Could life exist on a world with a methane rather than a water cycle? The nitrogen-rich atmosphere of Titan, laden with hydrocarbon smog, is a constant reminder of the question. Cassini has shown us the results of ultraviolet radiation from the Sun interacting with atmospheric methane, and we’ve had radar glimpses of lakes as well as the haunting imagery from the descending Huygens probe. Our notion of a habitable zone depends upon water, but adding methane into the mix would extend the region where life could exist much further out from a star. Chris McKay and Ashley Gilliam (NASA Ames) have been actively speculating on the possibilities around red dwarfs and have published a recent paper on the subject.
It’s intriguing, of course, that with methane we get the ‘triple point’ that allows a material to exist in liquid, solid or gaseous form at a particular temperature and pressure. That makes Titan ‘Earthlike’ in the sense that our initial view showed a landscape with the clear signs of running liquid, but this is a world where temperatures dip to 94 K (-179 Celsius) and water is the local analog of rock. In a fine essay on McKay’s work in Astrobiology Magazine, Keith Cooper notes an earlier McKay paper that suggested a potential life mechanism in this kind of environment: Local methanogens would consume hydrogen, acetylene and ethane while exhaling methane. That’s a mechanism useful for astrobiologists because it would show a particular signature in the depletion of hydrogen, acetylene and ethane at the surface.
Image: This composite was produced from images returned on 14 January 2005, by ESA’s Huygens probe during its successful descent to land on Titan. It shows the boundary between the lighter-coloured uplifted terrain, marked with what appear to be drainage channels, and darker lower areas. These images were taken from an altitude of about 8 kilometres with a resolution of about 20 metres per pixel. Credits: ESA/NASA/JPL/University of Arizona.
But the fact that Titan does show signs of such depletion isn’t necessarily indicative of life, for these signs are themselves dependent on atmospheric models that are still in play, and in any case we know little about other processes that could mimic the same characteristics without implications for life. Exo-Titans may be relatively common, for all we know, but to find them we are going to have to first establish that life can exist in this environment and then work out an atmospheric signature we can search for. Cooper quotes Lisa Kaltenegger (Max Planck Institute) on the issue:
“We just don’t know what the tell-tale signs for life would be in such an atmosphere because it is so vastly different from ours. That said, it will change in a flash if Chris [McKay] finds life on Titan and can tell us what it produces and what we could look for remotely with a telescope.”
That makes future probes of Titan all the more interesting, and adds to the desirability of a long-term presence on the moon, either through a surface rover or an aerostat that could range high over the surface and give us a highly-focused look. As for those red dwarfs McKay studied in his recent paper, a methane habitable zone should exist between 0.63 and 1.66 astronomical units (99 million and 248 million kilometers) around the star Gliese 581, that frequently invoked site of habitable planet speculation. Unfortunately, while we do have four planets confirmed in the system (with two others considered controversial), none exists in the methane sweet spot.
While Gliese 581 is an M2.5V dwarf, the authors also calculate the numbers for an M4 dwarf, finding a closer habitable zone between 0.084 AU and 0.23 AU (12.6 million kilometers to 34.4 million kilometers) in methane terms. The beauty of studying habitable environments — water or methane — around M-dwarfs is that these are systems where the orbital distances involved will be small and detection of planets through radial velocity and planetary transits somewhat easier. But what happens on the surface of such a planet is another matter. Much depends on how the atmosphere is affected by stellar conditions, as Cooper notes:
Titan’s atmosphere is opaque to blue and ultraviolet light, but transparent to red and infrared light, and red dwarfs produce more of the latter than the former. If Titan orbited a red dwarf, more red light would seep through to its surface, warming the planet and extending the range of the liquid methane habitable zone. (Interestingly, a red giant, which is close to the endpoint in the life cycle of a Sun-like star, produces light of similar red wavelengths. When our Sun expands into a bloated red giant in about five billion years, engulfing all the planets up to Earth and possibly Mars, Titan may well reap the benefits – for a short while at least before the red giant puffs away to leave behind a white dwarf star.)
