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Complex Reactions on Titan

Finding life on Mars would be a huge accomplishment, but finding life on Titan would be a fundamentally different kind of discovery. Martian life might well be related to us because of the exchange of materials between our two worlds, the inevitable result of planetary impacts and the scattering of debris. But Titan is a far more unearthly place than Mars, its chemistry exotic, its climate seemingly beyond the range of any life form we have ever discovered. Life on Titan should be evidence of that ‘second genesis’ planetary scientists dream of identifying.

Image: This artist concept shows a mirror-smooth lake on the surface of the smoggy moon Titan. Cassini scientists have concluded that at least one of the large lakes observed on Saturn’s moon Titan contains liquid hydrocarbons, and have positively identified ethane. This result makes Titan the only place in our solar system beyond Earth known to have liquid on its surface. Credit: NASA/JPL.

Now we have two papers based on Cassini data that show complex activity on the surface of Titan. Is there a precursor to life on the distant moon, or even an exotic life form based on methane? Darrell Strobel (Johns Hopkins) is the author of a paper now in press at Icarus that shows hydrogen molecules descending through the atmosphere and disappearing at the surface. Using data from Cassini’s composite infrared spectrometer and ion and neutral mass spectrometer, Strobel has studied the density of hydrogen in various parts of Titan.

Hydrogen molecules on Titan are thought to be a byproduct of ultraviolet sunlight breaking acetylene and methane molecules apart, a process that is understood and should theoretically distribute hydrogen evenly through the atmosphere. What Strobel has found is a flow to the surface at a rate of 10,000 trillion trillion hydrogen molecules per second. Says Strobel:

“It’s as if you have a hose and you’re squirting hydrogen onto the ground, but it’s disappearing. I didn’t expect this result, because molecular hydrogen is extremely chemically inert in the atmosphere, very light and buoyant. It should ‘float’ to the top of the atmosphere and escape.”

Is there an unknown mineral acting as a catalyst on Titan’s surface, converting hydrogen molecules and acetylene back into methane? The question lingers even as we look at the hydrocarbon mapping of Titan’s surface, which is being done by Roger Clark (U.S. Geological Survey), who notes that despite earlier predictions that acetylene would fall from the atmosphere to coat Titan’s surface, Cassini has detected no acetylene on the surface. Acetylene is important because it could be the best energy source for methane-based life forms on Titan. It is conceivable that surface acetylene is being consumed as food.

Moreover, the hydrogen flow to the surface is consistent with models showing life on Titan consuming hydrogen. Astrobiologist Chris McKay (NASA Ames), who has proposed a set of conditions for methane-based life on the moon, draws a parallel with our own planet:

“We suggested hydrogen consumption because it’s the obvious gas for life to consume on Titan, similar to the way we consume oxygen on Earth,” McKay said. “If these signs do turn out to be a sign of life, it would be doubly exciting because it would represent a second form of life independent from water-based life on Earth.”

Cassini’s spectrometer has detected an absence of water ice on the surface, but has found plentiful benzene and an apparently organic compound that is still unidentified. A film of hydrocarbons seems to be forming over surface water ice. All of this makes Titan an extremely lively place, as Clark notes:

“Titan’s atmospheric chemistry is cranking out organic compounds that rain down on the surface so fast that even as streams of liquid methane and ethane at the surface wash the organics off, the ice gets quickly covered again. All that implies Titan is a dynamic place where organic chemistry is happening now.”

On a surface where temperatures hover around 90 Kelvin (-183 degrees Celsius), liquid methane and ethane are the only substances available to serve as a liquid for life processes. It’s an exotic and fascinating place, this Titan, but we’re far too early in the game to rule out non-biological explanations. Chemical reactions involving mineral catalysts are very much in the running as we await further Cassini data from future flybys. On that score, note that Cassini will be making a close approach to Titan later today (early Saturday June 5 UTC), moving to within 2000 kilometers of the surface to view the north polar region and its lakes. Of special interest: Kraken Mare, Titan’s largest lake, covering an area larger than the Caspian Sea.

