Water is hydrogen and oxygen. To become a liquid known to us, they must combine, and in order for the liquid to be enough for the whole sea, there must be a lot of these substances. And, already, for the whole ocean …
In recent decades, scientists have been controversial about where on Earth the hydrogen in the required quantities came from. With oxygen, the issue is not so acute, since it is less volatile, and hydrogen at such a distance from the Sun is more or less surprising and requires an explanation.
A team of geologists from Arizona State University analyzed the isotopic composition of terrestrial hydrogen and the likely pathways for its accumulation. The focus was on two competing hypotheses, according to which hydrogen came to Earth either from asteroids or comets. It should be noted that in those days the Earth, or rather what will later be called by this name, itself was in general an asteroid, only more.
Natural hydrogen is composed of two isotopes. In the most common case, the nucleus of an atom consists of one proton. In rare cases (on the order of thousandths) a neutron is attached to a proton - this substance is called deuterium, and its combination with oxygen is called heavy water. There is also an option with two neutrons - tritium, it is radioactive, decays quickly and it is illogical to take it into account in these calculations.
The ratio of the number of deuterium atoms (D) to ordinary atoms (H) is called the D / H ratio, and it allows you to determine the source of hydrogen. For example, asteroid water has a D / H of about 140 parts per million (ppm), while comet water has between 150 ppm and 300 ppm.
Hydrogen on Earth has an approximately "asteroid" isotopic composition, but this is not the only issue.
Scientists believe that the gases of the nebula from which the Sun formed a little earlier could play a significant role in its accumulation. The key point is the concentration of hydrogen in the Earth's core with a simultaneous increase in the relative amount of deuterium in the mantle.
During the formation of our planet, planetesimals the size of the moon collided relatively often, forming more massive bodies, and a significant part of the future planet's surface was melted by the energy released during the collisions. This led to the fact that a significant part of the molecular hydrogen from the protoatmosphere of the planet was captured by heavier elements, especially iron, and, along with it, "drowned" in the bowels. This affected the isotopic composition of hydrogen - deuterium dissolves worse in molten iron and, in this situation, remained closer to the surface, in the mantle.
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To test this hypothesis, scientists studied samples of igneous rocks, considered mantle, in order to clarify the isotopic composition of the hydrogen present in them.
The analysis showed that hydrogen, then contained in the gas cloud surrounding the Sun, played a significant role in the formation of the Earth. It, dissolved in the depths of the Earth, at the initial stage of the formation of the planet should have been enough for seven or eight volumes of the modern World Ocean. Enough for two now.
Details can be seen in an article published in the Journal of Geophysical Research.