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Water - its origin

One obvious difference between Earth and other planets in the Solar System is that it's 70% covered in liquid water. There are at least five major (and very different) theories proposing the origins of such a large volume of water (estimated at around 1.3 billion cubic kilometres).

"The question of the origin of water on Earth, or the question of why there is clearly more water on the Earth than on the other planets of the Solar System, has not been clarified."

Source : Wikipedia

In addition, there are unexplained anomalies regarding the balance of water isotopes - i.e. the apparent 'excess' of deuterium ('heavy water')

See New Scientist (Nov.2010)

Ice-rich comets or asteroids from farther out in the solar system could have supplied it, but that raises a further problem. Comets are richer in deuterium, a stable heavy isotope of hydrogen, than Earth’s oceans. And asteroids should have brought more platinum and other rare elements than have been found. These mismatches are difficult to explain if most of Earth’s water came from impacts.

Now, it seems that water may after all have been present in Earth’s building blocks. Simulations by Nora de Leeuw of University College London and colleagues suggest that the dust grains from which Earth formed had such a tenacious grip on water that they could have held onto the molecules despite the high temperatures.

A 2021 paper in Nature Astronomy suggests that the Solar Wind might have influenced the isotope balance :

The production of this isotopically light water reservoir by solar wind implantation into fine-grained silicates may have been a particularly important process in the early Solar System, potentially providing a means to recreate Earth’s current water isotope ratios.
[…] the volatile isotopic composition of the Earth is an enigma, and it is likely that at least one other light isotope reservoir contributed to our planet’s water budget—the Sun, for example, and/or the solar nebula.

See : Solar wind contributions to Earth’s oceans. Nature Astronomy (2021)

Another theory for the water's origin, from University of California Los Angeles et al (2023) suggests that it could have formed due to complex reactions of atmospheric hydrogen with the molten magma of the Earth's core.

[…] we use a self-consistent thermodynamic model to show that Earth’s water, core density and overall oxidation state can all be sourced to equilibrium between hydrogen-rich primary atmospheres and underlying magma oceans in its progenitor planetary embryos. Water is produced from dry starting materials resembling enstatite chondrites as oxygen from magma oceans reacts with hydrogen. Hydrogen derived from the atmosphere enters the magma ocean and eventually the metal core at equilibrium, causing metal density deficits matching that of Earth."

Nature volume 616, pages 306–311 (2023) [ paywalled ]

Note that for the computer models to produce this result, it was assumed the Earth was only 20 - 30% of its current size.

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