Countering this warming is the effect of large stellar flares on evolving life, frequent on younger red dwarfs. McKay’s work suggests that such active M-dwarfs would dissociate atmospheric molecules on a Titan-like world, making the place more and more smoggy and reducing the surface temperature. The net effect would be to move the methane habitable zone closer to the star. Clearly we have a long way to go to be able to actively search for methane-based life outside our own Solar System, and probably decades to go before we get back to Titan.
For the time being, then, a methane habitable zone is sheer speculation, but it’s interesting to ponder the life that might appear on such worlds. One thing seems sure: The temperatures at which liquid methane exists would produce creatures with slow metabolisms. Will a future Titan probe find life? Given our relatively greater understanding of life’s relation to liquid water, we’re obviously going to keep the focus there, but a ‘second genesis’ on Titan would change the equation considerably as we ponder how frequently life can form and with what constituents.
The paper is McKay and Gilliam, “Titan under a red dwarf star and as a rogue planet: requirements for liquid methane,” Planetary and Space Science Volume 59, Issue 9, pp. 835-839 (July 2011). Abstract available.
“This composite was produced from images returned on 14 January 2005, by ESA’s Huygens probe during its successful descent to land on Titan. It shows the boundary between the lighter-coloured uplifted terrain, marked with what appear to be drainage channels, and darker lower areas.”
Do they know if the lower, darker areas in the image are liquid or not? Also,
is the light in the image from a long exposure ( is it really a lot darker)?
Is it known whether life is able to evolve in a solvent which is less dense in liquid form than solid? It’s often cited that a significant reason why life evolved on earth is because water, with its hydrogen bonds, expands and so floats; hence lakes tend to freeze over on the top rather than freeze from the bottom up as would be the case with methane. It would be hard for life – as we know it – to begin in an environment where seasonal changes are as severe as a change of state of the environment.
I’d certainly be happy if a mechanism has been found to overcome this drawback of methane based life.
Bob – The lower areas are “dry” gravel.
Two issues with the article:
Methane based life will have less energy for chemical reactions. It’ll be hard to tell the difference between a living critter and a rock that’s just reacting to a chemical change in its environment.
Titan is what it is because it’s at just the right distance from the Sun. Once the Sun becomes a red giant (actually, well before that) Titan will get a lot more energy from the Sun, raising its temperature, which will (depending on how hot it gets) cause a partial or complete loss of its atmosphere.
The same goes for Jupiter’s moons; if Jupiter were at 1AU the Galilean moons would quickly melt and the water would be lost to space in short order. The result would be much smaller, very dry rocky moons. There is no such thing as a habitable moon unless it’s considerably more massive than Mars.
Dan, if the liquid bodies are a mix of methane and ethane, then methane solids should float. The other point is that ice isn’t a medium for life, liquid water is, thus ice expansion is kind of irrelevant to life, unless it gets really cold.
Water has some properties that make it an ideal solvent for life, so it seems amazingly coincidental that it is the most common accessible liquid medium in our universe (metallic and liquid molecular hydrogen seem inaccessible to me). Its great abilities as a solvent, high heat capacity and wide temperature range seem indisputably useful. To add to this list excessively is to discard the principle of mediocrity. Water’s expansion on freezing is only useful in habitats that would otherwise be expected to freeze solid. Once an area is covered in ice, the high reflectivity of ice tends to keep it that way.
Adam, I think that your right in assuming that the concentration of ethane in Titans oceans is way below its eutectic point, so pure methane would be expected as freezing starts. What worries me is that the level of ethane here is so low that frozen methane would still be more dense than the ocean, and I doubt that any mix contains the perfect conditions to allow that floating (ie, for there to be enough ethane for methane to float in a mix, I suspect that ethane would preferentially freeze out)
I find it fascinating that, according to this article, no uv but plenty of red light reaches the ground on Titan due to the smog. Does this open the possibility of a photosynthetically driven biosphere, of which the hydrogen absorption only represents a small proportion to the biological activity?
