Origin of terrestrial water

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Water covers about 71% of the earth's surface

The origin of the terrestrial water has not yet been fully clarified:

Measurements of the hydrogen isotope ratio of deuterium and protium (H-1) (D / H ratio) rather indicate asteroids, since similar isotope ratios were found in water inclusions in carbonaceous chondrites as in ocean water. In contrast, the D / H ratio of comets and trans-Neptunian objects, according to previous measurements, does not agree well with that of terrestrial water.

For the current water resources in the solar system and especially on earth see water resources in the universe # solar system .

Earth Formation: Dry or Wet Accretion?

One of the main problems when trying to clarify the origin of the earth's water is the question of the water content of the planetesimals that made up the earth. There are two models:

  • the model of wet accretion (. English wet accretion ), alleging there is enough water in the planetesimals was present;
  • the model of dry accretion (Engl. dry accretion to the current amount of water to explain), according to the water content was low to the earth.

Depending on which model you adopt,

  • the origin can either be explained by pure volcanic outgassing from the earth's interior (with wet accretion) or
  • one needs extraterrestrial sources (with dry accretion).

Today's volcanoes emit water vapor , but most of it does not come from the interior of the earth, but from the earth's surface . So you could z. For example, volcanoes in Hawaii show that most of the water vapor comes from the groundwater reservoir .

Another important question is whether volcanic transport mechanisms are effective enough to transport any water inside the earth to the surface.

Origin from the interior of the earth via outgassing

Michael Julian Drake (1946–2011) was a representative of the origin of water from the earth's interior via outgassing . He justifies the earthly origin of the water with isotope studies of meteorites and material from the upper mantle of the earth . Accordingly, no later large impact of a body made of material, as it is represented by today's meteorites, can have contributed significantly to the composition of the upper mantle of the earth. On the other hand, Drake admits that a large “wet” planetary embryo from the asteroid belt or a comet with a corresponding element and isotope composition cannot be ruled out.

Drake tries to explain the problem of wet accretion, which cannot easily be explained at the temperatures in the earth's orbit, with the fact that the dust grains in the accretion disk , which agglomerated to the planetesimals, were fractal in nature. Because of the resulting large surface area, enough water could be adsorbed .

According to the wet accretion, there was enough water in the planetesimals. This water and other highly volatile substances such as carbon dioxide (CO 2 ), methane (CH 4 ) and nitrogen (N 2 ) outgassed the primordial earth, which largely consists of liquid magma , and formed an early primordial atmosphere rich in water vapor . According to today's model , this was carried away by a solar wind , which was much more violent at the time the earth was formed than it is today, and thus escaped from the earth.

By volcanism occurred later to form a new atmosphere, outgassed and from the earth's interior water vapor may have contained. With the formation of a solid earth crust and further cooling, water vapor condensed and the first oceans formed.

Extraterrestrial sources

ordinary chondrite

The dry accretion assumed in this model is justified by the fact that the planetesimals formed in an area of ​​the earlier solar system in which there was relatively little water. The smaller the distance from the sun, the higher the temperatures and the less water there was. Only outside the solar snow line , which was roughly in the middle of today's asteroid belt, was water available in large quantities. To show carbonaceous Chondrite , it is believed by those that they originated in the outer regions of the asteroid belt, a water content of sometimes more than 10% of their mass, while ordinary Chondrite or even enstatite Chondrite from the inner edge of the asteroid belt is less than 0.1 % of their mass in water. The planetesimals should therefore have contained even less water.

In addition, it is assumed that with the accretion of the planetesimals to the planets and the loss of the primordial atmosphere, large amounts of the originally existing water were lost again. That is why it is assumed by many planetologists today that the majority of today's terrestrial water comes from the outer areas of the solar system.

According to measurements of the isotope ratio of hydrogen in the three comets Halley , Hyakutake and Hale-Bopp by researchers such as David Jewitt, a purely cometary origin of the water was considered unlikely, since the ratio of deuterium to protium (D / H ratio) there is about double is as high as in oceanic water. In December 2014, the Rosetta space probe analyzed water vapor near the Churyumov-Gerasimenko comet ; These measurements also showed that the terrestrial water most likely does not come from comets.

Alternatively, the asteroid belt was suggested as a source of water, because water inclusions in carbonaceous chondrites show a D / H ratio similar to that of oceanic water. According to A. Morbidelli et al. Most of today's water comes from some of the protoplanets formed in the outer asteroid belt that crashed to Earth. A class of comets has now been identified that could have originated in this region. Two of these comets have so far been investigated for their D / H ratio: both C / 1999 S4 LINEAR and Hartley 2 carry 'earthly' water.

According to a study published in 2019, based on the isotope ratio of molybdenum on earth, it can be proven that the body ( Theia ), which created the moon when it hit the earth 4 billion years ago, came from the outer solar system and thus a large part of the water the earth could have brought.

Role of living beings

Hydrogen sulphide occurring in the primordial oceans and carbon dioxide present in the primordial atmosphere were used by autotrophic sulfur bacteria ( prokaryotes ) with the addition of light energy to build up organic compounds, producing methane, water and sulfur:

( Photosystem I).

literature

Web links

Individual evidence

  1. ^ MJ Drake, K. Righter: Determining the composition of the Earth. In: Nature . Volume 416, 2002, pp. 39-44.
  2. ↑ AP Boss: Temperatures in protoplanetary disks. In: Ann. Rev. Earth Planet. Sci. Volume 26, 1998, pp. 26-53.
  3. ↑ TO ISOTOPE HYDROLOGY STUDY OF THE KILAUEA VOLCANO AREA, HAWAII. US GEOLOGICAL SURVEY, Water-Resources Investigations Report 95-4213 ( PDF )
  4. ↑ Michael J. Drake: Origin of water in the terrestrial planets. In: Meteoritics & Planetary Science . Volume 40, No. 4, pp. 1–9, 2005, full text (PDF)
  5. ↑ Roland Meier et al .: A Determination of the HDO / H2O Ratio in Comet C / 1995 O1 (Hale-Bopp) , Science, Volume 279, 1998, pp. 842–844, doi : 10.1126 / science.279.5352.842 , Full text ( memento from September 19, 2009 in the Internet Archive ) (PDF; 319 kB).
  6. ↑ K. Altwegg et al .: 67P / Churyumov-Gerasimenko, a Jupiter family comet with a high D / H ratio. In: Science. Online advance publication of December 10, 2014, doi: 10.1126 / science.1261952
    Earthly water does not come from comets. On: zeit.de from December 10, 2014
  7. ^ A. Morbidelli, et al .: Source regions and timescales for the delivery of water to the Earth , Meteoritics & Planetary Science, Volume 35, 2000, pp. 1309-1329.
  8. ^ Henry H. Hsieh and David Jewitt: A Population of Comets in the Main Asteroid Belt. In: Science. Volume 312, 2006, pp. 561–563, doi : 10.1126 / science.1125150 , full text ( Memento from September 6, 2008 in the Internet Archive ) (PDF; 1.6 MB).
  9. ↑ NASA : A Taste for Comet Water , May 18, 2001.
  10. ↑ ESA : Did Earth's oceans come from comets? , October 5, 2011.
  11. ↑ Gerrit Budde, Christoph Burkhardt and Thorsten Kleine: Molybdenum isotopic evidence for the late accretion of outer Solar System material to Earth. In: Nature Astronomy. Online publication May 20, 2019, doi: 10.1038 / s41550-019-0779-y