Why Methane Can Be A Sign Of Life Outside Of Earth

Regardless of who we are or where we come from, everyone at some point in their lives ponders whether or not life exists beyond the Earth. For the first time in human history, we now possess the information and technology needed to detect life in other worlds. To do that, we must first identify the markers of life. Namely, what should we look for when determining if life exists in another world without actually traveling to other worlds? An often-cited chemical marker for living organisms is methane, an organic compound commonly associated with living things on Earth. However, methane can also be produced by processes that do not involve life. If we do detect methane in another world, should it be considered evidence of alien life? 

Where Does Methane Come From?

methane molecules
Structure of a methane molecule.

Methane is an organic compound composed of one carbon atom and four hydrogen atoms. Although methane isn’t overly common in the Earth’s atmosphere, it plays a vital role in our planet’s atmosphere. Like carbon dioxide, methane is a greenhouse gas that helps maintain a stable temperature across the Earth’s surface. On Earth, the vast majority of methane is produced by living organisms. Bacteria often produce methane in wetlands, and larger organisms will often produce it as a byproduct of digestion. When organisms die, methane is also released as they decompose. Most of the Earth’s methane is made through these biological processes, yet not all of Earth’s methane is produced by life. A small amount of methane is released through volcanic eruptions, a process that is likely common across countless worlds. The presence of methane alone is not evidence of life, yet how much there is and its existence alongside other chemicals can be. 

Why Detecting Methane Can Be Evidence Of Life

Methane is a fairly unstable compound. Sunlight will often break it apart quickly, and so if methane isn’t replenished over time, it will slowly disappear from a planet’s atmosphere. If not for biological processes, methane would be virtually nonexistent in Earth’s atmosphere. Although volcanoes do emit small amounts of methane, it is not nearly enough to replenish it over time. If a planet has a consistent amount of methane that is quickly replenished, it could mean that living organisms on the surface are producing that methane. Furthermore, other chemicals alongside methane could also offer evidence of alien life. On Earth, biological processes produce several chemicals alongside methane, the most notable of which are oxygen and carbon dioxide. If a planet has an abundance of methane alongside large amounts of oxygen and carbon dioxide, it could suggest that biological processes are occurring on the surface. If methane exists alongside other chemicals, most notably carbon monoxide, it would mean that volcanic eruptions or other geological processes produce methane. Volcanic eruptions emit large amounts of carbon monoxide, yet, on Earth, most of this carbon monoxide is absorbed by living things such as plants., If a planet’s atmosphere has a large amount of both methane and carbon monoxide, it would suggest that alien life is not responsible for the presence of methane. However, suppose a planet has an atmosphere abundant in methane but lacks an appreciable amount of carbon monoxide. In that case, it could not only suggest that there are living things producing methane, but also that there are living things absorbing the carbon monoxide. An observation such as this would be a strong indicator of alien life. 

On Earth, methane is a prime indicator of life, yet the existence of methane alone is not necessarily evidence of life. The amount of methane, how often it is replenished, and its existence alongside other chemicals would offer strong evidence for the existence of alien life in another world. For the first time in human history, technology has become advanced enough that scientists can now begin looking at other planets and determining their atmospheres' composition. It is likely only a matter of time before scientists uncover a world with an atmospheric composition that mimics the Earth’s. The answer to one of humanity’s most important questions may just hinge on whether or not we detect methane.