In our previous article, we discussed the existence of habitable planets and the difficulties associated with locating them. In this article, we will discuss the confirmation of habitability, through bio signatures.
Bio signatures is a term describing any substance, such as elements and molecules, that may provide evidence of past or current life. Elements such as Oxygen, which is produced by plants, and a molecule such as methane, which is produced by animals. Such signatures are highly indicative of life, however, they are not necessarily a proof of life.
For a substance to be considered a bio signature, it must be a reliable sign of life, such that it is not commonly produced by abiotic processes. Further more, it must be detectable and must have a long enough lifetime for observers to detect it. The reliability of a bio signature is a very difficult thing to determine, it is an active research field, with some of the latest developments challenging the reliability of Oxygen as a bio signature, more on that after we discuss more types of bio signatures.
Chemical Disequilibrium
Living creatures are sometimes broadly defined as systems that use energy to recede from equilibrium. After all, living organisms accumulate energy in their bodies in order to stay alive. In the process of acquiring energy, disequilibrium is created in the surroundings. In the case of earth, that disequilibrium is easily seen in the methane and oxygen levels in our atmosphere. The misbalance of elemental and molecular proportions in the atmosphere and surface is one of the most reliable bio signatures, which leads us to our next type of bio signatures.
Agnostic Bio Signatures
Life on other planets may not produce the same signatures as life on earth, where life produces substances such as oxygen and methane, but that is not necessarily the case in other planets. It is almost unpredictable what other forms of life can produce, which is why it is not the most reliable method to look for specific substances as a bio signature. However, studying the chemical disequilibrium should in theory detect all kinds of life. Whether they produce the same substances as on earth, or very different substances. Bio signatures that don’t assume any specific biochemistry are called agnostic bio signatures.
Morphology
Other than the atmosphere, it is beneficial to analyze the surface of the planet to look for other signs of life. Organisms can very often affect the shape and form of their surroundings. Think of how plants and fossils shape the soil, such observations can be very strong indications of life, yet are very difficult to make. It is very rarely possible to see the form and shape of a planet’s surface, requiring probes and spacecrafts. Furthermore, wind and asteroid impacts can sometimes create shapes that might be mistaken for shapes made by life forms.
Atmosphere
We now arrive at our last and most important bio signature, the atmosphere of the planet. Chemical disequilibrium and agnostic bio signatures can both be observed in the atmosphere of the planet. The atmosphere contains the most amount of possibly observable bio signatures, and its studies are of most importance.
We now go back to discussing the reliability of bio signatures, specifically, chemical bio signatures.
Substances such as oxygen and carbon dioxide are not produced exclusively by life forms, there are some abiotic processes that can produce these substances, as a matter of fact, carbon dioxide, despite being one of the most common products of life forms, is not considered to be a bio signature, due to it being commonly formed by abiotic processes. This makes it necessary not only to observe the bio signatures themselves, but also to observe the environment of the planet and surrounding the planet. The temperature of the planet, the kind of star the planet orbits, and the constituents of the planet, can all affect the chemical and physical processes occurring. A recent article titled “Abiotic molecular oxygen production—Ionic pathway from sulfur dioxide“ has concluded that high-energy radiation can ionize sulphur dioxide (SO2), causing the atoms in the molecule to be rearranged in a way that allows the two bound electrons to be ejected, resulting in one free sulphur atom (S) and a molecule of oxygen (O2). This is not the only abiotic way for oxygen to form, an article from 2014 titled "Evidence for direct molecular oxygen production in CO2 photodissociation" has provided evidence that a similar process can happen to carbon dioxide (CO2), resulting in a free carbon atom (C) and an oxygen molecule (O2).
This process is known as photolysis or photodissociation, it is a chemical process by which molecules are broken down through the absorption of light.
One of the most promising recent developments is the assembly theory, which is the attempt of measuring the molecular complexity, this method builds on both chemical disequilibrium and agnostic bio signatures. By taking the complexity of the molecules into consideration, it is not only possible to detect current life forms, but even previous ones that went extinct. However, the theory behind this method is yet to yield experimental results, due to current technological limitations. More development and research are needed.
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