Geoscientists discover an overlooked source for Earth's water

ASU geoscientists discover an overlooked source for Earth's water
Planet Earth -- or Planet Water? ASU geoscientists have found that our home contains within itself six or seven global oceans worth of hydrogen, in addition to the ocean seen here in a photo of the Pacific taken from the International Space Station. Credit: NASA

Where did Earth's global ocean come from? A team of Arizona State University geoscientists led by Peter Buseck, Regents' Professor in ASU's School of Earth and Space Exploration (SESE) and School of Molecular Sciences, has found an answer in a previously neglected source. The team has also discovered that our planet contains considerably more hydrogen, a proxy for water, than scientists previously thought.

So where is it? Mostly down in our planet's core, but more about that in a minute. The bigger question is where did all this come from in the first place.

"Comets contain a lot of ices, and in theory could have supplied some water," says Steven Desch, professor of astrophysics in SESE and one of the team scientists. Asteroids, he adds, are a source as well, not as water-rich yet still plentiful.

"But there's another way to think about sources of water in the solar system's formative days," Desch explains. "Because water is hydrogen plus oxygen, and oxygen is abundant, any source of hydrogen could have served as the origin of Earth's water."

In the beginning

Hydrogen gas was the major ingredient in the solar nebula—the gases and dust out of which the Sun and planets formed. If the abundant hydrogen in the nebula could combine with Earth's rocky material as it formed, that could be the ultimate origin of Earth's global ocean.

Jun Wu, the lead author of the paper the team has published in the Journal of Geophysical Research, is an assistant research professor in both SESE and the School of Molecular Sciences. He says, "The solar nebula has been given the least attention among existing theories, although it was the predominant reservoir of hydrogen in our early solar system."

But first, some geochemical detective work.

To distinguish between sources of water, scientists turn to isotope chemistry, measuring the ratio between two kinds of hydrogen. Nearly all hydrogen atoms have a nucleus that's a single proton. But in about one in 7,000 , the nucleus has a neutron in addition to the proton. This isotope is called "," or deuterium, symbolized as D.

The ratio of the number of D atoms to ordinary H atoms is called the D/H ratio, and it serves as a fingerprint for where that hydrogen came from. For example, asteroidal water has a D/H of about 140 parts per million (ppm), while cometary water runs higher, ranging from 150 ppm to as much as 300 ppm.

Scientists know that Earth has one global ocean of water on its surface and about two more oceans of water dissolved in its mantle rocks. That water has a D/H ratio of about 150 ppm, making an asteroidal source a good match.

Comets? With their higher D/H ratios, comets are mostly not good sources. And what's worse, the D/H of in the solar nebula was only 21 ppm, far too low to supply large quantities of Earth's water. In fact, asteroidal material is such a good match that previous researchers have discounted the other sources.

But, say Wu and co-workers, other factors and processes have changed the D/H of Earth's hydrogen, starting back when the planet was first beginning to form. Wu says, "This means we shouldn't ignore the dissolved solar nebula gas."

Scientists theorize new origin story for Earth's water
Artist's conception of the dust and gas surrounding a newly formed planetary system. Credit: NASA

Concentrating hydrogen

The key lies in a process combining physics and geochemistry, which the team found acted to concentrate hydrogen in the core while raising the relative amount of deuterium in Earth's mantle.

The process began quite early as the Sun's planets were starting to form and grow through the merger of primitive building blocks called planetary embryos. These Moon-to-Mars-size objects grew very quickly in the early solar system, colliding and accreting material from the solar nebula.

Within the embryos, decaying radioactive elements melted iron, which grabbed asteroidal hydrogen and sank to form a core. The largest embryo experienced collisional energy which melted its entire surface, making what scientists call a . Molten iron in the magma snatched hydrogen out of the developing primitive atmosphere, which derived from the solar nebula. The iron carried this hydrogen, along with hydrogen from other sources, down into the embryo's mantle. Eventually the hydrogen became concentrated in the embryo's core.

Meanwhile another important process was going on between and hydrogen. Deuterium atoms (D) do not like iron as much as their H counterparts, thus causing a slight enrichment of H in the molten iron and leaving relatively more D behind in the magma. In this way, the core gradually developed a lower D/H ratio than the silicate mantle, which formed after the magma ocean cooled.

All this was stage one.

Stage two followed as embryos collided and merged to become the proto-Earth. Once again a magma ocean developed on the surface, and once more, leftover iron and hydrogen may have undergone similar processes as in stage one, thus completing the delivery of the two elements to the core of the proto-Earth.

Wu adds, "Besides the hydrogen that the embryos captured, we expect they also caught some carbon, nitrogen, and noble gases from the early solar nebula. These should have left some isotope traces in the chemistry of the deepest rocks, which we can look for."

The team modeled the process and checked its predictions against samples of mantle rocks, which are rare today at Earth's surface.

"We calculated how much hydrogen dissolved in these bodies' mantles could have ended up in their cores," says Desch. "Then we compared this to recent measurements of the D/H ratio in samples from Earth's deep mantle." This let the team set limits on how much hydrogen is in Earth's core and mantle.

"The end result," says Desch, "is that Earth likely formed with seven or eight global oceans' worth of hydrogen. The majority of this indeed came from asteroidal sources. But a few tenths of an ocean's worth of hydrogen came from the gas."

Adding up the quantities cached in several places, Wu says, "Our planet hides the majority of its hydrogen inside, with roughly two global oceans' worth in the mantle, four to five in the core, and of course, one at the surface."

