written by: jessica wang and sorina andrei
graphics by: yunyi cui
The key molecule involved in life on Earth was already discussed in Water, but life makes many different compounds, so which ones are researchers looking for?
The atmospheres of exoplanets are filled with a variety of gases from the belching volcanos, geological processes, and most importantly life. The ones that could be “produced by life and accumulate in a planet’s atmosphere to detectable levels” are called biosignature gases (1). The most researched ones include oxygen, ozone, nitrous oxide and methane, but a more inclusive list was detailed by Seager, Bains, and Petkowski (2). However, since scientists do not know if all life in the universe is like Earth, the basic idea is to look for compounds in concentrations or amount in the atmosphere out of thermodynamic equilibrium, which means the amount is less or more than expected if only natural processes like volcanic activity are involved (1). There are some important considerations. For example, high levels of UV radiation from stars can produce OH on planets with hydrogen in its atmosphere and destroy nearly all biosignatures (3).
Types of Biosignatures
A paper by Seager, Bain and Hu classified biosignatures in three types in the context of metabolism (4).
Type I: These gases are involved in central functions crucial to obtaining energy from the environment. For example, for animals, these may include oxygen and carbon dioxide produced when breathing. These gases are abundant, so they are easily detected, but they often also lead to many false positives, since it can either be produced from organisms or from spontaneous natural processes.
Type 2: These gases are involved in chemical reactions needed for building biomass, such as carbon dioxide required when a human child grows up and gains more mass. However, these are not useful because they are not specific enough for life (5).
Type 3: These gases are very special and specific to the type of organism, and is unlikely to be produced from natural processes. For example, perfluorocarbons, used in industries as solvents and refrigerants, are almost entirely made by humans, as such the presence of such gases would strongly indicate that there is life on the planet. Another example would be dimethyl sulfide produced by plankton (1). However, because these are not crucial for survival and are so specific, they are often present in such low quantities that it is hard to detect.
Although gases produced by humans and animals are described, most of the time, researchers are looking for gases from microorganisms, which is more diverse. The most researched gases include the greenhouse gases nitrous oxide and methane.
Nitrous oxide
Nitrous dioxide or N2O is a Type I biosignature gas (3,4). On Earth, it is mostly produced organically by nitrifying bacteria in the soil oxidizing ammonia with atmospheric oxygen or hydrogen, but this process can also occur naturally by geochemical processes (3) or from flares from the host star (6).
It has been shown that it has a long atmospheric lifetime in Earth-like (6) and thin hydrogen-dominated atmospheres, because the rate of breakdown from UV is slower than the reaction with oxygen or hydrogen (3). As such, even for planets experiencing high levels of UV radiation, like the Earth, it can act as a biosignature gas (3).
Methane
Methane or CH4 is a type I biosignature gas (4). Methane is photochemically unstable, as such it needs to be continuously replenished or produced (1), so its presence can indicate lifeforms continuously producing it. On Earth, it is produced by humans and animals, but also by geological processes such as serpentinization, which is the process of changing minerals with water, or volcanic activity, which resulted in high levels of methane during the early days of Earth (6). It can also interact with other molecules, such as ozone, another biosignature, so methane’s concentration and interactions requires further study (6).
Conclusion
There are many atmospheric gases and other signatures in general that can indicate the presence of life on other planets. However, researchers still have a lot to learn about the concentrations and interactions of these various gases.
References
Seager S, Bains W. The search for signs of life on exoplanets at the interface of chemistry and planetary science. Sci Adv. Maart 2015;1(2):e1500047.
Seager S, Bains W, Petkowski JJ. Toward a List of Molecules as Potential Biosignature Gases for the Search for Life on Exoplanets and Applications to Terrestrial Biochemistry. Astrobiology. Junie 2016;16(6):465–85.
Seager S, Bains W, Hu R. BIOSIGNATURE GASES IN H 2 -DOMINATED ATMOSPHERES ON ROCKY EXOPLANETS. Astrophys J. 18 Oktober 2013;777(2):95.
Seager S, Bains W, Hu R. A BIOMASS-BASED MODEL TO ESTIMATE THE PLAUSIBILITY OF EXOPLANET BIOSIGNATURE GASES. Astrophys J. 12 September 2013;775(2):104.
Seager S. The future of spectroscopic life detection on exoplanets. Proc Natl Acad Sci. 02 September 2014;111(35):12634–40.
Grenfell JL. A review of exoplanetary biosignatures. Phys Rep. November 2017;713:1–17.
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