Over the years, many have speculated about the possibility of life on other planets. Since the 1950s, aliens have been a popular theme in sci-fi movies, namely Star Wars, Star Trek, E.T. the Extra-Terrestrial, and more. They have taken many forms, from the inky heptapods with seven long legs in Arrival to the strong hairy Wookies (like Chewie).
Astrobiology, the term first coined in the 1950s, is the study of life in the universe, exploring the biological impact of space missions and extra-terrestrial life in the universe. It wasn’t until Carl Sagan brought attention to this field that it quickly gained momentum and recognition. Astrobiologists ask near-philosophical questions like “what is life?”, “how did life begin on Earth?”, and “how can we search for life beyond Earth, if any?”.
And the million-dollar question: are we alone in this universe?
The Great Analogy Method: What are some “signatures of life”?
All of what we know about life is essentially based on one pale blue dot in the entire universe – planet Earth. Astrobiologists use this knowledge about our origins to predict how life could be formed on other planets.
Currently, scientists define “life” according to these three rules:
It must be able to utilize energy from various sources (e.g. redox reactions, sunlight, etc.) to maintain itself
The organisms should be able to perform homeostasis (internally maintain a relatively stable state)
It should be able to reproduce and be subject to evolutionary processes (natural selection)
Based on these rules, astrobiologists have identified some possible signatures of life:
A liquid solvent to originate from
Organic molecules
Sources of energy to use
Life in Liquids
Back then, the Earth hadn’t developed the ozone layer, making these living organisms susceptible to large amounts of solar radiation, and there wasn’t oxygen either. Therefore, astrobiologists postulate that the first life-forms were anaerobic (able to survive without oxygen) and were able to either tolerate or avoid radiation. Radiation doesn’t reach very far into the depths of the oceans, so it is very likely that when life emerged 3.8 billion years ago, it inhabited the deep oceans. Moreover, it was the oceans where the first organic molecules reacted with each other and performed abiogenesis (non-living matter spontaneously generates life), forming the first self-replicating structure.
Liquid water usually exists on a planet in the habitable zone, where it’s not too far or too close to the star, so the water doesn’t freeze (0ºC) or boil (100ºC). However, it is worth noting that in some extraneous cases, a planet with liquid water can exist outside the habitable zones. For example, the Jovian and Saturnian moons experience strong tidal forces that distort their shapes and produce friction inside them. The friction generates heat and the tidal heating melts the ice enough to create liquid water.
Two other reasons astrobiologists think that water could be a promising marker for the origin of life are:
Abundance in the universe The elements hydrogen (H) and oxygen (O) that make up water molecules (H2O) are the first and third most cosmic abundant elements respectively.
The physicochemical properties of water help mediate macromolecular interactions Water can be used as a solvent for polar molecules and compounds including carbohydrates and proteins, which then react in aqueous solutions within cells. Water also has a tendency to form hydrogen bonds, and often actively participates in metabolic/living processes (e.g. protein folding, protein substrate binding, and maintaining the structural stability of proteins and DNA/RNA, etc.).
In addition, note how the signatures of life include liquid “solvents”, not water. For life on Earth, water is a desirable candidate, because it’s amphoteric and acts both as a base and acid, facilitating acid-base reactions for biosynthesis. But the possibilities are virtually limitless. Ammonia could also be a medium for the origin of life, but this “life” would be stable at another pH range.
Carbon-Based Lifeforms
One of the most basic criteria for life is reproduction. To achieve propagation, highly specific interactions between molecules are required, and these chemical reactions would need to be directed by genetic information inside complex and large macromolecules. An example of this on Earth would be RNA, a molecular weight up to millions of daltons. Based on this information, we know living organisms are highly likely to be made of macromolecules.
The only two known natural atoms large enough to carry biological information are carbon and silicon. However, astrobiologists agree that carbon is more probable as a basis for biomass because…
Can readily form chemical bonds to produce complex molecules The versatile nature of carbon to react with various chemicals and form different types of compounds allows it to undergo necessary reactions for biological metabolism and reproduction. On the other hand, silicon only interacts with a few other atoms, limiting the variability of its compounds and products.
Energy capture and transformation Carbon holds electronic properties that facilitate chemical bonding with other atoms, allowing the capture and delocalization of electronic energy. Some organic compounds can transform the resonance energy to do work or perform life-sustaining processes.
Specific intermolecular interactions According to Pace, “the potential polarizability of organic compounds also contributes to the specificity of intermolecular interactions, because ionic and van der Waals (VDW) complementarities can shift to mesh with or to repulse one another.” To understand more about the VDW Forces and complementarities, check out THIS chapter (P. 114-120) in the book Intermolecular Forces in Biology.
The cosmic abundance of carbon Of the higher elements, carbon is one of the most abundantly found in space. Complex organic compounds have been consistently found and encounters with carbonaceous (carbon-rich) meteorites are common as well.
The CHNOPS elements (carbon, hydrogen, nitrogen, oxygen, phosphorus, sulfur) are the fundamental building blocks of life. These compounds can form higher-complex structures via various reactions and intermolecular interactions. It is up to speculation, however, whether an alternative set of “building blocks” could potentially be for life beyond our solar system. Silicon could be a substitute for carbon in structure, and arsenic may replace phosphorus.
In summary, it is surmised that extra-terrestrial life could highly likely be carbon-based and first emerge in aquatic conditions. Make sure to keep reading the next article of this series to learn about the biosignatures that astrobiologists use to look for life in the universe.
Works Cited
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