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Where did the World Ocean from Earth come from? A team of geoscientists from Arizona State University led by Peter Buseck, Professor of Regents at the School of Earth and Space Exploration (SESE) and the School of Molecular Sciences of the ASU, has found an answer to a previously neglected source. The team also discovered that our planet contains much more hydrogen, a substitute for water, than scientists thought.
So where is he? Mainly in the core of our planet, but more about it in a minute. The big question is where does all this come from?
"Comets contain a lot of ice and theoretically could have provided water," says Steven Desch, professor of astrophysics at SESE and one of the scientists in the team. Asteroids, he adds, are also a source of energy, not as rich in water as always abundant.
"But there is another way of thinking about water sources in the days of solar system formation," says Desch. "Because water is hydrogen plus oxygen, and oxygen is abundant, any source of hydrogen could have been the source of the Earth's water."
In the beginning
Hydrogen was the main ingredient of the solar nebula – the gases and dust from which the Sun and the planets were formed. If the abundant amount of hydrogen in the nebula could combine with the Earth's rock material as it formed, it could be the ultimate origin of the Earth's global ocean.
Jun Wu, the main author of the document published by the team in the newspaper Journal of Geophysical Research, is an Assistant Professor of Research at SESE and the School of Molecular Sciences. "The solar nebula has received the slightest attention among the existing theories, although it is the predominant hydrogen reservoir in our early solar system," he said.
But first, some work of geochemical detectives.
Scientists use isotope chemistry to measure the relationship between two types of hydrogen. Almost all hydrogen atoms have a nucleus composed of a single proton. But in about one in every 7,000 hydrogen atoms, the nucleus has a neutron in addition to the proton. This isotope is called "heavy hydrogen" or deuterium, symbolized by D.
The ratio between the number of atoms of D and the ordinary atoms of H is called the ratio D / H, and serves as a fingerprint on the origin of this hydrogen. For example, asteroidal water has a D / H ratio of about 140 parts per million (ppm), while cometary water is higher, ranging from 150 ppm to 300 ppm.
Scientists know that the Earth has an ocean of global water on its surface and about two other oceans of water dissolved in mantle rocks. This water has a D / H ratio of about 150 ppm, which is good for an asteroid source.
Comets? With their higher D / H ratios, comets are usually not good sources. And even worse, the D / H of hydrogen gas in the solar nebula was only 21 ppm, which is far too low to provide large amounts of water from the Earth. In fact, the asteroid material is such an agreement that previous researchers ignored the other sources.
But, say Wu and his colleagues, other factors and processes have changed the hydrogen D / H of the Earth, from the moment the planet began to form. Wu said, "That means we should not ignore the dissolved solar gas of the nebula."
Concentration of hydrogen
The key lies in a combination of physics and geochemistry, which, according to the team, has been able to concentrate hydrogen in the nucleus while increasing the relative amount of deuterium in the Earth's mantle.
The process began early enough as the planets of the Sun began to form and develop through the fusion of primitive building blocks called planetary embryos. These Moon-to-Mars-sized objects developed very rapidly in the early solar system, colliding and accumulating material from the solar nebula.
Within the embryos, decaying radioactive elements melted iron, which seized asteroid hydrogen and sank to form a nucleus. The larger embryo experienced a collision energy that melted its entire surface, making what scientists call an ocean of magma. Melted iron in the magma removed the hydrogen from the developing, primitive atmosphere, which came from the solar nebula. Iron carried this hydrogen, as well as hydrogen from other sources, into the mantle of the embryo. Finally, the hydrogen concentrated in the nucleus of the embryo.
Meanwhile, another important process was taking place between molten iron and hydrogen. Deuterium (D) atoms do not like iron as much as their H counterparts, which results in a slight enrichment of H in the molten iron and leaves relatively more D in the magma. In this way, the core progressively developed a lower D / H ratio than the silicate coat that formed after cooling the ocean magma.
All this was the first step.
