Why The Search For Life, Probably, Requires The Search For Water? (Planetary Science)

Friends, there are several papers you may have read, which focuses on the search for extraterrestrial life that claim, either directly or indirectly, that life (including Earth’s) forms in the absence of water. But according to Modirrousta–Galian and Maddalena, this argument is incompatible with most lines of evidence and data.

“Liquid water is essential to life and should be the direction to follow in future space flight missions to improve the likelihood of finding extraterrestrial life.”

In order to tackle this argument, they breakdown water properties to the physical, chemical and biological level and demonstrated that it is the most adequate medium for the formation of life.

1) Cosmochemical level

By shedding light on the cosmochemical composition of our solar system, they showed, how common water is in the Universe as the two elemental constituents of water, hydrogen and oxygen, are the first and third most common elements (as you can see in table below).

Table 1: The cosmochemical composition of our solar-system according to Lodders. © Modirrousta-Galian & Maddalena

“It follows that from a purely statistical perspective a strong argument can be made that if life were to exist elsewhere in the universe, it would have most probably interacted with water in a certain manner.”

2) Planetary Level

In our solar system the condensation temperature of H2O is ∼ 150 K, which corresponds to a distance of approximately ∼ 3 AU from the Sun. This location is called the ‘frost line’ and its properties will vary depending on the nature of the protoplanetary disc from which the planet forms as well as the spectral type of the host star.

According to Modirrousta–Galian and Maddalena, for planets that formed at temperatures below ∼ 150 K, there is a very high probability that water is present in large amounts, such as Uranus and Neptune that are rich in water ice. In contrast, planets that formed at hotter temperatures are unlikely to have a significant abundance of water unless it came from external sources.

“This is the strong evidence that the delivery of water by comets and asteroids could be common not just in our solar system, but elsewhere in the universe. In other words, due to the copious cosmic presence of water, whether a planet formed within or outside of the frost line appears to have little influence on whether it has come in contact with water.”

But, though the delivery of water is common in the universe, the presence of water on planetary surfaces depends on their thermodynamic stability, how close they orbit to their host star etc etc.

3) Micropalaeontological Level

They also mentioned that the proportion of Earth’s ‘dry’ surface has been investigated is much greater than the equivalent for Earth’s oceans meaning that there is a bias that has to be accounted for. Even still, the oldest known microfossils have been found in the oceans. In addition, considering that most of Earth’s surface is covered by water (approximately 70%), it must be acknowledged that there is a strong statistical argument for life’s oldest fossils being located underwater.

4) Physiochemical level

The ‘wet-dry’ hypothesis suggests that water was a catalyst for the formation of primordial cells in the early-Earth due to the prevalence of hydrolysis-condensation reactions found in nature. However, according to Modirrousta–Galian and Maddalena, this description is unable to fully encapsulate the unique physicochemical properties of water, which may be crucial for abiogenesis. For this, they attained more holistic analysis by considering the electronic properties of a H2O molecule.

Figure 2: The physicochemical properties of water can be explained by the high electronegativity of oxygen, and the low electronegativity of hydrogen, which give it two dipoles capable of hydrogen bonding. The polar nature of the molecule is due to the 104.5° angle which forms. © Modirrousta-Galian & Maddalena

According to authors, the highly polar nature of water makes it a universal solvent allowing it to dissolve a large range of inorganic and organic molecules. Wet-dry cycles taking place on mineral deposits would benefit from this property as they can solubilise metal ions such as iron and arsenic, which were relatively abundant in the early Earth.

“The varying redox states of these metals allows them to transfer electrons to other molecules and act as a source of energy. Dissolution of these metal ions is an example of how the polar behaviour of water facilitated primordial reactions.”

The dielectric constant of water is also very high (with a value of 81) and it is responsible for weakening the bonds between other molecules such as metal salts in minerals.

Hydrophobicity is another physicochemical property that is important in sustaining life. This behaviour is crucial to forming an enclosed membrane, a prerequisite to forming enclosed cellular life. A semi-permeable barrier provides a means to control the movement of molecules, either produced inside the cell, such as proteins, or carried into the cell, such as metal ions. In aqueous solutions, amphiphilic molecules (containing one charged hydrophilic end and one hydrophobic moiety) will form liposomes, micelles or bilayers as the most thermodynamically favourable structure (Fig. 3). This is because these shapes reduce the surface area in contact with water, minimising the free energy of the system.

Figure 3: The most thermodynamically favourable shapes for amphiphilic molecules to take in water are micelles, bilayer sheets and liposomes. These forms are driven by the hydrophobic effect and may have facilitated the evolution of cellular membranes. © Modirrousta-Galian & Maddalena

The high electronegativity of oxygen, in the molecular structure of water, allows it to form hydrogen bonds (Fig. 3B). These can form between other water molecules, or with other molecules containing highly electronegative atoms, such as peptides. Hydrogen bonds play a chaperone-like role in biomolecular topography, meaning that they assist in the process of folding biological molecules such as proteins and DNA. The shape of these molecules are key to regulating processes inside and outside of the cell. For example, an enzyme has a very precise and defined shape that dictates the biochemical functions it has. As protein folding takes place, water forms hydrogen bonds with peptides in a rapid cycle of breaking and reforming, following a folding-funnel mechanism. This mechanism causes the potential and configurational energy of a protein to decrease as its native state is approached, following a funnel-like direction as various folding conformations are trialled.

“It is the polar nature of water that allows for this process to take place. Without hydrogen bonding and water as a solvent to chaperone, proteins would have an extensive number of probable folding patterns, leading to imprecise folding or no folding at all. This may result in a complete loss of function.”

For more, read..

Darius Modirrousta-Galian, Giovanni Maddalena, “Of Aliens and Exoplanets: Why the search for life, probably, requires the search for water”, arxiv, pp. 1-5, 2021. https://arxiv.org/abs/2104.01683

Copyright of this article (editing) totally belongs to our author S. Aman. One is allowed to reuse it only by giving proper credit either to him or to us

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