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We have comets and asteroids to thank for Earth's water, according to the most widely-held theory among scientists. But it's not that cut-and-dried. It's still a bit of a mystery, and a new study suggests that not all of Earth's water is delivered to our planet.
Hydrogen is the most abundant element in the Universe, and it's at the center of the Earth's water issue.
This new study was co-led by Peter Buseck, Regents' Professor in the School of Earth and Space Exploration and School of Molecular Sciences at Arizona State University.
In it, the authors suggest that the hydrogen cam, at least partially, from the solar nebula, a cloud of gas and dust left over the Sun formed.
Before we dig into the details of this new study, it is helpful to look at the long-held theory that it may replace.
Earth's Water: The Widely-Held Theory
For a long time, most of the people of the water-from-comets-and-asteroids. It all starts with the formation of the Sun.
When the Sun is formed out of a cloud, it sweeps up most of the material in the cloud, leaving a little left over for everything else: planets, asteroids, and comets.
Once the Sun burst into life with fusion, a powerful solar wind feels a lot of hydrogen from its outer layers out beyond where the inner rocky planets – Mercury, Venus, Earth, and Mars – are today.
This is the realm of the gas giants, and more importantly, comets and asteroids.
Comets are here, rocky bodies, thought to contain significant amounts of hydrogen by the early Sun, and asteroids too, although to a lesser extent. They became a significant reservoir for hydrogen.
When Earth was formed, it was a molten ball, its surface kept in that state by repeated collision with asteroids. So far, so good, since the early Solar System has been much more chaotic than it is now.
Asteroids and comets this earth, the water and the hydrogen in boiled off into space. As Earth cooled over time, water from comet and asteroid collisions was allowed to condense on Earth, and not be boiled off into space. The water stuck around.
The evidence for this lies in isotope ratios. The ratio of heavy hydrogen isotope deuterium to normal hydrogen is a chemical signature.
Two bodies of water with the same ratio must have the same origin, the thinking goes. And Earth's oceans have the same ratio as water on asteroids. That's a very simplified version of the widely-held theory of how water got to Earth.
Earth's Water: A Leaky Theory With Holes In It
But scientists are misguided, always trying to have a better understanding of things. They were questioning the "water from comets" theory before this newest study came out.
Back in 2014, some scientists studied the issue by looking at meteorites of different ages. (Meteorites are just asteroids that have struck Earth.) First they looked at what are known as 'carbonaceous chondrite meteorites'.
They're the oldest ones we know of, and they formed about the same time as the Sun did. They are the primary building blocks of Earth.
Next, they studied meteorites that we think originated from the wide asteroid Vesta. Vesta formed in the same region as Earth, about 14 million years after the solar system was born.
According to this 2014 study, the ancient meteorites resembled the bulk Solar System composition and have a lot of water in them, so they have been widely considered to be the source of Earth's water.
The measurements in this 2014 study showed that these meteorites have the same chemistry as the carbonaceous chondrites and rocks found on Earth. They conclude that carbonaceous chondrites are the most likely common source of water.
At the time, Horst Marschall, one of the authors of the study, said, "The study shows that Earth's water is most likely accreted at the same time as the rock.
The team behind that study acknowledges that some of our water did come from impacts.
Which brings to this new study, which reinforces the conclusions from the 2014 study.
Earth's Water: All About Hydrogen
The authors of this new study say that their isotope ratios can not tell the whole story. "It's a bit of a blind spot in the community," said Steven Desch, a professor of astrophysics in the School of Earth and Space Exploration at Arizona State University in Tempe, Arizona.
"When people measure the [deuterium-to-hydrogen] ratio in ocean water and they see it asteroids, it was always easy to believe it all asteroids. "It's hard to blame them, it's a pretty compelling piece of evidence.
Desch and the other authors of this new study point to research published in 2015 showing that Earth's oceans may not be representative of Earth's primordial water.
The oceans may be cycled between the surface and a deeper reservoir of water, deep in the Earth. This may have changed the ratio over time, and it may mean that this earth is true primordial water.
And that water can be directly from the solar nebula, rather than through comet and asteroid impacts.
The study develops a new theoretical model of Earth's formation to explain these differences between hydrogen and earth.
This new model shows large water-logged asteroids formed into planets billions of years ago in the solar nebula swirling around the Sun. These planetary embryos had a sequential collision and they grew quickly.
Eventually, they say, a powerful enough collision melted the surface of the largest embryo into an ocean of magma. This largest embryo became Earth.
This large embryo had enough gravity to hold an atmosphere, and it attracted gases, including hydrogen, the most abundant one, from the solar nebula to form one.
The hydrogen in the solar nebula is less deuterium and is lighter than asteroidal hydrogen. It is about the molten iron of the magma ocean on Earth.
The hydrogen is pulled to the center of Earth by a process called isotopic fractionation. Hydrogen is attracted to earth's core by the iron.
Deuterium, the heavy hydrogen isotope, remained in the magma, which cooled to form the Earth's mantle. Continuing impacts brought to the earth and mass to Earth
The key point in this new model is that hydrogen in the earth's core is different than in the mantle and in the oceans. Core water has much less deuterium. But what does it mean?
The new model has been estimated to increase the size of the earth as a result of earthquakes.
Their conclusion?
"For every 100 molecules of earth's water," said Jun Wu, assistant research professor in the School of Molecular Sciences and School of Earth and Space Exploration at Arizona State University and co-lead author of the study.
Conclusion: It's About More Than Just Earth's Water
This study is a new perspective on planetary training, development, and early life could flourish on a young planet.
"This model suggests that the inevitable formation of water would be likely to occur in the future." I think this is very exciting. " – Jun Wu, School of Molecular Sciences and School of Earth and Space Exploration at ASU, co-lead author.
Previously, we thought that the planets that could have life on a solar system as well as water-bearing asteroids and comets. But that may not be the case. In other solar systems, not all Earth-like planets have access to asteroids loaded with water.
The new study has any possible exoplanets might have gotten water from the solar nebula in their system. Earth hides most of its water in its interior. Earth has roughly two continents in its mantle, and 4 or 5 in its core. Exoplanets may be similar.
"This model suggests that the inevitable formation of water would be likely to occur in a broad rocky exoplanet in extrasolar systems," Wu said. "I think this is very exciting."
There is one cautionary point in this new model though, and that involves the hydrogen fractionation.
It is not well-understood how the deuterium-to-hydrogen ratio changes when the element dissolves in iron, which is at the center of this new model. It was to be estimated in this new study.
Overall, the new study fits in with Earth's water. Once more work is done on hydrogen fractionation, the new model can be tested more rigorously.
This article was originally published with Universe Today. Read the original article.
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