The hydrogen paper is Strobel, “Molecular Hydrogen in Titan’s Atmosphere: Implications of the Measured Tropospheric and Thermospheric Mole Fractions,” in press at Icarus. Clark’s hydrocarbon mapping paper will appear in the Journal of Geophysical Research.


Comments on this entry are closed.

  • Zen Blade June 4, 2010, 10:00

    I did not know this was happening. Very interesting.
    I look forward to hearing more about this.

  • Tulse June 4, 2010, 12:15

    Cassini continues to impress with the vast amount of science it is doing. I think it is arguably the most productive space probe ever produced by humanity, and barring misfortune it will keep going until (at least) 2017. Simply amazing.

  • Malcolm Ramsay June 4, 2010, 12:36

    How fascinating- and oddly reminiscent of Stephen Baxter’s “Titan”; watch out for big rocks, everybody…

    This would be a revolutionary discovery, more seriously- while “life as we know it” would be an epochal discovery, “life as we don’t know it” could be even more so. Here’s hoping!

  • Eric June 4, 2010, 13:01

    Wow! These are very exciting results. What a strong motivation to go back there, land, and find out more.

    However, I would be a little worried about these results. Going back to earlier threads about a “Great Filter” (or even Doomsday arguments) to explain the apparent Fermi Paradox, life on Titan would have disturbing implications for our future.

    If life so easily emerges that it can form on Titan, at 90 Kelvin, then it is probably really ubiquitous in the universe. Unlike Mars, life on Titan would really have to be independent, since if we find like on Mars, there’s always (a strong?) chance that it will be related to Earth life, given the similar environments and long-term exchange of rocks. So life on Titan would be a more momentous discovery than life on Mars.

    However, it means that the Great Filter that would not be at the stage where life first emerges. That means that the Great Filter lies somewhere between life emerging and life spreading and colonizing the galaxy. Maybe the really hard hurdle is the evolution of complex life, or intelligence. In which case, we’d be lucky, since that puts the Great Filter behind us and not ahead of us.

    But discovery of life on Titan would eliminate one potential Great Filter that we’ve already crossed, which is the emergence of life itself. That elimination makes me worry that the Great Filter is more likely to lie ahead of us, stopping us short of spreading out into space.

  • kurt9 June 4, 2010, 13:48

    But discovery of life on Titan would eliminate one potential Great Filter that we’ve already crossed, which is the emergence of life itself. That elimination makes me worry that the Great Filter is more likely to lie ahead of us, stopping us short of spreading out into space.

    I disagree. Titan life would be very different from our kind of life. For one thing, it would be very slow and simple, unlikely to have evolved beyond the simplest of single-celled organism. If Titan life is common but we are rare, that Titan life has not evolved to space travel says nothing about the prospects of our kind of life.

  • Enzo June 4, 2010, 17:36

    I remember reading about this possibility in this CHARM presentation:


    On pg 30 it mentions that Huygens’ GCMS can detect hydrogen depletion in the lower atmosphere, but they needed more time to analyze the results as the carrier gas was H2.
    I never heard about this any more and the article I’ve seen do not mention Huygens.

  • NS June 4, 2010, 22:17

    A truly alien lifeform might not even have cells — maybe that’s just a local feature of the type of life that got going on Earth. For all we know Titan’s life could be like the ocean in “Solaris”. That’s the problem when you only have one example to work from — you can’t be sure what’s basic and what’s just a quirk. I’m looking forward to further discoveries.

  • Procyan June 5, 2010, 3:43

    Could someone dummy this down please? Or perhaps smarten up … Hydrogen should be going up but instead moves down and disappears. Therefore LIFE? How does that follow?

    With respect, this reads more like USA today than your usual intellectual style.

  • Administrator June 5, 2010, 8:43

    Procyan, read the piece again and I think we won’t be in any disagreement. I’m not saying there’s life on Titan. I’m saying that the complex reactions studied in these new papers are not inconsistent with life. They’re also not inconsistent with chemical reactions involving mineral catalysts, and that point is in the article, too. The hydrogen issue is provocative but no one is claiming it to be a definitive discovery of life.

