Water is essential for life as we know it, and having water all around us seems to be perfectly natural. Despite this, Earth is the only world believed to have oceans. Do we know where it got its water from?
Because of its proximity to the Sun and the high temperatures at the time the solar system formed, it was long assumed that Earth formed dry – without water. Comets or asteroids colliding with the Earth, according to this model, may have carried water to Earth. With such a complicated origin for water, our world is almost certainly unique in the universe.
However, we demonstrated in a 2020 study that water – or at least its elements, hydrogen and oxygen – may have been present in the rocks that shaped the Earth. If this is the case, other "blue planets" with liquid water are more likely to occur.
More than 70% of the Earth's surface is covered by liquid water, with 95.6 percent of it in oceans and seas and the remaining 4% in glaciers, ice caps, groundwater, lakes, rivers, soil humidity, and the atmosphere.
The majority of Earth's water, however, is found deep underground: the mantle contains between one and ten times the amount of the oceans.
At the Earth's surface, "water" refers to hydrogen incorporated in minerals, magmas, and fluids (H20), while "water" in the mantle refers to hydrogen incorporated in minerals, magmas, and fluids. At the right temperature and pressure, this hydrogen will combine with the oxygen in the air to form water.
Although water makes up less than 0.5 percent of the Earth's mass, it is critical to the planet's evolution and existence on its surface.
There was a lot of hydrogen in the early solar system, either in the form of dihydrogen gas (H2) or bound with oxygen atoms to form water (H2O). Earth, as well as the other rocky planets (Mercury, Venus, and Mars), formed close to the Sun, where it was too hot for water to freeze into rock as ice, and it would have simply evaporated. So, why is there so much water on the Earth's surface and in its mantle?
The widely held belief is that hydrated asteroids carry hydrogen to Earth.
Chondrites are meteorites that come from small asteroids that, unlike planets, have not evolved geologically since their formation. They are excellent witnesses to the solar system's early millions of years.
Carbonaceous chondrites, for example, evolved far enough away from the Sun to produce water ice at first (all of which has since been incorporated in hydrated minerals through hydrothermal alteration). Ordinary and enstatite chondrites, on the other hand, developed closer to the Sun, where water was gaseous and incorporated in large quantities into rocks: ordinary and enstatite chondrites, like rocky planets, are considered "dry."
Until recently, the accepted theory was that Earth was made from dry materials and that its water was supplied by celestial bodies that formed further away from the sun: hydrated meteorites, such as carbonaceous chondrites, or comets – though the ESA space probe Rosetta recently debunked this last hypothesis.
Is there another source for Earth's water?
Their research, on the other hand, tells a different storey. The hydrogen in enstatite chondrites was studied. Since these rocks are among the best analogues for the rocks that formed Earth, the hydrogen concentrations in these "dry" rocks suggest the presence of water during the formation of the planet.
They looked at the quantities of various isotopes in enstatite chondrites to equate the Earth's composition to that of enstatite chondrites (atoms of the same element but containing different numbers of neutrons). They discovered that, while enstatite chondrites lack hydrated minerals, they do contain small quantities of hydrogen with an isotopic ratio similar to that of the Earth. Hydrogen is believed to have been present in trace quantities (0.1 percent) in the minerals and organic compounds that agglomerated to form enstatite chondrites, explaining where the majority of the water in the Earth's mantle and a portion of the oceans came from. The majority of Earth's water (specifically, hydrogen and oxygen) may have been present from the beginning.