As to that smog itself, is it mostly absorbing or is it mostly reflecting that uv. If it is absorbing much uv, I would expect it to be releasing much hydrogen, and that brings me to my next problem.
The very narrow part of the uv spectrum that directly results in methane releasing hydrogen is completely absorbed on Titan at about 1000km altitude. It is only a tiny fraction of the uv energy available, but even so, it is sufficiently well understood that the downward flow of hydrogen measured from there created a deep mystery. Hydrogen released from uv initiated reactions within this smog could be at least an order of magnitude higher (if, indeed, they do happen).
Another thing, life at Titan would almost certainly be based around free radical reactions, and so would only be expected to be fractionally more sluggish than life on Earth. If you were a writer of very hard science fiction, it would still be allowable to have our first astronauts to that moon greeted by a large enthusiastic group of natives.
“Our notion of a habitable zone depends upon water” — No. Our notion of an Earth-Analogue Habitable Zone, or of a Surface Liquid Water Zone, depends upon water.
Since carbon-based life can live sub-surface (Europa is a favourite possibility), the term Habitable Zone is meaningless, as the zone of space which is potentially habitable by carbon-based life is coextensive with all of interplanetary and interstellar space. I would suggest that scientific precision in the use of these terms to express what one actually means is always helpful in aiding communication.
I’m curious if anyone out there is aware of any studies or predictions regarding what TYPE of life could possibly arise out of methane-based chemistry? Could there be something analogous to cells, or are we likely just talking about very simple, self-replicating molecules (or, at best, entities similar in complexity to viruses)? What I’m really interested in is whether there is any chance that methane-based multicellular life could exist.
Indisputably? There are many better solvents than water, depending on what you mean by a “great” solvent, it is not clear how solvent heat capacity impacts life, and a narrower temperature range could be argued to be of advantage.
To me, “water is uniquely suited” is just another one of these Earth-centric conjectures that somehow became dogma without the usual process of evidence gathering and sound reasoning.
Another thing occurred to me – life exists in a medium full of solutes. Salt water is significantly denser than pure water and I imagine a methane medium might be similar.
There’s also the curious fact that methane’s critical point is higher than the eutectic melting point of ammonia/water – in theory the three liquids could co-exist.
@Scott: Cell membranes serve to keep the components of an organism together. It is hard to imagine a form of life that does not have such a mechanism. However, there may be alternatives to membranes: Circular RNA-like macromolecules could be held together and form a unit by interlocking like links of a chain. This would serve the purpose of holding things together, but it would not form a physical barrier separating inside from outside. It is interesting to speculate how far organisms could evolve with such a scheme. Then again, it is likely that methane chemistry permits something analogous to lipid bilayers that could fulfill all the the functions of cell membranes.
Viruses are a bad model for primitive life, they are evolutionary dead ends that have become an inextricable part of their hosts.
Worse than ultra-slow reaction rates
is the dearth of cryogenic chemical species
with both the combinatorial complexity
and catalytic flexibility of proteins,
let alone fats, carbohydrates, and nucleic acids.
Silicon doesn’t cut it for that job.
Even if you did have all those cryogenic chemical analogs,
the best you could expect is rock-pore proto-life
so ultra-slow that it would need a trillion years to make true life.
I am very skeptical that you could have recognizable “life” without water.
Having said that, ideas on the origin of life on earth as electron transport “metabolism”, bounded within pores in rocks might be a possible model for places like Titan. You would not see mobile independent organisms, just complex chemical metabolism occurring in the out layers of porous rocks (or other suitable substrates).
If that were possible, Titan would be a fascinating example of an independent genesis preserved at a very early stage of evolution.
I don’t think we know enough about the totality of cryogenic chemical species to assert that there is such a dearth. And reaction rates depend not only on temperature, but also on the nature of the reaction. Again, I am not sure we have explored this area of chemistry enough to conclusively exclude the existence of molecules that react fast and are capable of combinatorial complexity.