  • Anders Feder June 5, 2010, 8:46

    What data will be required to tentatively rule out non-biological explanations, when will Cassini acquire them – and how long will it take scientists to process them? :D

  • James M. Essig June 5, 2010, 13:25

    Hi Paul;

    Perhaps any Titan life might consist of crystals. I can imagine thermodynamic processes by which crystals could self replicate. While my siblings and I were in elementary schoold, my mother during the Holiday season used to mix ammonia with a solid powder and McCormick brand food coloring, I forget which, thus resulting in crystal growth. For those of you with children, be very carefull with any such displays, because if you mix ammonia with the wrong chemicals, extremely toxic gases can be produced. Luckily, the above recipe was completely benign.

    I can imagine life of similar topological characteristics developing on Titan. Now meeting any ETI persons based on such physiology would be down right freaky, but cool, no pun intended. I wonder what sort of space craft they would use.

  • Athena Andreadis June 6, 2010, 16:30
  • Administrator June 6, 2010, 17:49

    Good post, Athena, and I want to quote this from it, as it’s nicely put and gets across the possibilities these new papers suggest:

    Titan, unlike Mars, has enormous lakes – although they contain liquid hydrocarbons, rather than water. The lightest in that family (methane and ethane) are poor solvents because they’re non-polar, unlike water and ammonia. Nevertheless, they do act as solvents for the rich organic soup churned by Titan’s thick atmosphere of ammonia and methane (Carl Sagan’s “tholins”, from the Hellenic word for murky). And although chemical reactions will be slow in Titan’s ambient temperature of –190 Celsius (room temperature is 24 Celsius), all bets are off once enzymes are involved.

  • NS June 6, 2010, 21:36

    Presumably it’s worlds with life like ours that would be most in danger of infection/destruction by us. The shear “alien-ess” of Titan life might be its best defense.

  • Joe June 7, 2010, 10:09

    One thing I don’t understand about this article. Particularly, “… a process that is understood and should theoretically distribute hydrogen evenly through the atmosphere.”

    Uh, lets think about this. You folks tell me what I am missing, because this article has me perplexed.

    If hydrogen is being created in Titan’s thick atmosphere by the effects of UV radiation from the Sun, is not this gas (H2) far lighter than the main component of Titan’s atmosphere, Nitrogen?

    One would expect no H2 to be at the surface for more than a short time anyway, and it would simply fly up to the upper atmosphere quickly, just as it does here on our own Earth.

    So if you are measuring H2 at the surface, I’d expect it to be minimal, just do to simple physics.

    Plus, one would expect less UV at the surface as well, due to the simple fact that it has to travel all the way though the thick smog to get down there in the first place. The UV reactions should take place at a far greater rate in the upper atmosphere than near the surface. So this is yet another simple physical reason as to why you see less H2 down at the surface. And H2 would not easily migrate down due to simple buoyancy.


  • Eniac June 7, 2010, 11:27

    If I understand correctly, the energy source for such “titanic” life is postulated to be the combining of hydrogen and acetylene into methane or other carbohydrates, which would indeed yield net energy, I think. This energy would originate from sunlight through inorganic processes in the upper atmosphere. This would be interesting, as it eliminates the need for organic photosynthesis, thus representing a shortcut compared with how Earth life uses light. Organic photosynthesis would be a much taller order to evolve on Titan, anyway, because of the low light levels.

    Thus, Titanic life could be all animals, no plants needed. Has anyone done a study on how hydrogen/acetylene metabolism would work? Would it support mobility, or would Titan animals behave more like Earth plants?

  • Eniac June 7, 2010, 14:54

    Joe: My understanding is that atmospheric gases behave independent from each other, with each assuming the exponential equilibrium density dictated by the balance of pressure and gravity. This would have the partial pressure of hydrogen be greatest on the ground. True, hydrogen density would decrease more slowly with height than that of the other gases, so the relative abundance of H2 would increase with height. However, the absolute density (or partial pressure) would still be maximal on the ground. If you removed hydrogen at the ground by some process, the ensuing disequilibrium would have hydrogen diffuse downwards, indeed. Gravity would win out over the reduced partial pressure.

    Your other point, about less UV at the surface, is true, and whether that would be reflected in the density depends on the dynamics. If hydrogen diffusion is much faster than the UV generation, you would expect the equilibrium distribution, otherwise, something more top-heavy (but still monotonously decreasing with height). An “inversion”, i.e. higher density above than below, can only happen if there is a sink below, which is presumably what they are arguing.

