All living beings, from microbes to humans, live in water or contain water. All chemical reactions that take place inside cells are carried out in water.
Liquid water is an extraordinary compound, with
strange properties. It has one
of the highest specific heats of all substances. This means that when heat is added to or removed
from a body of water, the temperature varies more slowly than in any other
liquid. This is important for living beings, as water acts as an environmental
stabilizer. Also, it is the
non-metallic liquid with the highest thermal conductivity, so local temperature variations are balanced very
quickly.
Most liquids contract when they solidify, but water is an exception. It has its maximum density at a temperature of 4°C. The density of ice is lower, 0.92 times lower, so ice floats on water. So, when the temperature drops, water freezes from top to bottom, while other liquids solidify from bottom to top. This also has important biological consequences. In polar seas and in fresh waters of cold regions, when the temperature drops below the freezing point, the surface layer of ice insulates the water below from the cold, so that it never freezes and the beings that live there can remain alive and active, despite the harsh environmental conditions.
Liquid water is one of the substances with highest dielectric constant. This means that, inside it, two electric charges
of different signs attract each other with less force than in any other liquid
or gas. As a result, water is one of the best solvents. Many substances
dissolve in it, as can be seen in the composition of sea water, where almost
all natural elements are found, to a greater or lesser extent. The biological
significance of this is evident.
Should we conclude that water is an essential
component of living matter? If the answer is affirmative, we must conclude that
the appearance of life would be only possible in planets where there is liquid
water. But first we must consider two preliminary questions: Is a liquid
medium, whether water or not, necessary for life? What substances can we find
in a liquid state in other places in the Universe?
As every chemist knows, the liquid state is the
most suitable for carrying out many chemical reactions. One of the fundamental
principles of chemical analysis is that solid substances, whose composition is
to be analyzed, must first be dissolved, that is, turned into the liquid state.
This gives them great mobility, which allows atoms, molecules or ions to meet frequently,
while the compounds that should react are enclosed within a fixed volume, which
would be more difficult if they were in the gaseous state. It seems reasonable
to assume that many of the spontaneous reactions that led to the origin of life
on Earth were possible because our planet has a large liquid mass on its
surface.
As for the second question, let us recall that the
most common volatile substances in the primitive atmospheres of the solar
system were water, ammonia and methane. Under the current conditions on Earth,
ammonia is a gas, but it would be possible to find oceans of ammonia on planets
with a denser atmosphere or whose surface is at a lower temperature. At the
pressure of one atmosphere, ammonia condenses at —40° C. At a pressure seven
times greater, it is liquid at ordinary temperatures. Methane condenses at lower
temperatures than ammonia: —160° C at the pressure of one atmosphere.
Therefore, the temperature of a planet would have to be significantly lower
than that of the Earth for a methane ocean to be possible. Under these
conditions, the speed of chemical reactions would be lower, which could be a
disadvantage for the appearance or evolution of life.
A priori, it is not impossible that ammonia or
methane, or even other liquids, could replace water as the liquid component of
living beings. Their properties are not so favorable, but they should not be eliminated.
Life will not appear on a planet if there is not a
source of energy that makes possible the chemical reactions that generate
increasingly complex organic compounds. The obvious source is the radiation
from the star around which the planet revolves. But this energy may not be
enough if the planet is very far from its star, or if it is isolated by a layer
of clouds. If the atmosphere is dense, atmospheric electrical discharges may
play a role. Perhaps the energy of volcanoes can also be used.
In conclusion, for life to appear, the following
factors seem to be necessary:
First: a
planet with three phases: solid, liquid and gas.
Second: an
abundance of carbon compounds in the atmosphere, such as methane and/or carbon dioxide.
Third: a
source of energy
capable of stimulating the organic chemical reactions that precede life.
In any case, without excluding other possibilities,
the existence of liquid water in great amounts is generally considered one of the most
useful conditions for the apparition of life.
Thematic Thread about Life in Other Worlds: Previous Next
Manuel Alfonseca
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