Are we alone in this universe? Many have dedicated their lives to answering this question. In the process, they have developed numerous ways to identify possible signifiers of life, specifically biosignatures. Biosignature is an “object, substance, and/or pattern whose origin specifically requires a biological agent”, or something only biological processes produce, which can be used as an indication of living organisms. Continuing our last article, we investigate different biosignatures, and where/how to find them!
Biosignatures 101: The Basics
Astrobiologists attempt to find biosignatures of all shapes and forms on Earth, other worlds, and deep space as an indication of the habitability of a planet and the probability of life on it. There are numerous biosignatures developed and used in the field, but the ultimate goal is to search for a universal feature that is common to all lifeforms.
It is important to note two things when looking at biosignatures:
The significance of a potential biosignature depends on how likely it was produced by biological processes, as opposed to other abiotic (non-living) activities. For this reason, it’s important to evaluate the risk of false-positive detections and understand how the biosignature may be produced by abiotic natural phenomena.
When investigating biosignatures of varying levels of complexity in nature, it is likely that those simpler and less complex can be observed more frequently across multiple environments. For example, astrobiologists often use spectral signatures of a planet’s atmosphere due to their relative simplicity (compared to biopolymers) and the increased conservation across organisms.
While many biosignatures exist in nature (see Fig. 1), only the organic, geological, and spectroscopic (chemical) aspects will be discussed in this article.
Organic Biosignatures: We’re Left-Handed!
Biosignatures show definite signs of life including biological macromolecules (lipids, carbohydrates, nucleic acids, and proteins or DNA). For example, lipids from the cell’s membrane are typically used to show where early life has formed on Earth.
In our last article, we explored the significance of water and carbon being the fundamental building blocks for life. Interestingly, the left-handedness of an amino acid and the right-handed feature of sugars may be a way to identify life.
What do we mean by left and right-hand? Well, whether you realize it or not, each amino acid and sugar have a mirror twin! Molecules can assemble in two ways resulting in reflected structures. Imagine a glycine molecule, a carbon atom with four bonds. Let’s say it bonds with two hydrogen atoms, then the molecule remains symmetric regardless of which two bonds it interacts with. But if you “swap a hydrogen for a heavier atom”, then the molecule becomes asymmetrical and forms two mirrored versions, also known as “chirality”.
To the extent of human knowledge, natural selection seems to favor left-handed (“L”) amino acids and right-handed (“D”) sugars, including carbohydrates. This phenomenon is referred to as “homochirality”. This informs astrobiologists of the environments in which life may form and elicits many hypotheses regarding the possibility of varied homochirality and even stereoisomer (isomers with the same molecular formulas and atom arrangement but different spatial orientation) biology.
Geological Biosignatures: Rocks Will Remember You
Any physical mark left behind by a living organism, including biomolecules, cells, tissues, and waste can be preserved by rocks and minerals.
There are many ways geological biosignatures are used to offer insight into the living processes, for example:
Extractions from clay/silica-rich rocks and sediments can demonstrate the presence of ancient biomolecules
Mold-and-cast fossils: Even though they may not have any chemical residues useful, they can still be an important biosignature if their structure is inexplicably complex, functionally adaptive to the environment, and highly organized, i.e. dinosaur skeletons Likewise, a biosignature becomes more ineffective when it comes to simpler structures like bacteria as it can be replicated by other abiotic processes (as mentioned above).
Not only does this reveal the history of early life on Earth, but this method can also be applied to any samples collected from otherworldly sources, rendering a highly useful albeit uncommon tool in the search for extraterrestrials.
Spectroscopic Biosignatures: The Gassy Method
When you can’t analyze samples up close as the previous approach requires, what should you do?
Well, spectroscopy can confirm the presence of life from space. Probes can record the reflectance spectra of gasses in a planet’s atmosphere as light is absorbed and subsequently transmitted by them. Using this method, the atmospheric composition can be determined.
There are four gasses that are linked to likely signs of life:
1. O₂ - Oxygen
Produced by photosynthesis, a metabolic pathway for living organisms. However, oxygen can also be produced from abiotic factors like photolysis from water vapor.
2. O₃ - Ozone
A product of oxygen-light reactions in the atmosphere.
3. CH₄ - Methane
May indicate methanogenesis by bacteria and microorganisms, even though it is also produced by other abiotic geochemical pathways.
4. N₂O - Nitrous Oxide
Produced by microbial denitrification and lighting.
Despite possible abiotic sources for these gasses, they are promising biosignature gasses that allow astrobiologists to ascertain the probability of an exo- or extrasolar planet being capable of life.
WATCH this video to learn more about how gasses produced by biology are explored!
Furthermore, the reflectance spectra can also be used to suggest potential biomass and terrain on the planet’s surface. If we look at Earth’s reflectance spectrum (Fig. 4), there’s a sharp increase in reflectance rate at the wavelengths 680-720 nm. This is known as the “vegetation red edge” because there’s a prominent absorption of red light by chlorophyll in plants.
To summarize…
Astrobiologists use varying potential biosignatures to identify likely exoplanets holding life. From chiral biomolecules to rocks and the reflectance spectra, signs of life can be found in many aspects, all of which are promising and huge if true. Watch out for our next and final article in this series to learn more about extremophiles, and the current ongoing Search for Extra-Terrestrial Intelligence (SETI)!
Works Cited
Chemistry LibreTexts. (2019). 2.7: Isomerism Introduction. [online] Available at: https://chem.libretexts.org/Courses/Sacramento_City_College [Accessed 28 Oct. 2023].
Hays, L.E., Graham, H.V., Des, D.J. and Lynch, K.L. (2017). Biosignature Preservation and Detection in Mars Analog Environments. [online] ResearchGate. Available at: https://www.researchgate.net/publication/313494550_Biosignature_Preservation_and_Detection_in_Mars_Analog_Environments#pf5 [Accessed 28 Oct. 2023].
McMahon, S.M. (2021). Astrobiology (Overview). [online] doi:https://doi.org/10.1093/acrefore/9780190647926.013.1.
NASA, ESA and Hustak, L. (2021). Reflectance Spectra of Materials on Earth’s Surface. [online] Webb Space Telescope. Available at: https://webbtelescope.org/contents/media/images/01F8GFAGTM98YTKDS0FZAAWWV2 [Accessed 28 Oct. 2023].
Open Education Edinburgh (2014). 8.2. ASTROBIO - How to look for Biosignatures. YouTube. Available at: https://www.youtube.com/watch?v=PDe8vq5t-sI [Accessed 28 Oct. 2023].
Seaton, K.M., Cable, M.L. and Stockton, A.M. (2021). Analytical Chemistry in Astrobiology. Analytical Chemistry, 93(15), pp.5981–5997. doi:https://doi.org/10.1021/acs.analchem.0c04271.
Sedbrook, D. (2016). Must the Molecules of Life Always be Left-Handed or Right-Handed? [online] Smithsonian Magazine. Available at: https://www.smithsonianmag.com/space/must-all-molecules-life-be-left-handed-or-right-handed-180959956/ [Accessed 28 Oct. 2023].
Tasker, E. (2015). Why is life left-handed? The answer is in the stars. [online] The Conversation. Available at: https://theconversation.com/why-is-life-left-handed-the-answer-is-in-the-stars-44862 [Accessed 28 Oct. 2023].
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