  • Anders Feder June 7, 2010, 19:56

    “If I understand correctly, the energy source for such “titanic” life is postulated to be the combining of hydrogen and acetylene into methane or other carbohydrates, which would indeed yield net energy, I think. This energy would originate from sunlight through inorganic processes in the upper atmosphere. This would be interesting, as it eliminates the need for organic photosynthesis, thus representing a shortcut compared with how Earth life uses light.”

    Interesting point. This would vastly improve the odds of life elsewhere in the universe. Is the whole notion of an circumstellar habitable zone (CHZ) not implicitly based on the assumption that life is dependent on photosynthesis?

    If I may cite Wikipedia, it defines the habitable zone as: “… the region in a star-centered orbit where an Earth-like planet can maintain liquid water on its surface and Earth-like life. ” Why the surface? Because both sunlight and liquid water is required to do photosynthesis, and the surface is the only place on a planet where both is (potentially) abundant.

    If this ‘pseudo-photosynthesis’ happening in the upper atmosphere of Titan generates sufficient chemical energy to support life based on other fluids than liquid water, the CHZ may be much wider than previously thought – even when discounting exotic sources of energy, such as tidal forces. It’s speculation, but with this process, a planet with liquids in the orbit of our Saturn (way beyond the CHZ as currently defined) now could potentially sustain life.

    This wider CHZ, in turn, would qualify way more exoplanets (and thus host stars) as worthy SETI targets.

    Hoping to learn a lot more about this phenomenon soon! Go Cassini.

  • Bounty June 7, 2010, 19:59

    The extreme environments here continue to serve up interesting lifeforms. http://www.theregister.co.uk/2010/06/07/canadian_mars_microbes/

  • Anders Feder June 7, 2010, 21:15

    Chris McKay of NASA Ames, who is mentioned in the article, has published a good response to the whole thing on the website of the Cassini Imaging Science Team: http://www.ciclops.org/news/making_sense.php?id=6431&js=1

  • Enzo June 7, 2010, 22:27


    If you listen to the talk given by McKay for CHARM :

    When it gets to pg 28 of the presentation I previously posted, it specifically mentions that the amount of energy produced by hydrogen and acetylene is bigger than the energy generated by methanogens on Earth.
    McKay goes further and he seems to equate hydrogen depletion the surface very close to proof of life on Titan.

  • Eniac June 8, 2010, 13:35

    Anders: I think you are right. Of course, we are already dumping the CHZ just by talking about life on Titan. Also, we know Earth life that is independent of sunlight at the deep sea thermal vents, which could theoretically exist the same way deep within cold bodies such as Enceladus, with or without atmosphere, with or without light, driven by geothermal or geochemical energy. The latter is ultimately derived from the radioactive decay of heavy elements, as on Earth, or from tidal forces, as on some moons. Such life not only defies the CHZ, it could exist on some brown dwarfs and “starless planets”, if such exist.

  • Athena Andreadis June 8, 2010, 13:53

    I’m surprised how little this has shown up in the media… but I’m doing my part!


  • kzb June 9, 2010, 8:13

    I like how the article says “10,ooo trillion trillion molecules of hydrogen per second”. Sounds like a lot, however I make that 33 kilogram per second.

    The surface area of Titan is 8.3E+07 sq km. So the rate works out to 0.4mg/sec/sq km. Not as impressive sounding is it?

  • Tobias Holbrook June 11, 2010, 7:14

    If life can start easily and survive on places ranging from Terra to Titan at the least, though – where are the aliens, as Fermi famously said? If we do find life there, the biggest thing it will do is make Fermis paradox an even more pressing matter. Unless the creatures can’t develop technology, I suppose…

    The energy source exists, but there’s still the question of how the life actually operates. Are their any conceivable mthods for an organism to use Hydrogen and Acetylene to reproduce and grow? What would such creature use instead of DNA?