Dan wrote: “Is it known whether life is able to evolve in a solvent which is less dense in liquid form than solid? It’s often cited that a significant reason why life evolved on earth is because water, with its hydrogen bonds, expands and so floats; hence lakes tend to freeze over on the top rather than freeze from the bottom up as would be the case with methane. It would be hard for life – as we know it – to begin in an environment where seasonal changes are as severe as a change of state of the environment.”
On the other hand, part of the reason freezing kills Earthly life is because water expands when it freezes, thereby rupturing cells and tissues from the inside. If there were methane-based life in Titan’s lakes, say, then it would freeze along with them, sure, but it might not be damaged by the freezing; it would just go dormant until it thawed. So the fact that methane doesn’t expand when it freezes could actually be a net positive.
Scott G. wrote: “I’m curious if anyone out there is aware of any studies or predictions regarding what TYPE of life could possibly arise out of methane-based chemistry?”
Asimov’s famous “Not as We Know It” essay suggested that life in a methane solvent could be based on complex lipid molecules:
Asimov’s is a great essay, very mind-opening about the possibilities. Read it before you say “too cold” or “not suitable”.
I bugs me that above I wrote that “If you were a writer of very hard science fiction, it would still be allowable to have our first astronauts to that moon greeted by a large enthusiastic group of natives”, yet the likely surface hydrogen concentration, as indicated by that reverse hydrogen flow data should only be about 0.5%. Any active intelligent forms would have to have massive lungs, and spend hours recovering from a little activity if that activity was limited by the diffusion rate of hydrogen. Luckily, I think I have the answer.
On Earth many creatures produce hydrogen peroxide, and as complex a creature as a beetle produces it in high concentrations (20%). Houtkouper and Schulze-Makuch showed that if Martian life employed this chemical it would nicely explain most of the unexplained Viking life experiment data. If Martians did use H2O2, it would have been a rather expensive (and rather too uv sensitive for Martian conditions) to have been produced just for its antifreeze properties. If Houtkouper and Schulze-Makuch were indeed right its production would have been much more useful as a store of energy, and under Titanian conditions it would be exceptional in that role. If so the high availability of this powerful oxidant in the local food would absolve us of the need for metabolic rates to be limited by hydrogen uptake. Perhaps then, we can still entertain the possibility of being greeted with wild enthusiasm.
Maybe an open door, but the very low temperatures and low metabolism are also likely to result in very slow evolutionary rates. So, even if life arises in such an environment, it is not likely to develop much.
Ronald, it seems to me that we are forced to use a completely different class of reactions to those employed by Earthly biochemistry if there is any measurable rate of biochemistry on Titans surface. Also, if that hydrogen flow data holds up, it must be due to chemical reaction rates that belie Titans cold condition. The real limit then by these indications should be the typical molecular speeds and diffusion rates, which are at least 50% as high as those typical for Earth,so if this flow is due to Titanian biochemistry, we still have a possibility of metabolism and evolution at about 60% Earth’s rate. You can’t dismiss my reception committee that Easily!
At least three things are being conflated n this conversation: metabolic products (oxygen/carbon dioxide versus methane), solvents (water/ammonia versus methane and other hydrocarbons) and temperature ranges.
Hydrocarbons make terrible solvents, because they only dissolve non-polar molecules. You can also have exotic metabolites while being pedestrian otherwise, as most terrestrial thermophiles are. Hydrocarbons also are the only carbon compounds that don’t produce the type of complexity conducive to life. So, yes, life beyond earth may not use water as its main solvent — but it will still have to use a polar compound instead of it, like ammonia. Open brains, falling minds and all that jazz.
Athena, surely we need further information before discussion can be bought to a productive focus, but for the moment allow me to take you back to methane as a solvent and dissolved hydrocarbons.
Normal hydrocarbons are definitely too to inert and limited in reaction scheme to be the basis of high metabolism life, even at our temperatures. I am not absolutely certain that the same could be said for free radical hydrocarbons at 90K (I suspect that a little Nitrogen is also involved, since where does all that thiolin bound N that is NOT on Titans surface end up?). My guess is that most hydrocarbon free radicals should have a low enough dipole moment to dissolve, and peroxides seem a decent guess as to the sort of chemicals that might be involved in generating them in intermediate metabolism.
