Astrobiology studies the origin, evolution, and possible distribution of life in the universe. One important aspect of astrobiology is the search for biosignatures and bioindicators. Biosignatures are byproducts of living organisms that indicate life may exist in an environment. Bioindicators, in contrast, are characteristics of environments that show their potential to harbor life. To learn more about the difference between the two, we spoke to Dr. Margarita Safonova of the Indian Institute of Astrophysics. She also explained why argon may be a bioindicator for intelligent life, and she furthermore described pathways for individuals who would like to work in astrobiology.
What is the difference between “biosignatures” and “bioindicators”?
Actually, these terms are often used interchangeably in the literature. But it is good to go by their literal meanings. A biosignature is an actual bioproduct of living organisms. It can be metabolic gases – like methane in Earth’s atmosphere, 90% to 95% of which is biological in origin. Or it could be like the phosphine reportedly being detected in Venusian clouds – phosphine is produced biologically on Earth and is known to be quickly destroyed in rocky planets’ atmospheres or on their surface. Therefore, its continuous presence indicates a constant supply and, since we do not currently know of any inorganic source, phosphine could be considered a biosignature.
Biosignatures could also be surface features, like the vegetation red edge, so called because of the steep rise in Earth’s reflection spectrum at around 720 nm. The primary reason for this is the molecule chlorophyll. Such reflection characteristics could be useful for detecting photosynthesizing organisms on distant planets. Since all planets where we are searching for life are very far away, spectroscopy is an important tool. We can use either direct spectroscopy, like when we look at Earth from orbit, or we can use transmission spectroscopy, like when stellar light passes through another planet’s atmosphere.
Bioindicators, on the other hand, are parameters that indicate life is possible on a planet. A bioindicator could be water, for example. It is an indicator of potential habitability since water is essential to our kind of life. Oxygen in this sense is both a biosignature and a bioindicator. It’s a biosignature because it is a byproduct of oxygenic photosynthesis by living systems. And it’s a bioindicator because it indicates the possibility of complex life since complex, highly organized life on Earth requires high levels of oxygen.
Why do you propose that argon could be an appropriate bioindicator?
Apart from biosignatures, which would indicate the presence of life, people also discuss technosignatures – meaning signatures of developed intelligent civilizations. The pollution in our atmosphere, for example, could be considered a technosignature. Some astrobiologists have proposed that once multicellular life arises, intelligence is the next logical step. But the only “laboratory” we have to derive the results from is here on Earth. We are trying to understand what environmental or physical conditions on a planet can facilitate development of intelligence and how we can infer it.
We have noticed two seemingly disconnected facts. One is that argon is a third major constituent in the atmospheres of planets that are considered potentially habitable (this includes Titan). Interestingly, argon is sometimes more abundant than oxygen. The second fact is that argon is increasingly being reported to exhibit neuroprotective properties, especially for organisms under hypoxic conditions. Humans exposed to hypoxia, for instance, have reportedly been able to perform complex manual and mental tasks when they are located in an argon-enhanced atmosphere (compared to nitrogen-enhanced). Additionally, argon protects kidney, heart, and lung cells; it has organoprotective properties.
Argon’s most potent function, however, is its neuroprotective function. It has been discovered that argon attenuates brain injury, reducing brain inflammation and preserving neurons from damage due to ischemia. Studies have shown that argon is especially active in the cells responsible for higher functions – such as microglia, neurons, and astrocytes.
Based on the special neuro-related properties of argon described above, we have suggested that argon in the atmosphere of a rocky planet may be a bioindicator of highly organized life. By this we mean life similar to “intelligent” life here on Earth, akin to humans or bees, with brain activity and the capacity for sophisticated behaviors such as learning, decision making, planning, and social interaction. Argon may be indicative of such life, just as water indicates rocky planets’ potential habitability or oxygen – with the advanced age of a planet – may indicate complex life has developed. For argon to be a bioindicator though, the planet also has to be potentially habitable, with water, an atmosphere, and an age sufficiently long enough to allow complex life to have evolved.
What advice do you have for people who want to study biosignatures and bioindicators?
That is a very interesting question. Astrobiology is a transdisciplinary subject integrating astrophysics, chemistry, biology, planetary science, and even ecology. It has also now become a challenging problem in astroinformatics, which is part of computational astrophysics. So, there are many pathways for people who want to study biosignatures or bioindicators. One can be an astronomer because biosignatures are detected spectroscopically and devising and understanding detection methods requires understanding astronomical techniques. Or one can be a biologist or chemist, since understanding the specifics of living organisms and the byproducts of their metabolisms requires knowing biochemistry. One can even be a geologist or planetary scientist, as it is necessary to know about abiotic production to be able to either confirm or discard signatures’ biotic origins.
What is important is to have a good background in one of these fields. Then it is possible to expand your knowledge into a neighboring field or to work with collaborators to combine knowledge. This subject of biosignatures is rather new, and I do not know of any textbooks yet. But there are some good reviews on the topic of biosignatures. One is right from when the concept was developed: an article from 2002 by Des Marais and others called “Remote sensing of planetary properties and biosignatures on extrasolar terrestrial planets.” There’s also another more recent one I would recommend checking out: an article from 2018 by Walker and others called “Exoplanet Biosignatures: Future Directions.”