    I can’t decide whether to hope for Titanic life or not – on the one hand, it gives us something to bioengineer that can be used on extreme worlds; on the other, it puts Titan off limits :)

  • Tobias Holbrook June 11, 2010, 7:23

    Could carbon-based life be surviving there, by any chance? We have methane feeders here on Terra, after all, that are all based on Carbon.

  • Eniac June 11, 2010, 23:13

    Are their any conceivable methods for an organism to use Hydrogen and Acetylene to reproduce and grow? What would such creature use instead of DNA?

    Could carbon-based life be surviving there, by any chance? We have methane feeders here on Terra, after all, that are all based on Carbon.

    In some ways Titan is friendlier to carbon based life, as there is no shortage of carbon. On Earth, carbon has to be extracted from dilute CO2 at high energy cost, on Titan it is readily available in dense hydrocarbons that require comparatively little energy to be processed into more complex biomolecules. Thus, Titan life is quite likely carbon based. We don’t know what it uses for RNA/DNA, but it may well be very much like ours.

    On Titan, the essential element requiring the most energy to obtain is oxygen, which has to be liberated from water ice. This would lead to a tendency to use less of it in biochemistry, making Titan life more “fatty” than Earth life. Improved solubility in liquid carbohydrates also should have this effect.

  • Eniac June 11, 2010, 23:21

    The surface area of Titan is 8.3E+07 sq km. So the rate works out to 0.4mg/sec/sq km. Not as impressive sounding is it?

    This is indeed disappointingly low, and probably not enough to support anything other than very small clusters of sessile organisms with very large dead areas in between. Life would thus be very hard to find on Titan. It would certainly not be running around and waving to us.

    Most likely there exists an inorganic explanation for such a minuscule flux, we just have not found it yet.

  • Anders Feder June 13, 2010, 1:40

    I’m not sure an average for the entire surface of the moon makes any sense in this context. Even humans inhabit only .3% of the surface of Earth. Some microscopic cultures of microbes could be much more localized than that.

    Let’s have some fun. Some of the smallest known bacteria are around .3 micrometers long. That’s about 5 million times shorter than the average human being. Now let’s very unscientifically assume that because these bacteria are 5 million times smaller than humans, in total, they occupy 5 million times less space than humans. That’s .3% divided by 5 millions. That’s suddenly a very small fraction of the planetary surface.

    Equally unscientifically, if we assume that these bacteria would occupy the same fraction of the surface of Titan, this would all translate into 747.8 kilograms of hydrogen per square kilometer of habitation per second. (Just ask Wolfram.)

    That’s more like, right?

    Essentially, I don’t think this hydrogen flow figure says much about whether or not there is life on Titan. It asks us to look closer.

  • ljk September 5, 2010, 8:12

    About the possible role of hydrocarbon lakes in the origin of Titan’s noble gas atmospheric depletion

    Authors: D. Cordier, O. Mousis, J. I. Lunine, S. Lebonnois, P. Lavvas, L.Q. Lobo, A.G.M. Ferreira

    (Submitted on 22 Aug 2010)

    Abstract: An unexpected feature of Titan’s atmosphere is the strong depletion in primordial noble gases revealed by the Gas Chromatograph Mass Spectrometer aboard the Huygens probe during its descent on 2005 January 14. Although several plausible explanations have already been formulated, no definitive response to this issue has been yet found.

    Here, we investigate the possible sequestration of these noble gases in the liquid contained in lakes and wet terrains on Titan and the consequences for their atmospheric abundances. Considering the atmosphere and the liquid existing on the soil as a whole system, we compute the abundance of each noble gas relative to nitrogen.

    To do so, we make the assumption of thermodynamic equilibrium between the liquid and the atmosphere, the abundances of the different constituents being determined via {\bf } regular solution theory.

    We find that xenon’s atmospheric depletion can be explained by its dissolution at ambient temperature in the liquid presumably present on Titan’s soil. In the cases of argon and krypton, we find that the fractions incorporated in the liquid are negligible, implying that an alternative mechanism must be invoked to explain their atmospheric depletion.

    Comments: 11 pages, 1 figure

    Subjects: Earth and Planetary Astrophysics (astro-ph.EP)

    Cite as: arXiv:1008.3712v1 [astro-ph.EP]

    Submission history

    From: Daniel Cordier [view email]

    [v1] Sun, 22 Aug 2010 18:32:07 GMT (17kb)


  • ljk September 21, 2010, 20:23

    Have We Already Discovered Alien Life—on Mars?

