Origin of water on Earth: Scientists say it probably came from the asteroid belt between Mars and Jupiter. Scientists have long wondered about the origin of the Earth’s water. Until now, the official version was that the volatile elements (such as hydrogen, nitrogen, or carbon), and even the first organic matter, arrived here from the outside, brought by comets and by a class of meteorites of very ancient origin, the carbonaceous chondrites. Both classes of objects formed beyond Jupiter’s orbit and then “migrated” into the Solar System. Now, a new study has just revealed that this was not the case.
Of course, determining exactly where these essential elements came from means being able to understand how water, and life, got to Earth.
Water on Earth and its origin
Now, a group of researchers from the Carnegie Institution of Washington has just published an article in Science according to which the water on our planet is not the same as that which was distributed, in the form of ice, throughout the Solar System, but came much later. And not aboard distant comets from the confines of the Solar System, but much closer: from the asteroid belt between Mars and Jupiter.
For Conel Alexander, principal investigator of the study, a large amount of water ice was distributed throughout the early Solar System. However, that ice was never part of the materials that were added to form the Earth. That early ice can be found today in objects such as comets or the aforementioned carbonaceous chondrites. But their analysis shows, according to the researchers, that the two types of objects were not born beyond the orbit of Jupiter, as was believed. Instead, they were born, they were born in two very distant areas. Comets are formed in the outer regions of the Solar System. The carbonaceous chondrites formed much closer.
Alexander established this difference by calculating the proportion of deuterium (an isotope of hydrogen) found in the water ice of 85 carbonaceous chondrites. The farther away from the Sun an object has formed, the more deuterium it contains. And if comets and carbonaceous chondrites formed in the same place, their ice should include the same, or similar, amount of deuterium. But this is not the case.
The ice of the meteorites analyzed contains much less deuterium than comets, suggesting that the latter formed much farther from the Sun. This result also contradicts the most widely accepted models of how the Solar System acquired its current architecture.
For Alexander, the deuterium content of the carbonaceous chondrites indicates that these meteorites must have formed much closer to the Sun than comets. Specifically, in the asteroid belt, the large ring of rocks lies between Mars and Jupiter’s orbits. And that, the study suggests, is precisely where Earth’s water comes from. Alexander is convinced that the results of his research will force the revision of current theories. In addition, they will contribute, in passing, to resolving a variety of questions that have so far resisted science.
Where Did Earth’s Water Come From?
If we stick to what we know, Earth’s water came from comets and asteroids. That’s the most widely accepted theory. But a new study suggests that perhaps not all water reached our planet through this system.
Searching for the Origins of Earth’s Water
Hydrogen is the most abundant element in the universe (after all, it is composed of a single proton and an electron). In the study, a group of researchers suggests that some of this element could come from the solar nebula. That is the cloud of gas and dust in which the Sun formed. If the approach is correct, it could replace the best-known theory about how water arrived on Earth.
It has always been thought that water arrived through the impact of comets and asteroids until now. After the formation of the Sun, the young star expelled most of the material from the cloud in which it was located. When its fusion began, the solar wind sent a lot of hydrogen from its outermost layers into the interior of the solar system. To the vicinity of Mercury, Venus, Earth, Mars, and the outer solar system.
Here, we find the giant planets (Jupiter, Saturn, Uranus, and Neptune) and a multitude of comets and asteroids. As you may know, Comets are frozen, rocky objects that can contain large amounts of hydrogen. Asteroids, although to a lesser extent, may also contain hydrogen. This is because it was a sphere of molten magma when the Earth formed. Its surface was kept in that state by the collision of asteroids.
The cooling process of the Earth
With the Earth in this molten state, the water from the impacts of comets and asteroids evaporated into space. But, over time, our planet cooled down. Earth’s water may have begun to arrive from asteroid and comet collisions. Little by little, the element condensed on the surface, instead of evaporating into space. This scenario is supported by the isotope ratio.
The amount of the isotope of deuterium, compared to normal hydrogen, is a signal. If two objects with water have the same ratio, it is reasonable to assume that their origin must be the same. The Earth’s oceans have the same proportion as the water of the asteroids. All this would seem unappealable, but the approach has always offered some doubts. In 2014, some scientists analyzed meteorites of different ages.
Among them, are carbonaceous chondrites. They are the oldest known meteorites, having formed at about the same time as the Sun. As well as meteorites that may have originated from the asteroid, Vesta. It formed in the same region as Earth, about 14 million years after the formation of our planet. They stand out for having a lot of water. So it is believed that they may be the primary source of water on Earth.
Clues of the formation of the planets
What does this mean, and why does water near the core contain less deuterium? With this model, researchers can calculate how much water comes from asteroid impacts. They also compare how much water comes from the solar nebula. They conclude that for every 100 water molecules on Earth, one or two originate from the solar nebula. The study offers a fascinating perspective on planet formation.
So does its evolution and how life might have arisen on a planet. The model, the researchers say, suggests that water formation is inevitable on any sufficiently large rocky exoplanet. This contradicts earlier conclusions. Earth’s water and its origin could be a very significant clue. According to an earlier hypothesis, the only planets on which life could form would be those with asteroids and comets that contain water.
However, if this model is correct, it would not be necessary. In other star systems, planets could have received enough water from their solar nebulae to have enough water for life to evolve. Most of Earth’s water is found below the surface. There could be up to twice as many oceans in Earth’s mantle as on the surface. It could be up to 4 or 5 times as much near the core.
The model
However, the model is not as robust as we would like. The hydrogen fractionation is not well understood. It is not clear how the ratio of deuterium to hydrogen changes when the element dissolves in iron. This is an essential point because it is an essential part of this new study, and the researchers had to guess what that ratio was.
In any case, the study fits with what was already known about the origin of water on Earth. The great novelty is that it gives a new insight into the possible origin of this water. It is very interesting, both to better understand how it came to be on our planet and to understand what the rocky exoplanets we have discovered might be like. If there need not be water-rich comets and asteroids, the prospects are more promising.
This in no way implies a greater likelihood of finding life. After all, we know that water is a necessary prerequisite. It is an essential building block of life on Earth. But there are other factors to consider—for example, the atmosphere’s composition. But studies like this will, in time, give us a much more complete picture, not only of the solar system but of other places in the galaxy.
The study is J. Wu, S. Desch, L. Schaefer; “Origin of Earth’s Water: Chondritic Inheritance Plus Nebular Ingassing and Storage of Hydrogen in the Core.” Published in Journal of Geophysical Research: Planets on October 9, 2018.
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