What an idiot I am. How could I simultaneously postulate that Titanian life was based on free radicals and to also say that we could at the very least use diffusion limitations to hypothesise that life could be expected to be a few ten’s of percent slower there. Superoxide dismutase is one of the few enzymes that I can think of that has a free radical substrate and, low and behold, we find that catalytic rates are sometimes a hundred times greater than the rate at which diffusion can be expected to find the active site!
The key is that free radicals can be guided to the active site by magnetic fields. Free radical catabolism based intelligent life on Titan is bound to come to the mistaken belief that, even if life in Earth’s oceans is possible, the fact that it is too warm to use many free radicals would make it hundreds of times more sluggish than life on Titan due to the sever constraints of diffusion limitations!!!
The predominant biochemical cycle of a biosphere will be constrained by the redox state of the planetary environment. Recent papers that have appeared concerning proposed super-Earths, massive enough to hold onto a lot of hydrogen and to be warmed by a “hydrogen greenhouse,” got me thinking about what kind of biochemical cycle involving both respiration and photosynthesis could work on such a world with a highly reducing environment. It turns out that one can construct a plausible alternative from various biochemical reactions we already know about on the Earth, but the overall result is quite different: heterotrophs that would “breathe” and autotrophs that would excrete hydrogen, rather than oxygen.
Assuming carbohydrate for energy storage, then fermentation gives:
2 (CH2O) –> CO2 + CH4.
(Parenthesis marks used because the editor here does not interpret regular brackets correctly)
Methane is exhaled and the CO2 further reduced with inhaled hydrogen in a
similar manner to methanogenesis:
CO2 + 4 H2 –> CH4 + 2 H2O.
The overall reaction for respiration, where hydrogen is inhaled and methane
and water are exhaled is therefore:
(CH2O) + 2 H2 –> CH4 + H2O.
Photosynthesis would work the other way around, although unlike on the Earth the photochemical splitting of water would be used to provide an oxidant, rather than a reductant, so hydrogen would be evolved rather than oxygen.
CH4 + H2O –> (CH2O) + 2 H2.
The overall biochemical cycle predicted to occur within such a highly
reducing environment can therefore be summed up on one equation:
CH4 + H2O < --> (CH2O) + 2 H2,
where reading to the right gives you photosynthesis and reading to the left
gives you respiration. It is worth noting that since these are anaerobic
reactions, there is a much lower energy flux embodied in this cycle than
within the oxygenic biochemistry we have on Earth.
Martyn Fog, how much of a problem is it really that “there is a much lower energy flux embodied in this cycle than within the oxygenic biochemistry we have on Earth.”
Firstly, in anaerobic metabolism on Earth, hydrogen is rare, and so the difference in energy available to organisms of aerobic v anaerobic metabolism here is very great. To remove this guilt by association, I note that reducing formaldehyde (I’m unsure what carbohydrate extraterrestrial biochemistry will utilise, and methanal is the simplest carbohydrate with the most data) with hydrogen releases fully a third the power of its oxidation with oxygen.
Secondly, carbohydrate would also be three times easier to synthesise, so plant would just make three times as much, and an equal amount of energy would thus be available per sunlight energy captured.
Thirdly, starch typically holds about ten times its weight in in vivo water of hydration here on Earth. If its energy density were a problem, we could use (unsaturated) fats more liberally as our store of energy. Alternatively, if our medium of life were nonpolar (as it would be for methane based life) sugars would be naturally anhydrous, and thus would be expected to have a much higher effective energy density than they do here on Earth!
Fourthly, the metabolism of large active animals on Earth is limited by the rate at which oxygen can be absorbed. Per mole oxygen might give three times the energy of hydrogen, but it also diffuses four times more slowly than hydrogen, so it is a slightly more potent gas per atmospheric mole fraction. If we do the same calculations by weight hydrogen would win the contest potential to deliver energy flows to active life by a factor of twenty.
So which is really the better gas for intelligent life forms?