    Planets and moons do not give up their secrets willingly or easily — they make us work for every clue we get. That seems particularly true when it comes to the search for extraterrestrial life. Even then, some bodies in the Solar System make us work harder than others.

    Take Titan, for example. Two weeks ago, I wrote that observations of Titan from Cassini have been interpreted by some as possible signs of life, in particular:

    Now it turns out that computer simulations based upon Cassini observations, simulations which hint at depletions of various chemical species at Titan’s surface may again hint at the possibility of life on Titan. The results are very preliminary, but fascinating nevertheless.

    It’s highly unlikely that we’ll ever be able to make a positive determination if there’s life on Titan based upon Cassini data alone. Cassini is, after all, an orbiter, and its observations of Titan’s surface come from hundreds, even thousands, of kilometers away–limited to those that can be attained during flybys. To ascertain the presence of life, we’ll need what scientists in the field of remote sensing call “ground truth”–we’ll have to wait until we are able to send a followup probe to the surface of Titan. Perhaps we’ll send a probe to Titan similar to Tiny–the Titan rover who has guest-starred in episodes of this season’s Eureka.

    Full article here:


  • Rob Henry April 12, 2011, 23:10

    As more is learned about this amazing world that is Titan I feel the less I know. As new information emerges, I am beginning to loose my grip on how the pieces of this Titanic puzzle fit to create the whole. Surely a world with such a low gravity could never retain a primordial atmosphere left over from the turmoil of Saturn’s formation. If its atmosphere is volcanic, most of that nitrogen must have been erupted as ammonia, and if so, wouldn’t it have been accompanied by at least an order of magnitude more methane? If so where are all the organic tars or deep hydrocarbon oceans?

    In an earlier article on these pages, the unusual concentration profile of hydrogen with altitude on Titan was featured. An experiment had been done with the assumption that hydrogen loss through the exosphere changed this profile. This would have been far easier to calculate than hydrogen loss into the ground, since Titan’s atmosphere gets complex and smoggy in its lower reaches. Thus I strongly suspect that the 30 odd kilograms calculated to be disappearing into the ground each second must have been a lower estimate (if anyone can confirm or correct that belief, I would be grateful). Either way it is obvious that hydrogen loss through the exosphere must also occur, and must be added to this figure to find the grounds true hydrogen absorption rate. If we assume that that rate was also 30 kg/s we find that it takes at least 2 billion years just to generate today’s inventory of atmospheric nitrogen – and that assumes that none of it has been lost to space.

    Titan’s core does not seem to be well enough differentiated to allow any past epoch of high volcanic activity let alone allowing it today, so I suppose that that atmosphere must really somehow be primordial, yet then where are all the noble gasses? I admit that a drowning man does clutch at straws, but this situation reminds me of Lovelock’s Gaia hypothesis. Could life really have engineered the maintenance of high methane levels without use of colossal inventory of carbon? If so it is at least clear why life needs to do so by Gaian principles.

    In the comments section of Paul’s original précis of the hydrogen altitude data, the consensus was that the ground absorption of hydrogen was so low when expressed by weight as to be insignificant, and preclude the possibility that our first probe to study its surface might be met by an enthusiastic reception committee of natives. This was in error since a little bit of hydrogen provides much energy (especially when reacted with acetylene). Contributors to those comments correctly calculated that the measured H adsorption rate (that I believe to be a minimum), was 0.4 mg/s/sq km, but this is about 20 Watts. By comparison, over all Earth, plants capture 175 000 W/sq km.

    In that discussion it was also floated that this activity could be the seen as more similar to that of animals than plants. I have no figures for Earths animal activity, but I would guess that it would total 10 000 W/sq km, so the minimum animal biological activity given by hydrogen flow might be only a few hundred times less than Earth’s.

    I agree that idea of a thriving biosphere on Titan sounds crazy, even as a possibility, and would appreciate it if someone would correct me, yet I do not see an obvious mistake, and an beginning to see signs that it exists.