The origin of water on Earth

The issue of the origin of terrestrial water is subject to speculation. Traditionally it was thought that water was part of the "original" composition of the planet and that its volume had remained relatively unchanged.

According to certain more recent approaches (Fred Hoyle and others), it is maintained that the amount of water has increased over time due to the contribution of comets and meteoroids.

Thomas Gold goes further, arguing that an important part of the water is the result of the bacterial oxidation of rising methane gas in the degassing process of the planet. The methane hydrogen would be oxidized by hyperthermobacteria that live in depth forming CO2 and H2O as a result of their metabolism.

While liquid water has not yet been found on the surface of any known planet, it is likely to exist in considerable volumes in depth on many planets and satellites.

Anyway, water in the form of ice was identified on the surface of many bodies of the Solar System.

It is known that there is ice water on Mars, several satellites of Jupiter and Saturn and on the farthest bodies of the system, for example, Triton (a satellite of Neptune), Pluto and most comets.

From water brought by comets, asteroids and meteorites, and formed by the bacterial metabolic process and / or due to other processes of oxidation of hydrogen, seas and lakes were formed in the depressions of the crust, a part remained or was infiltrated into the rock formations and the rest froze in the colder areas near the poles or high mountains.

The oceanic waters were exposed to solar radiation leading to widespread evaporation processes along its contact surface with the atmosphere.

The solar warming of the atmospheric water and on the continental surfaces caused, and still causes, convection phenomena of the lower tropospheric layers, raising the water vapor to the levels of condensation, forming clouds.

Atmospheric circulation generated by temperature differences pushes the clouds towards the continents, where part of the water falls in the form of rain or snow.
This process, which is so familiar to us, is the product of the Earth's particular thermal and atmospheric pressure conditions, which allow most of the water to be in a liquid state and to develop evaporation and condensation phenomena, with formation of clouds and their fall in the form of rain.

In fact, this process occurred simultaneously with the expansion of life. Whether from subterranean or extraterrestrial bacterial forms, the seas were "colonized" by innumerable organisms, they later spread to the bodies of continental water, and finally to the rest of the subaerial environments.

The water cycle was (and still is) influenced by life. The organisms modify the physical-chemical properties of the waters where they live. There is no hydrological parameter that is not modified by the presence of living beings in the water: the albedo (reflectivity), the surface tension, the viscosity, the turbidity, the tenors in salts and dissolved gases, the chemical composition, etc.

Due to the complexity of the process it is very difficult to reconstruct the processes that gave rise to the planetary evolution, and in particular to the changes at the level of the hydrosphere.

The geological record gives us with fragmentary information. The dimensions and shape of the oceans changed. There were times when part of the water remained frozen in the coldest zones (glacial epochs), decreasing the level and extension of the oceans, and others in which all the ice melted rising the sea at its maximum levels.

The shape of the continents, and therefore of the oceanic basins, also varied.

Some continents were divided, the fragments thus formed migrated slowly and, in some cases, merged with other fragments forming new continental masses of different contours. Concomitantly, the oceans changed shape. Some oceanic depressions, such as the Atlantic Ocean,

They were formed in relatively recent geological times (about 100 million years ago). Others are much older (the Pacific Ocean, whose era of formation is unknown.

During the billions of years that passed, the oceanic waters received enormous volumes of salts until they stabilized in the current composition.

Some of these salts were immobilized and buried in the bottom of the sea for a long time. Some reappeared in the new mountains formed in the orogenetic margins of the continents.

Also from the beginning, the groundwater was exposed to the gas and heat sources of the interior of the planet. These ascending currents of predominantly radioactive origin1 were a major factor in terrestrial dynamics. Many geological processes occur in the presence of water. The mineralogical registry includes numerous hydrated minerals originated in environments

Aqueous underground: micas,  amphiboles, clays, etc. Liquid or gaseous water is introduced through the fissures, dragging varied solutes that finally crystallized in the form of minerals. A large part of the minerals in rocks originate in this way (eg, feldspars and quartz).

Volcanic phenomena also occur in the presence of water. A precipitating cause of some volcanic eruptions may be the vaporization of hot water as the pressure that kept it in a liquid state decreases. The steam "bubbles" released from boiling water, along with other gases, act as a "piston" that pushes lavas and volcanic clasts along fractures and chimneys, and ends up spilling them out. At the same time, the mixture of water vapor and other gases with liquid water has a lubricating effect that facilitates the flow of the lavas. If not, these, whose viscosity is very high, could not drain

through the narrow effusion channels. The large columns of "smoke" that come out of the volcanic craters are formed, among other components, by water vapor emitted during the effusive processes. In the same way, geysers and fumaroles are hot aqueous and gaseous ejecta common in the

volcanic zones. Water is also the main factor in the genesis of sedimentary rocks. With very few exceptions, sediments are formed due to the entrainment of particles and materials by liquid water currents (rivers, marine and lacunar currents, etc.) or solid (glaciers).

When the sediments are buried, they undergo compaction and dehydration processes. Part of the water, subjected to conditions of high pressures and temperatures, migrates out of the sediments, reducing the water content of

the same.

In spite of this, the sedimentary material retains an important content of water, part of which can be incorporated to the new minerals that are formed during the diagenetic processes.

As you can see, water plays a fundamental role in the dynamics of the earth's crust and in the formation of rocks. Not only is water the central factor in the hydrological cycle, but also in the petrogenetic cycle.

Introduction to the book "Acuíferos de América Latina". Danilo Antòn, Piriguazú Editions