The second step followed when the embryos collided and merged to become Proto-Earth. Once again, an ocean magma has developed on the surface and, once again, the iron and hydrogen remains may have undergone processes similar to those of the first stage, thus completing the delivery of the two elements in the heart of the proto-Earth.
Wu adds, "In addition to the hydrogen captured by the embryos, we expect that they will also capture carbon, nitrogen and rare gases from the original solar nebula. These should have left isotopic traces in the chemistry of the deepest rocks, which we can look for. "
The team modeled the process and verified its predictions using samples of mantle rocks, rare today on the surface of the Earth.
"We calculated the amount of dissolved hydrogen in the bodies of these bodies that could have been in their nuclei," says Desch. "Next, we compared this to recent D / H measurements in samples taken from the deep mantle of the Earth." This allowed the team to set limits on the amount of hydrogen contained in the core and mantle of the Earth.
"The end result," says Desch, "is that the Earth probably formed with seven or eight oceans of hydrogen. Most of this comes from asteroid sources. But a few tenths of the value of hydrogen from an ocean came from solar gas from the nebula. "
By adding the quantities stored in several places, Wu: "Our planet hides most of its hydrogen inside, with roughly two global oceans in the mantle, four to five in the core, and of course, a global ocean on the surface. "
Not only for our solar system
This new discovery, says the team, fits perfectly into the current theories on the formation of the Sun and the planets. This also has implications for habitable planets beyond the solar system. Astronomers have discovered more than 3,800 planets orbiting other stars, and many seem to be rocky bodies not very different from ours.
Many of these exoplanets may have formed far from areas where water-rich asteroids and other building blocks may have appeared. Yet, they could still have captured the hydrogen gas from the solar nebulae of their own stars, like the Earth.
The team concludes: "Our results suggest that water formation is probably inevitable on sufficiently large rocky planets in extrasolar systems."
More information:
Jun Wu et al., Origin of Earth's Water: Chondritic Legacy and Nebular Ingestion and Storage of Hydrogen in the Heart, Journal of Geophysical Research: Planets (2018). DOI: 10.1029 / 2018JE005698
Where did the World Ocean from Earth come from? A team of geoscientists from Arizona State University led by Peter Buseck, Professor of Regents at the School of Earth and Space Exploration (SESE) and the School of Molecular Sciences of the ASU, has found an answer to a previously neglected source. The team also discovered that our planet contains much more hydrogen, a substitute for water, than scientists thought.
So where is he? Mainly in the core of our planet, but more about it in a minute. The big question is where does all this come from?
"Comets contain a lot of ice and theoretically could have provided water," says Steven Desch, professor of astrophysics at SESE and one of the scientists in the team. Asteroids, he adds, are also a source of energy, not as rich in water as always abundant.
"But there is another way of thinking about water sources in the days of solar system formation," says Desch. "Because water is hydrogen plus oxygen, and oxygen is abundant, any source of hydrogen could have been the source of the Earth's water."
In the beginning
Hydrogen was the main ingredient of the solar nebula – the gases and dust from which the Sun and the planets were formed. If the abundant amount of hydrogen in the nebula could combine with the Earth's rock material as it formed, it could be the ultimate origin of the Earth's global ocean.
Jun Wu, the main author of the document published by the team in the newspaper Journal of Geophysical Research, is an Assistant Professor of Research at SESE and the School of Molecular Sciences. "The solar nebula has received the slightest attention among the existing theories, although it is the predominant hydrogen reservoir in our early solar system," he said.
But first, some work of geochemical detectives.
Scientists use isotope chemistry to measure the relationship between two types of hydrogen. Almost all hydrogen atoms have a nucleus composed of a single proton. But in about one in every 7,000 hydrogen atoms, the nucleus has a neutron in addition to the proton. This isotope is called "heavy hydrogen" or deuterium, symbolized by D.
The ratio between the number of atoms of D and the ordinary atoms of H is called the ratio D / H, and serves as a fingerprint on the origin of this hydrogen. For example, asteroidal water has a D / H ratio of about 140 parts per million (ppm), while cometary water is higher, ranging from 150 ppm to 300 ppm.
