Featuring image: Titan’s atmosphere is rich in organic molecules, but we still don’t know if there is life on Saturn’s icy moon. With JWST and the coming generation of telescopes, we will be able to observe the atmospheres of exoplanets. Is there a way to search for life on these distant worlds? NASA/JPL, public domain (CC0).
Authors: M. A. Thompson, J. Krissansen-Totton, N. Wogan, M. Telus and J. J. Fortney
Visiting and exploring exoplanets for extraterrestrial life still belong to the realm of science fiction. However, the coming generation of telescopes will enable us to look into the atmospheres of exoplanets and search for possible biosignatures, chemical compounds that could indicate the presence of life.
Searching for life on a planet is not a trivial task. Since the first Mars landing in 1976, scientists still search for recent or ancient traces of life. It becomes even more difficult on planets that we cannot directly visit. The next telescope generation will enable us to observe the atmosphere of distant planets remotely. Are there ways to find evidence of life in a planet’s atmosphere? A new study suggests that the freshly launched James Webb Space Telescope (JWST) could help us to search for life on other worlds.
Life changed Earth in many ways. One well-known example is the evolution of photosynthesis, which drove the oxygenation of Earth’s atmosphere atmosphere by cyanobacteria. But looking for other planets with an oxygen-rich atmosphere might not be the best path to finding extraterrestrial life. Oxygen is not uniquely a photosynthesis byproduct, but can also originate from abiogenetic sources. We also do not know how likely it is that photosynthesis is likely to happen on other planets.
Before cyanobacteria were able to harness sunlight to produce energy, most life used a chemical process called methanogenesis to feed. Moreover, in contrast to photosynthesis, methanogenesis was invented not only once, but multiple times independently by different organisms. The end product is always the same: methane. While there are other promising biomarkers like ammonia, none will be detectable in the atmosphere of exoplanets by the current or next generation of telescopes.
If we would find a planet with methane in its atmosphere, would it be strong evidence for life? Maggie Thompson and her coworkers tried to settle this question by considering different possible geological and astrophysical settings for terrestrial planets.
Multiple geological processes can lead to the accumulation of methane in a planet’s atmosphere, so the presence of methane alone doesn’t imply that a planet hosts life. But methane is not very stable to ultraviolet radiation. Depending on the planet’s host star, atmospheric methane lasts a few tens of millions to 100 million years. This decay means that methane has to be constantly produced to maintain substantial quantities in an atmosphere.
Volcanos can produce methane, but to outgas a similar amount of methane to that generated by biological processes, the oxygen content in its interior has to be much lower than in the Earth. Thompson looked into the melt reaction that would take place in such a low-oxygen mantle and concluded that most of the carbon will precipitate as graphite before it could outgas as methane in a volcano.
Serpentisation is another way to produce methane without life. When fresh oceanic crust comes in contact with water, the olivines in it reacts to become serpentinite. A similar water-rock reaction occurs when iron is reduced in metamorphic rocks or if fresh, iron-rich material of meteorites is altered during an impact. But all these processes on Earth produce not even 1\% of the methane that is produced by organisms. Only a giant impact could elevate the methane concentration in a planet’s atmosphere to higher levels for a while until it decays. Moreover, these rock-water interactions would lead to an atmosphere rich in methane and CO, but low in CO2, because methane and CO2 usually don’t exist in equilibrium. On the other hand, biogenic methane production can lead to an atmosphere rich in methane and CO2.
Thus, one biomarker might not be sufficient to decide if a planet is inhabited or not. But if we are able to find to two or more markers which cannot be produced by physical processes in the same planetary environment, chances are good that life might produce one of them. Life has the remarkable ability to push the inanimated world out of it equilibrium. This fundamental property of life might be the key in our search for extraterrestrial life and JWST will help us to look out for it.