Oops. I forgot that it takes twice as many molecules of hydrogen to reduce carbohydrates as it takes oxygen to oxidise it. That makes hydrogen only ten times better, per atmospheric weight fraction, at energy delivery.
Researchers build computer model that explains lakes and storms on Saturn’s moon Titan
January 4, 2012 by Marcus Woo
Saturn’s largest moon, Titan, is an intriguing, alien world that’s covered in a thick atmosphere with abundant methane. With an average surface temperature of a brisk -297 degrees Fahrenheit (about 90 kelvins) and a diameter just less than half of Earth’s, Titan boasts methane clouds and fog, as well as rainstorms and plentiful lakes of liquid methane. It’s the only place in the solar system, other than Earth, that has large bodies of liquid on its surface.
The origins of many of these features, however, remain puzzling to scientists. Now, researchers at the California Institute of Technology (Caltech) have developed a computer model of Titan’s atmosphere and methane cycle that, for the first time, explains many of these phenomena in a relatively simple and coherent way.
Full article here:
Dan raised the question whether methane lakes would freeze seasonally from the top or the bottom? On Titan, I doubt whether they freeze seasonally at all — if methane ice occurred anywhere, you’d expect it at the north and south poles, but they are exactly where those big liquid lakes are found. Titan may seem cold to us earthlings, but it’s probably too warm for methane ice hockey.
FrankH suggested that life on Titan could be mistaken for a rock…
I’d agree that an organism (or colony of organisms) could conceivably look like a rock from outside. However, if it uses a liquid solvent to do its internal chemistry, surely it will be wetter inside than the average rock? If it resembles any mineral, it may be more like a hunk of mud, or perhaps a dripping stalactite.
Nice web page Colin Robinson.
What I can’t help noticing is there seems no reason to believe that that hydrogen altitude variation data might be just the tip of the iceberg. If confirmed in extent and as a biological phenomenon, it caps Titans minimum biological activity at 20W/sqkm. If life there covers its entire surface and is as efficient at extracting energy from sunlight as sugarcane under ideal laboratory conditions, it would be 100,000W/sqkm. This is as active as our oceans, even though Titan has just 1% our sunlight levels. By contrast, I can think of two scenarios for an Europan ecosystem that cap its activity at 0.0001W/sqkm (one being that it is fed by hydrothermals). How are we to believe Europa interesting and Titan not even worth a peek?
Of cause, I would be astonished if it was anything like as active as Earth, but I am anxious to dispel the notion that all but the most sophisticated such searches would be a complete waste of time.
After saying all that, I would still wager against there being any life there, but don‘t tell those NASA budget trimmers.
How sophisticated would a search have to be to find methane-based organisms, if they are there?
One point to consider is that methane is a weaker solvent than water; which logically means you need more solvent to do a given amount of chemistry. (Ethane, which is also a feasible solvent on Titan, is similar to methane in this regard.)
The cells would have a more dilute cytoplasm, compared with water-based life. To explain the apparent amount of hydrogen/acetylene consumption, they would need to be either larger or more numerous, than if they were water-based.
That may make them easier to find and photograph, by means of a robot lander with a magnifying camera.
Could Dust Devils Create Methane in Mars’ Atmosphere?
by Nancy Atkinson on September 10, 2012
Methane on Mars has long perplexed scientists; the short-lived gas has been measured in surprising quantities in Mars’ atmosphere over several seasons, sometimes in fairly large plumes.
Scientists have taken this to be evidence of Mars being an ‘active’ planet, either geologically or biologically. But a group of researchers from Mexico have come up with a different – and rather unexpected – source of methane: dust storms and dust devils.
“We propose a new production mechanism for methane based on the effect of electrical discharges over iced surfaces,” reports a paper published in Geophysical Research letters, written by a team led by Arturo Robledo-Martinez from the Universidad Autónoma Metropolitana, Azcapotzalco, Mexico.
“The discharges, caused by electrification of dust devils and sand storms, ionize gaseous CO2 and water molecules and their byproducts recombine to produce methane.”
Full article here: