Is There a Methane Habitable Zone?

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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.