Why do people always look for liquid water in the search for extraterrestrial life?

Answer

Indeed, it does not have to be a priori that the evolution towards self-organizing complex molecules, which has happened here on Earth in an aqueous environment, must happen everywhere in the same way. That is why we are really curious about what complexity exists on the Saturn satellite Titan, where lakes with hydrocarbons have been found.

But on the other hand, water is very special in that context, and perhaps therefore unique:

– after H2, and in some places CO, H2O is the most abundant molecule in the universe: it consists of the most abundant element (H) and the third most abundant element (O), and number 2 (He) does not participate in the chemistry;

– it is a very good solvent, because it has a fairly large dipole moment;

– the heat capacity of water is very large, as we see in the fact that the temperature of the oceans fluctuates much less than that of the continents; in that way water is a stabilizing factor;

– water also plays a role as a greenhouse gas, thus participating in the thermostat that has kept the temperature of the planet fairly constant despite a brightening sun;

– water is the only substance that is lighter in the solid state (ice) than in the liquid state; therefore a pond, or possibly the sea, freezes at the top, and the ice puts a plug on it which retards further cooling of the water; if ice were heavier, ponds would sometimes freeze completely, and perhaps the oceans would have done so in earlier stages, with fatal effects to life;

– water interacts with rocks – hydration – and that makes them softer. It is thanks to the hydration of the earth’s crust that the rocks are soft enough to allow continental drift, whereby the different partitions of the earth’s crust can slide under each other. This continental drift is essential to regularly degas the earth’s crust, so that gases (which are removed from the atmosphere by rain) are released again to keep the greenhouse effect going and as food for photosynthesis.

An impressive list of properties, from which one can conclude that water is the best choice. “The only choice” may be a step too far, but anyway, if life is thriving in the universe, it will mostly be in aquatic environments. So it does make sense to use the presence of liquid water as a good selection criterion to search for life.

It is difficult enough for nature to produce life, so it is likely that she chooses the easiest way. A similar reasoning is the following. It is also sometimes said that we are too anthropocentric in that we assume that life elsewhere must be based on carbon chemistry, just like here. Well, if we look at what molecules have been identified in the interstellar medium, it is found that out of about 100 molecules with five atoms or more that we have been able to find there, there is only one (1!), namely SiH4 , which does not contain a carbon atom. Why? Because C is the champion of chemistry: it is quite common, light and therefore mobile, and has by far the richest catalog of possible bonds (one-dimensional chains, rings, chains of rings, amorphous variants, graphite, graphene, diamond, self buckyballs). If you want to make complex molecules, you’d be crazy not to work with carbon, and nature isn’t crazy.