The Role of Carbon and of Liquid Water

One chemical element is the basis of life as we know it: carbon, which is the basis of all organic molecules. What are this atom's specific properties? First, it is one of the most abundant in the Universe; it has four valence electrons, which enable it to create multiple bonds with itself and with other atoms (particularly with hydrogen, oxygen, and nitrogen, but also with sulphur and phosphorus). More than one hundred gaseous-phase molecules have been discovered in the interstellar medium, containing up to fifteen atoms, most of which are carbon. Carbon is also present in the Universe in the form of more complex molecules, containing tens or even hundreds of atoms, such as the polycyclic aromatic hydrocarbons (PAH).

Could we imagine another form of life based on an atom other than carbon? According to Mendeleyev's periodic table (Table 9.1), the most favourable element would be silicon, which also has four valence electrons. But this element is only one-tenth as abundant in the Universe as carbon. To this day, no interstellar molecule containing more than one atom of silicon has been discovered, whereas interstellar molecules have been observed to have as many as 11 atoms of carbon. So it appears reasonable to assume that if extraterrestrial life exists, it would also be based on carbon chemistry.

Liquid water is another element essential for life on Earth, because it provides a medium in which molecules may dissolve and react with one another. The H2O molecule is a polar solvent, that is to say that is has a bipolar moment, with the hydrogen atoms carrying the positive charge and the oxygen atom the negative charge. Polar solvents such as water are able to dissolve polar organic molecules, known as hydrophils, whereas non-polar molecules, known as hydrophobes, do not react with water. The hydrophilic or hydrophobic properties of different molecules are used by living organisms to produce specific functions.

Could we imagine the development of a form of life in another solvent than liquid water? A priori, nothing forbids it. Ammonia, NH3, is also a polar molecule which could play the same part; chemical reactions within a liquid solution would, however, be slower, because the temperature would be lower, but, in principle, they could take place. However, we should note that water possesses a certain number of trumps that may explain the role that it has played in the development of life

Table 9.1 Mendeleyev's periodic table of the elements

Table 9.1 Mendeleyev's periodic table of the elements

on Earth. Formed from hydrogen and oxygen, both of which are particularly abundant in the Universe (see Sect. 1.4.5), water is omnipresent in planetary, stellar, and galactic sources. In the interstellar medium, oxygen is primarily in the form of H2O; a few percent are in the form of carbon monoxide, CO, methanol, CH3OH, or other oxygenated molecules; molecular oxygen is notoriously absent. These abundances are repeated in the composition of cometary material. Water, then, because of its cosmic abundance, is particularly well placed to play the role of solvent. We should also, finally, mention that water possesses a unique property that may have proved decisive in preserving life, once the latter had appeared in the liquid medium. Water is, in fact, the sole molecule where ice formed by condensation from the liquid phase has a density less than that of the liquid itself. This is why icebergs float on the ocean. This feature undoubtedly played a key role in the history of life on Earth. During cooling episodes, the oceans became covered with a layer of ice, which allowed the oceans to survive, together with the living species within them. If the oceans had solidified from the bottom, as would be the case with hydrocarbons, for example, the solid phase would, bit by bit, have invaded the whole of the oceans, thus destroying any complex life forms that they may have sheltered. To summarize, although water is perhaps not a unique solvent capable of supporting the appearance and development of life, it obviously possesses specific features that render it particularly suitable for this role.

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