Origin of water on Earth

The origin of the ground water, in particular the question of why on earth a lot more than water can be found on Venus and Mars, is not fully understood.

Part of the water is caused by outgassing of the magma, thus comes from the Earth's interior. Another portion has come by impacts ( Impacts ) of comets, trans-Neptunian objects or water-rich asteroids ( protoplanets ) from the outer regions of the asteroid belt to Earth. Measurements of the quantity ratio of the two hydrogen isotopes deuterium and protium (D / H ratio ) indicate that the material rather asteroids out there similar ratios were found in water inclusions in carbonaceous chondrites as in oceanic water, whereas previous measurements of this isotope ratio in comets and trans-Neptunian objects poorly matched with earthly water.

For the current water resources of the solar system and especially of the earth, see the section deposits in the article water.

Earth was: Dry or wet accretion?

One of the main problems when trying to clarify the origin of the ground water is the question of the water content of planetesimals that formed the earth. There are two models, one model of dry accretion (English dry accretion ), according to which the water content was too low to explain the current amount of water on Earth, and the model of wet accretion (german wet accretion ), according to which enough water was present in the planetesimals. Depending on which model it is assumed, one can either explain the origin by pure volcanic outgassing from the earth or you need extra- terrestrial sources.

Although modern volcanoes emit steam, but this mostly does not come from the earth's interior, but from the earth's surface. So you could show, for example, to volcanoes in Hawaii, that the water vapor largely comes from the groundwater reservoir.

Another important question is whether volcanic transport mechanisms are effective enough to possibly carry any water in the Earth's interior to the surface.

Caused by outgassing from the earth's interior

Representative of a terrestrial origin of the water by outgassing from the Earth's interior is MJ Drake. Drake founded the underground water source with isotopic studies of meteorites and material from the upper mantle of the earth. Accordingly, no later large impact of a body of material, as represented by today meteorites, have contributed significantly to the composition of the upper mantle of the earth. On the other hand, Drake admits that a large " wet " planet embryo from the asteroid belt or even a comet with corresponding elemental and isotopic composition were ultimately not be excluded. The problem of wet accretion, which is not easy to explain at the temperatures in the Earth's orbit, Drake tries to explain the fact that the dust grains were fractal nature in the accretion disk, which together curled into the planetesimals. Due to the resulting large surface therefore could be adsorbed enough water.

According to the accretion wet enough water was present in the planetesimals. This water and other volatile substances such as carbon dioxide (CO2 ), methane (CH4 ) and nitrogen ( N2) from outgassed from the primordial earth mostly made ​​of molten magma and formed an early, steam- rich primordial atmosphere. This was by today's model concepts, carried away by a solar wind, which was at the time the Earth much more violent than today and thus departed from the earth. By volcanism occurred later in the formation of a new atmosphere, which should have also included within the earth outgassed water vapor. With the formation of a solid crust and the further cooling it was therefore for the condensation of water vapor and the formation of the first oceans.

Extraterrestrial sources

The assumed in this model dry accretion is motivated by the fact that the planetesimals formed in an area of ​​previous solar system, in the relatively small amount of water was present. The smaller the distance from the sun, the higher the temperature and the less water was present. Only outside the solar " snow line ", which was about the middle of today's asteroid belt, water was present in larger amounts. To view carbonaceous chondrites, of which it is believed that they originated in the outer regions of the asteroid belt, a water content of sometimes more than 10 % of their mass, while ordinary chondrites or even enstatite chondrites from the inner edge of the asteroid belt is less than 0.1 contain % of their mass of water. The planetesimals should therefore contain less water. It is also assumed that in the accretion of planetesimals to the planet and the loss of the primordial atmosphere again lost large amounts of water originally present. Therefore, is now thought by many planetary scientists that the vast majority of today's terrestrial water from the outer regions of the solar system originated.

A purely cometary origin of the water was maintained by measuring the isotopic ratio of hydrogen in the three comets Halley, Hyakutake and Hale -Bopp by researchers like David Jewitt unlikely, since there the ratio of deuterium to protium (D / H ) ratio of comets about twice as high as in oceanic water Alternatively, the asteroid belt has been proposed as a source of water, because water inclusions in carbonaceous chondrites show a similar D / H ratio as oceanic water. According to A. Morbidelli et al. for the great majority of today's water from some formed in the outer asteroid belt protoplanets, which fell to the earth.

Meanwhile, a class of comets has been identified that could come from this region. Two of these comets have so far been investigated for their D / H ratio towards: Both C/1999 S4 LINEAR and Hartley 2 wear ' underground ' water.

Role of living things

In the Urozeanen occurring hydrogen sulfide and carbon dioxide present in the primordial atmosphere was of autotrophic sulfur bacteria ( prokaryotes ) used with supply of light energy to the structure of organic compounds, water and sulfur emerged:

( Photosystem I).