How Methane Tells a Story of its Origin

A flare burns excess methane from an energy-producing well near Karnes City, Texas.

Aaron M. Sprecher/Associated Press

Methane is important, not only because it's the major component of natural gas -- a widely-used clean energy that can power the electric lamp, but also because it's a powerful greenhouse gas that traps the heat of the sunlight and is the second-largest contributor to global warming after carbon dioxide. Given its great importance, we may wonder where and how was it formed? What was the temperature of its formation? What kind of journey did it experience before it is finally exposed to the atmosphere? The answer might have recorded by a small group of rare methane molecules, waiting for us to explore.

Radiative forcing due to major greenhouse gases (GHGs). The y-axis represents the energy trapped by the gas per square meter area, which is termed radiative forcing. The longer the bar, the stronger the contribution to the greenhouse effect. The red bar corresponds to carbon dioxide. The green bar is for methane. Please note that despite the atmospheric methane concentration is about 200 times lower than carbon dioxide, the contribution of methane is equivalent to about 1/3 of carbon dioxide. (Source: National Academies of Sciences, Engineering, and Medicine, 2018.)

The Rare Methane Molecules

Methane (CH4) is a chemical with one atom of carbon (C) and four atoms of hydrogen (H).  Although still composed of one carbon and four hydrogens, rare methane molecules have uncommon flavors for atoms. For simplicity, imagine yourself stepping into a Starbucks, "I'd like a cup of Latte, triple espresso, almond milk, and strawberry sauce." Now multiple uncommon flavors are gathering in a cup of Latte, which makes it a "rare Latte". The same thing can be done for methane, "I'd like a molecule of methane, with the carbon atom slightly heavier than usual, and one normal hydrogen atom replaced by a twice heavier hydrogen atom." These heavier atoms you required are termed "isotopes". Anyway, now you get a rare methane molecule. 

 A real photo of common Latte (left panel) and a conceptual structure diagram of common methane and rare methane (right panel). A common methane molecule has four hydrogen atoms with a relative mass of 1 and one carbon atom with a relative mass of 12. A rare methane molecule here has three hydrogen atoms with a relative mass of 1, one hydrogen atom with a relative mass of 2, and one carbon atom with a relative mass of 13. This rare methane molecule is an example of methane clumped isotopologue.

In a parcel of methane that contains a huge number of methane molecules, the proportion of rare methane molecule is normally quite small because the chance for two rare isotopes meeting and combining on one methane molecular is very small. 

"How do rare methane molecules tell a story?" It's complex.

Generally speaking, it's the proportion of rare methane molecules (or how clumping the parcel of methane is) that conveys information. To get an idea of what is "how clumping", let's first check our university.

UMD is a diverse campus. Students have different genders, ages, nationalities, and majors. If socializing happens completely randomly on campus, you will have the same probability of becoming friends with anyone. However, a student majoring in geology may find himself or herself surrounded by friends from the geology department because they both like rocks. A Chinese student may find that (s)he usually chats with Chinese because of their common language and background. These are examples of the clumping phenomenon. The clumping is determined by the inherent attributes of students, such as major and nationality. The combination of each person's social circle shapes the social network of UMD that has a unique clumping signal.

Social network visualization plot. The circles represent the members inside the network. The lines indicate the interactions (e.g. emails) between two members. Larger circles correspond to more interactions for the individual. The areas with large and dense circles indicate the social center, or more clumping area. The areas with small and sparse circles belong to people who are less involved in social activities, or less clumping area. (Source: Grandjean, 2014.)

However, this balance can be broken by certain processes. COVID is a good example. Due to the outbreak of the epidemic, everyone follows social distancing. The clumping signal is significantly reduced.

The same principle can be applied to methane. The substitution of normal atoms with rare atoms will change the energy of the methane molecule, just like using strawberry sauce instead of caramel syrup will make a Latte with a different flavor. The energy difference, combined with the ambient temperature, determines the specific characteristics of clumping which may directly reflect the formation temperature of methane. However, processes such as microbial activity and mixing will change the clumping signal in a specific direction. In these cases, although we lost the information of temperature, we may infer the processes that methane went through. Taken a sip of the Latte, you can tell the syrup it uses. Given a sample of methane, I may tell you the story of its origin.

References:

National Academies of Sciences, Engineering, and Medicine. 2018. Improving Characterization of Anthropogenic Methane Emissions in the United States. Washington, DC: The National Academies Press. doi: https://doi.org/10.17226/24987.

Grandjean, Martin (2014). "La connaissance est un réseau". Les Cahiers du Numérique 10 (3): 37-54. DOI:10.3166/LCN.10.3.37-54.