Not just for our solar system

The new finding, says the team, fits neatly into current theories for how the Sun and planets formed. It also has implications for habitable planets beyond the solar system. Astronomers have discovered more than 3,800 planets orbiting other stars, and many appear to be rocky bodies not greatly different from our own.

Many of these exoplanets might have formed far from the zones where water-rich asteroids and other building blocks might have arisen. Yet they still could have collected gas from their own stars' solar nebulas in the way that Earth did.

The team concludes, "Our results suggest that forming is likely inevitable on sufficiently large rocky planets in extrasolar systems."

Explore further

Water in the solar system predates the Sun

More information: Jun Wu et al, Origin of Earth's Water: Chondritic Inheritance Plus Nebular Ingassing and Storage of Hydrogen in the Core, Journal of Geophysical Research: Planets (2018). DOI: 10.1029/2018JE005698
Journal information: Journal of Geophysical Research

Citation: Geoscientists discover an overlooked source for Earth's water (2018, November 7) retrieved 16 July 2019 from
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Nov 07, 2018
"And God said, "Let there be an expanse in the midst of the waters, and let it separate the waters from the waters." And God made the expanse and separated the waters that were under the expanse from the waters that were above the expanse. And it was so. And God called the expanse Heaven.And there was evening and there was morning, the second day.And God said, "Let the waters under the heavens be gathered together into one place, and let the dry land appear." And it was so. God called the dry land Earth,and the waters that were gathered together he called Seas. And God saw that it was good."
Genesis 1

Nov 07, 2018
Oh begging, which stuporstitious loon are you sockpuppeting for?

The primitive fetish taboos of the tiny trailer park of inbred cultists who spawned you. Have failed in their mission to plague Humanity with their dregs of plagiarized ravings.

Nov 07, 2018
This article and the Paper MOST accurately reflects the true beginnings of the H2O that lies within the Core and up through the 2 mantles of the Earth. It makes great sense that asteroids, with their complement of Hydrogen, were the main source of the waters, where the asteroids themselves were incorporated into each of the levels of the fledgling Earth after collisions with the surface of the Earth. As each level built up, more and more collisions with the surface of Hydrogen-carrying asteroids brought so much more of the gas, which eventually mixed with Oxygen to form water.
I have often wondered how it was possible that such an enormous Flood of Noah's time could have occurred - and where the water had come from. There are many possibilities, but a world-wide flooding of the whole Earth didn't seem possible.
But this Paper gives the possibility of a world-wide flood a tad more credibility - that is, if the upwelling of waters took place ALL OVER Earth, and not just one region

Nov 07, 2018
The next thing to wonder is: How and why did such an upwelling of the Hydrogen within the rocks in the mantles of Earth mix with Oxygen to form water, and then emerge onto the Earth's surface to flood the world? There are some cultures in various parts of Earth who tell of a time when their regions were flooded even in high places. The stories are prevalent enough so that it can't be mythical if human lives and cattle were lost due to such a flood.
Some say that the Flood was confined to the Middle East. It is possible that the 2 rivers in the M.E. flooded over the land if the Persian Gulf had risen high enough to back flow at the confluence of the Tigris and Euphrates. But then, how and why would the Persian Gulf rise at such a level, and what other countries were affected? Did the Black Sea and others also rise to flood the land areas? One alternative is that the ice at the poles melted. But there is also the possibility that G-d caused the waters to rise to flood

Nov 07, 2018
Makes sense, Many ways of forming water, Hot meets Cold is one.

Nov 08, 2018
Our planet is saturated with water.

Nov 08, 2018
"And God said, "Let there be an expanse in the midst of the waters, and let it separate the waters from the waters." And God made the expanse and separated the waters that were under the expanse from the waters that were above the expanse. And it was so. And God called the expanse Heaven.And there was evening and there was morning, the second day.And God said, "Let the waters under the heavens be gathered together into one place, and let the dry land appear." And it was so. God called the dry land Earth,and the waters that were gathered together he called Seas. And God saw that it was good."
Genesis 1

Genesis (continued):

And on the seventh day, Man said, "let there be God, and let us create Him in our own image."

And on the eighth day, each man saw that his God hated exactly the same people he did.

And on the ninth day, they began to exterminate one another in His name.

Nov 08, 2018
V Larin HYDRIDIC EARTH proposes that hydrogen is constantly being expended upward and indeed CLUSTER satellites show large emissions of hydrogen from Earth. One can also consider that the pressures encountered allowed the combination of elements that our geologic history shows formed one after the other as our geologic ages indicate.
The information gathered from the ICON satellite to be launched will be informative altho we already know UV is responsible for the Airglow.

Nov 08, 2018
oh lordy and the looneytunes keep coming

Nov 09, 2018
oh lordy and the looneytunes keep coming

...led by rrwilliejoe as drum major out in front with a big fur hat

Nov 10, 2018
"oh lordy and the looneytunes keep coming"

Virgin water is created when a major solar proton storm allows such protons to interact with the oxygen in Earths atmosphere. The Great Deluge was such an even, and when the H2O molecule is formed it is at -170C. This is supported by northern native legends which tell of the Great Snow, and is the reason flash frozen animals and plants are found in northern Canada and Russia, as the protons enter at the North pole. Ice and snow in the North, torrential rain further south.

Nov 11, 2018
Nice 4-2-1 progression by "following the hydrogen"!

"oh lordy and the looneytunes keep coming"


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