by Robby Berman
November 16, 2018

from BigThink Website

New Research

identifies an Unexpected Source

for some of Earth's Water...

 

• A lot of Earth's water is asteroidal in origin, but some of it may come from dissolved solar nebula gas.
   

• Our planet hides majority of its water inside: two oceans in the mantle and 4-5 in the core.
   

• New reason to suspect that water is abundant throughout the universe.

Scientists have puzzled for some time over how the Earth first acquired water.

Some have theorized it arrived in cometary ice, or possibly aboard asteroids crashing onto the planet's surface.

"But there's another way to think about sources of water in the solar system's formative days," says Arizona State University's Steven Desch, a member of the team of geoscientists led by Peter Buseck, Regents' Professor at ASU's School of Earth and Space Exploration.

 

"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 a paper (Origin of Earth's Water - Chondritic Inheritance Plus Nebular Ingassing and Storage of Hydrogen in the Core) published in Journal of Geophysical Research, the researchers suggest that the H in our early H²0 may have come from the rocky center of the planet itself, left there during its formation.

If so, it may be indicative of what happens on other rocky planets in our solar system.

Heavy hydrogen
 

Image source:

gritsalak karalak

Shutterstock
 

Lead author of the paper, Jun Wu, tells ASU,

"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."

Earth has three major areas of water, the most visible of which is the ocean.

There are two more "oceans," however, under the ground dissolved into the mantle. While both hold the liquid, the water above and below ground is not quite the same. It has to do with the presence of heavy hydrogen.

While far and away most hydrogen atoms have a nucleus containing a single proton, the nucleus of about 1 in 7,000 hydrogen atoms has a neutron as well.

These isotopes - exceptions to the single-proton norm - are considered "heavy" hydrogen atoms, or deuterium, abbreviated as "D."

Scientists can ascertain the source of hydrogen by determining its ratio of D to A atoms, or its D/H ratio. The hydrogen in water from comets has a D/H ratio ranging from 150 ppm (parts per million) to 300 ppm. Water from asteroids comes in at about 140 ppm. The D/H ratio down deep in the earth that comes from solar nebulae is a measly 21 ppm.

Most ocean water is about 150 ppm, suggesting that a lot of it is asteroidal in origin, and, indeed, that's the going assumption.

But Wu suggests that Earth's hydrogen has changed since it first got here, and that,

"This means we shouldn't ignore the dissolved solar nebula gas."

Hydrogen to the core, deuterium to the mantle
 

Image source: Wu, et al
 

The change to which Wu refers is what happened to the Earth's earliest hydrogen.

His team studied the planet's likely history as it collided with and collected cosmic materials to expand from a moon-to-Mars-sized "planetary embryo" to the form in which we find it.

Wu's team asserts that the iron in our planetary embryo became molten and sank to become the Earth's core. As it did, it pulled asteroidal hydrogen down with it.

Energy from later cosmic collisions created a magma ocean at the surface, and the iron there also pulled hydrogen, this time from solar nebulae, out of the primitive atmosphere, and sank.

Eventually, Earth collided with other planetary embryos, and each time a similar process repeated. And down went all this hydrogen into the Earth's core.

At the same time, deuterium, which isn't attracted as strongly to iron, was left behind in the magma, mantle, and atmosphere. The net result is that there's a lower D/H ratio in the core than elsewhere, and that's where the hydrogen from cosmic nebulae is.

It probably had a much higher ratio when it got here.

"Such processes not only delivered countless hydrogen atoms from the mantle to the core," says the research, "but also generated an appreciable difference in hydrogen isotopic composition. . . between the mantle and core."

It's this dissonance in D/H ratios that makes the current belief in asteroidal hydrogen alone as the source of our water perhaps too simple an explanation.

Water, water, everywhere
 

Jurik Peter

Shutterstock
 

Steven Desch says,

"We calculated how much hydrogen dissolved in these bodies' mantles could have ended up in their cores. Then we compared this to recent measurements of the D/H ratio in samples from Earth's deep mantle."

Their calculations revealed that, Wu explains,

"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 global ocean at the surface."

"The end result is that Earth likely formed with seven or eight global oceans' worth of hydrogen," concludes Desch.

 

"The majority of this indeed came from asteroidal sources. But a few tenths of an ocean's worth of hydrogen came from the solar nebula gas."

Apparently about 1 in 100 of the water molecules we have on Earth today came from solar nebulae hydrogen.

A particularly provocative conclusion the paper draws is that the production of water may be an inevitable result of the process that forms rocky planets such as ours.

As the paper says,

"These results suggest inevitable formation of water on sufficiently large rocky planets in extrasolar systems."

There may be water all over the universe...

ASU Geoscientists Discover an Overlooked...

Source for Earth's Water
by Robert Burnham
November 07, 2018
from ASU Website

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.

Photo by 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," said Steven Desch, professor of astrophysics in SESE and one of the team scientists.

Asteroids, he added, 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 explained.

 

"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 (Origin of Earth's Water - Chondritic Inheritance Plus Nebular Ingassing and Storage of Hydrogen in the Core) the team has published in the Journal of Geophysical Research, is an assistant research professor in both SESE and the School of Molecular Sciences.

"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," Wu said.

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 hydrogen atoms, the nucleus has a neutron in addition to the proton.

This isotope is called "heavy hydrogen," 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)

   

• 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 hydrogen gas 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, said 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 said,

"This means we shouldn't ignore the dissolved solar nebula gas."

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 that melted its entire surface, making what scientists call a magma ocean.

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 molten iron and hydrogen. Deuterium atoms 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 added,

"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," said 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," said 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 solar nebula gas."

Adding up the quantities cached in several places, Wu said,

"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 global ocean at the surface."

Not just for our solar system

The new finding, the team said, 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 hydrogen gas from their own stars' solar nebulas in the way that Earth did.

The team concluded,

"Our results suggest that forming water is likely inevitable on sufficiently large rocky planets in extrasolar systems."

The authors of the paper are,

• Jun Wu

• Steven Desch

• Laura Schaefer

• Linda Elkins-Tanton

• Kaveh Pahlevan

• Peter Buseck,

...all affiliated with SESE.

Wu and Buseck are also affiliated with ASU's School of Molecular Sciences. The research was funded by the Keck Foundation. Based in Los Angeles, the W.M. Keck Foundation was established in 1954 by the late W.M. Keck, founder of the Superior Oil Company.