Scientists know that the Earth has an ocean of global water on its surface and about two other oceans of water dissolved in mantle rocks. This water has a D / H ratio of about 150 ppm, which is good for an asteroid source.
Comets? With their higher D / H ratios, comets are usually not good sources. And even worse, the D / H of hydrogen gas in the solar nebula was only 21 ppm, which is far too low to provide large amounts of water from the Earth. In fact, the asteroid material is such an agreement that previous researchers ignored the other sources.
But, say Wu and his colleagues, other factors and processes have changed the hydrogen D / H of the Earth, from the moment the planet began to form. Wu said, "That means we should not ignore the dissolved solar gas of the nebula."
Concentration of hydrogen
The key lies in a combination of physics and geochemistry, which, according to the team, has been able to concentrate hydrogen in the nucleus while increasing the relative amount of deuterium in the Earth's mantle.
The process began early enough as the planets of the Sun began to form and develop through the fusion of primitive building blocks called planetary embryos. These Moon-to-Mars-sized objects developed very rapidly in the early solar system, colliding and accumulating material from the solar nebula.
Within the embryos, decaying radioactive elements melted iron, which seized asteroid hydrogen and sank to form a nucleus. The larger embryo experienced a collision energy that melted its entire surface, making what scientists call an ocean of magma. Melted iron in the magma removed the hydrogen from the developing, primitive atmosphere, which came from the solar nebula. Iron carried this hydrogen, as well as hydrogen from other sources, into the mantle of the embryo. Finally, the hydrogen concentrated in the nucleus of the embryo.
Meanwhile, another important process was taking place between molten iron and hydrogen. Deuterium (D) atoms do not like iron as much as their H counterparts, which results in a slight enrichment of H in the molten iron and leaves relatively more D in the magma. In this way, the core progressively developed a lower D / H ratio than the silicate coat that formed after cooling the ocean magma.
All this was the first step.
The second step followed when the embryos collided and merged to become Proto-Earth. Once again, an ocean magma has developed on the surface and, once again, the iron and hydrogen remains may have undergone processes similar to those of the first stage, thus completing the delivery of the two elements in the heart of the proto-Earth.
Wu added, "In addition to the hydrogen captured by the embryos, we expect that they will also capture carbon, nitrogen, and rare gases from the early solar nebula. These should have left isotopic traces in the chemistry of the deepest rocks, which we can look for. "
The team modeled the process and verified its predictions using samples of mantle rocks, rare today on the surface of the Earth.
"We calculated the amount of dissolved hydrogen in the bodies of these bodies that could have been in their nuclei," says Desch. "Next, we compared this to recent D / H measurements in samples taken from the deep mantle of the Earth." This allowed the team to set limits on the amount of hydrogen contained in the core and mantle of the Earth.
"The end result," says Desch, "is that the Earth probably formed with seven or eight oceans of hydrogen. Most of this comes from asteroid sources. But a few tenths of the value of hydrogen from an ocean came from solar gas from the nebula. "
By adding the quantities stored in several places, Wu: "Our planet hides most of its hydrogen inside, with roughly two global oceans in the mantle, four to five in the core, and of course, a global ocean on the surface. "
Not only for our solar system
This new discovery, says the team, fits perfectly into the current theories on the formation of the Sun and the planets. This also has implications for habitable planets beyond the solar system. Astronomers have discovered more than 3,800 planets orbiting other stars, and many seem to be rocky bodies not very different from ours.
Many of these exoplanets may have formed far from areas where water-rich asteroids and other building blocks may have appeared. Yet, they could still have captured the hydrogen gas from the solar nebulae of their own stars, like the Earth.
The team concludes: "Our results suggest that water formation is probably inevitable on sufficiently large rocky planets in extrasolar systems."
More information:
Jun Wu et al., Origin of Earth's Water: Chondritic Legacy and Nebular Ingestion and Storage of Hydrogen in the Heart, Journal of Geophysical Research: Planets (2018). DOI: 10.1029 / 2018JE005698
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