:08

        Quite frankly given the immensity of the universe coupled with the Earth and what’s here … I, personally, cannot phathom anything other than ubiquity throughout, say one in a trillion bodies harbor Life … I think even at those odds the odds are great. The paradox I believe is due to the enormity of the space we exist within.

        I really wonder though given Venus, Earth and Mars if there is a system out there somewhere where not one but several bodies give rise to Life given the likenesses of their formation process.

        “You may say I am a dreamer, but I am not the only one”

  • RobFlores April 29, 2016, 12:58

    Regarding the ubiquity of life:
    While finding single celled organisms or even colonial mats on Titan
    does increase the odds of finding planets amenable to life, there is a
    bit of a catch:

    At near Ceres and beyond, potential abodes for life can only exist in deep oceans, as most of the surfaces are cold and have no atmospheres. There is no accumulation of any ‘liquids’ on the surface (And I am not convinced that
    on the smaller moons seafloor’s there is enough chemical activity to sustain a biosphere)

    The exception is Titan.
    Titan is of the right composition to entertain liquid hydrocarbons
    It’s atmosphere is protected by distance from the sun. The sun’s light
    has just enough power to create a hydrocarbon cycle there, and keep the
    atmosphere from freezing. Even the balance between hydrocarbons as gas and liquid rests on a knife edge, more so than liquid water on Earth.
    At the spot where Huygens landed, the temperature was about -179c.
    at this temperature Methane is a liquid, the range of temperature that it remains so is only about 20 C degrees making it far more volatile than liquid H2O.
    My point is that Titan may turn out to be one those worlds that is MORE
    rare than a terrestrial world with H20. So yes, there will be lots of ice worlds
    but not many that feature liquids. I rule out places were nitrogen is a liquid since, where nitrogen is liquid the only gases are H2 and He, and there are no other liquids for chemistry to occur at a reasonable rate.

  • Michael April 30, 2016, 13:10

    The only thing I see when I look at Titan is raw materials for huge world ships and the like, we could never live there, it is far to cold. The total nitrogen it its atmosphere is enough for many world ships and the hydrocarbons for plastics. I would love to see a world ship around Saturn, the views would be astounding.