Is There a Methane Habitable Zone?

Article written: 15 Nov , 2011
Updated: 24 Dec , 2015
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For a long time now, we have heard the mantra “follow the water” when it comes to searching for life elsewhere. Life as we know it here on Earth requires liquid water, whether it is tiny microbes or elephants. It has thus been assumed that carbon-based life somewhere else that is basically similar to ours in its chemical makeup (another assumption) would also require water for its survival and growth. But is that necessarily true? In recent years, more consideration has been given to the possibility that life could develop in other mediums as well, besides water. A liquid is still ideal, for allowing the necessary molecules to bond together. So what are the alternatives? Well, one of the most interesting possibilities is something we have already seen now elsewhere in our solar system – liquid methane.

It should be noted that the importance of water cannot be overlooked. According to Chris McKay, an astrobiologist and planetary scientist at NASA’s Ames Research Center, “We live on a planet where water is a liquid and we have adapted and evolved to work with that liquid. Life has very cleverly used the properties of water to do things not just in terms of solution, but in using the strong polarity of that solution to its advantage in terms of hydrophobic and hydrophilic bonds, and using the very structure of water to help align molecules.”

But McKay also published a paper In the journal Planetary and Space Science last April, postulating how life on some worlds could use liquid methane in place of water. There could be planets orbiting red dwarf stars, which are smaller and cooler than our Sun, and could have a “liquid methane habitable zone” where methane could exist as a liquid on the surface of planets orbiting within that zone. They could also exist around Sun-like stars, although they would be easier to detect around the smaller, dimmer red dwarf stars. But there is already one methane world that we know of, much closer to home…

Orbiting the sixth planet out from the Sun, Saturn, is a moon which in some ways is eerily Earth-like, with rain, rivers, lakes and seas – Titan. It is the first world we’ve found so far that has liquid on its surface like Earth does. But there is one major difference; the liquid is not water, it is liquid methane/ethane. With temperatures far colder than anywhere on Earth at –179 degrees Celsius, water cannot exist as a liquid, it is frozen as hard as rock. But methane can exist as a liquid under those conditions and indeed does on Titan. Beneath an atmosphere that is thicker than ours (but also made primarily of nitrogen), the surface of Titan has been modified in much the same way as Earth’s; liquid methane plays the same role there as water does here, with a complete hydrological cycle. It is like a familiar-looking but colder version of our planet, which has raised the question of whether an environment like this could even support life of some kind.

McKay had also previously suggested that methane-based life could consume hydrogen, acetylene and ethane, and exhale methane instead of carbon-dioxide. This would result in a depletion of hydrogen, acetylene and ethane on the surface of Titan. Interestingly, this is just what has been found by the Cassini spacecraft, although McKay is quick to caution that there could still be other more likely explanations. There is still a lot we don’t know about Titan. Whatever the explanation, there is some interesting chemistry going on.

At the very least, Titan is thought to represent conditions similar to those on the early Earth, a sort of primordial Earth in deep-freeze. That alone could provide vital clues as to how to life took hold on our planet. If there are other planets or moons out there that are similar, as now seems likely, they could also reveal valuable insights into the question of the origin of life, whether there is anything swimming in those cold lakes and seas or not. While water is still considered the primary liquid medium of choice, liquid methane could be the next best thing, and if we have learned anything, it is how amazingly adaptive and resourceful life can be, perhaps even more than we think.



22 Responses

  1. Anonymous says

    Since chemical reactions are much slower at this temperatures: how long would life take to envolve there?

    • Anonymous says

      It’s had 4 billion years or so. I think that’s more than enough time.

      • Ray Fowler says

        Spoken like a true Earthling

      • Anonymous says

        See “Arrhenius equation”. The reactions might take twice the time if the temperatur is 10 K less. So we talk about the factor 1:1.000.000.

      • Ray Fowler says

        Are you suggesting that the soup of complex hydrocarbons and the methanological cycle we see on Titan would have taken only 4,500 years to develop on Earth?

      • Ray Fowler says

        ok, I looked it up. Here’s what wiki says about it:

        “A historically useful generalization supported by the Arrhenius equation is that, for many common chemical reactions at room temperature, the reaction rate doubles for every 10 degree Celsius increase in temperature.”

        Operative phrase: “at room temperature”

      • Torbjörn Larsson says

        It is arguable.

        The historical observations, as Ray Fowler points out, are few and seem to conflict with modern ones. Especially for a few rapid reactions, rates increase much faster with temperature.

        In Arrhenius theory, the leading factor (A) is the collision frequency, which in this case of high pressure liquid is likely fairly comparable with water at room temp. (I haven’t time to estimate, sorry.)

        The exponential factor is the probability that a collision will give a reaction. It depends on the activation energy, which can do the damnedest things, barrierless reaction’s rates can _decrease_ with temperature.

        We need observations of reaction rates to go further. Notice that we can’t use the above estimates on atmosphere lifetime, since the tholin production happens by photoactivation high up in the atmosphere.

      • Torbjörn Larsson says

        Unfortunately we don’t know that. If it was a once only release and no further supply, the methane could be less than 10 – 50 Ma old, which is the time it would take to form more complex hydrocarbons out of it.

    • Tom Litton says

      I have the same question, i just want to ask it a different way:

      What are the chances that life would develop, but not evolve to the point that it would be obvious on the pictures that we have seen already?

  2. Anonymous says

    Wouldn’t it be amazing to land and shake the hand of an ice-being, meanwhile introducing him to fire-beings! Does Methane still burn at those frigid temps? Could we light a match and watch a whole lake go up?
    Something that has always intrigued me is how the lake shores on Titan are akin to a reservoir, not a natural, long lived lake. A lake tends to make its shoreline with currents, ice and wave action. The Titanese lakes seem very still in comparison.

  3. Torbjörn Larsson says

    A technical point: McKay points to the modeling of Titan’s atmosphere as a likelier explanation to a surface hydrogen sink. However astrophysicists I have spoken with, as well as the paper itself IIRC, claim that it is very hard to explain the deficit in any realistic atmosphere model. The competing hypotheses is some inexplicably efficient catalyst or other inorganic surface sink.

  4. Anonymous says

    It might be one thing for liquid methane to have a geological analogue with water. It is an entirely different thing with biochemistry. The polar water molecule plays a vital role in the mechanism of bio-molecules, in particular that most important one DNA. The structure of water and its interaction with other molecules is very unique. Methane is the complete opposite; it is nonpolar.

    The temperature is another factor. Clearly at these low temperatures the rate of chemical reactions is much slower. I suppose I am pretty much in the skeptics camp on the prospects for this type of alternative biochemistry.

    LC

  5. Prof. Michael O. Zimmermann Ph says

    …what bout life based on silicon, or even energy? Just because we have not researched the infinite possibilities we cannot just dismiss the concepts by using our form of organic life as the only example from which to deduct other concepts.
    Titan is an intriguing world, which should be on the list of a human led exploration besides the moon, Mars, and the asteroid belt. Maybe it should be elevated to No. 1 on that list.

    • squidgeny says

      Nobody’s dismissing those concepts… but it’s better to start by searching for the kind of life we know can exist. There’s no point wasting money on searching for silicon-based life until we have some good theories about how silicon-based life can develop and exist.

  6. John Anderson says

    The McKay article in Planetary and Space Science is July 2011, not April.

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