|Maapallo kuvattuna Apollo 17:sta 7.12. 1972|
Ehkä tähän mennessä hienoin portretti maapallosta
Veden alkuperä maapallolla on tieteelle edelleen hämärän peitossa. ("Selvä kuin vesi" ei siis ole kovin onnistunut vertaus.)
Ensinnäkin on todettava, että syvässä avaruudessa ja aurinkokunnssa on paljon vettä.
Much of the universe's water is produced as a byproduct of star formation. When stars are born, their birth is accompanied by a strong outward wind of gas and dust. When this outflow of material eventually impacts the surrounding gas, the shock waves that are created compress and heat the gas. The water observed is quickly produced in this warm dense gas.Mutta mistä ja milloin tulee maan pintakuoresta 71% peittävä vesimäärä?
On 22 July 2011 a report described the discovery of a gigantic cloud of water vapor containing "140 trillion times more water than all of Earth's oceans combined" around a quasar located 12 billion light years from Earth. According to the researchers, the "discovery shows that water has been prevalent in the universe for nearly its entire existence".
Water has been detected in interstellar clouds within our galaxy, the Milky Way. Water probably exists in abundance in other galaxies, too, because its components, hydrogen and oxygen, are among the most abundant elements in the universe. Based on models of the formation and evolution of the Solar System and that of other star systems, most other planetary systems are likely to have similar ingredients.
Some of the most likely contributory factors to the origin of the Earth's oceans are as follows:
- The cooling down of the primordial world to the point where the outgassed volatile components were held in an atmosphere of sufficient pressure for the stabilization and retention of liquid water.
- Comets, trans-Neptunian objects or water-rich meteoroids (protoplanets) from the outer reaches of the main asteroid belt colliding with the Earth may have brought water to the world's oceans.
- Measurements of the ratio of the hydrogen isotopes deuterium and protium point to asteroids, since similar percentage impurities in carbon-rich chondrites were found in oceanic water, whereas previous measurement of the isotopes' concentrations in comets and trans-Neptunian objects correspond only slightly to water on the Earth.
- Biochemically, during the Great Oxygenation Event, via redox reactions and photosynthesis.
- Gradual leakage of water stored in hydrous minerals of the Earth's rocks.
- Photolysis: radiation can break down chemical bonds on the surface.
- Planetesimals heated by the decay of aluminum. This could cause water to rise to the surface.
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