Oceans of Water Locked Up in Mantle Rocks 400 Miles Deep
© 2014 by Linda Moulton Howe
“I can tell you that if the amount we have observed is spread out over the entire Earth, there's the potential for there to be more water inside the Earth than in the oceans.
- Steve Jacobsen, Ph.D., Geophysicist, Northwestern Univ.
Earth's mantle is a silicate rocky shell about 1,800 miles thick (2,900 km).
400 miles down from the surface crust is a “transition zone” where the pressure
and temperature cause rocks such as ringwoodite and wadsleyite to “sweat” water.
For the first time, evidence is showing those rocks might hold as much
water as twice the oceans on the surface.
Earth's solid iron-nickel inner core is 760 miles in diameter (1,220 km) and its surface
temperature is as hot as the sun's surface, 10,800 degrees Fahrenheit (6,000 degrees Celsius).
The solid inner core's diameter of 2,159 miles (3,474 km) is about 70% the diameter
of Earth's moon. The mass of the inner core is about one hundred million million million tons -
which is about 30 per cent greater than the mass of the moon.
Illustration 2013 Wikipedia by Kevinsong.
June 26, 2014 Evanston, Illinois - 71% of Earth's surface area is water. Earth ocean depths can be as much as five to six miles — and even 6.8 miles deep in the Pacific's Mariana Trench. How did so much water end up on a planet that began as boiling hot with lava all over its surface? This question has baffled scientists for a long time. Theories have included comets crashing into Earth and leaving their ice to melt. But could comets provide all the water that's filled Earth's oceans and seas? No one knows exactly when the first ocean filled, but estimates are as long ago as 3.8 billion years.
Now in the June 13, 2014, issue of Science, geophysicists have reported the first hard evidence that 400 miles below the surface of the Earth's crust in the mantle transition zone there are pressures and temperatures high enough to cause rocks to literally “sweat” water. Surprisingly, those deep mantle rocks might hold more water than all the oceans and seas combined on the Earth's surface. The research came from Northwestern University's high pressure studies of rocks, such as ringwoodite and wadsleyite, joined with seismic studies by the University of New Mexico from a dense array of 2,000 seismometers in the western half of the United States.
The scientists hypothesize that these water-rich mantle rocks might finally be the answer to the ancient mystery: where did all of Earth's ocean waters come from?
Steve Jacobsen, Ph. D., Geophysicist and Assoc. Professor of Earth and Planetary Science, Northwestern University, Evanston, Illinois: “It looks like from this study we are seeing, at least beneath the United States, an amount that would add up to about as much water as in the oceans. And if that is the case spread out all over the globe at these depths, this could potentially reach up to three times as much water as found in the oceans.
What we have found is that the water cycle extends much deeper into the planet than we thought before. Now the water cycle that we experience every day involves rain and the formation of lakes and oceans, and water is evaporated back into the sky and it's condensed in the form of clouds and it rains again. That water can also go into rocks. So the ground is also part of the water cycle. And we can extend this water cycle deeper into the Earth if we think about volcanoes. Water is coming out of volcanoes in the form of steam. So, where did that water come from? Well, it probably came from inside the Earth? How did it get there? It got there through a process called plate tectonics. We don't know the driving forces, but it has to do with heat inside the Earth. The Earth's interior is moving around in a process that we call convection. That's one way heat moves through the Earth. Sometimes these rocks on the surface are taken down into the Earth.
400 miles below the surface of the Earth's crust in the Lower Mantle transition zone
(grey line between Upper Mantle and Lower Mantle in above illustration). That's where
pressures and temperatures are high enough to cause high-pressure forms of olivine rocks such as ringwoodite and wadsleyite to literally “sweat” water. Those olivines can hold up to 1wt% of H2o.
Surprisingly, those deep mantle rocks might hold more water than all the oceans
and seas combined on the Earth's surface. Approximate to scale
illustration by Steve Jacobsen, Northwestern University.
The study we reported last week, we explored the idea that the Earth's water cycle extends much deeper into the planet. This is important because it just makes the total amount of water on and in the Earth much larger than we thought before.
SO WE HAVE IN FRONT OF US ON THE SURFACE OCEANS AND SEAS AND LAKES AND WHAT YOU ARE SAYING IS THAT IF WE GO DOWN INTO THE MANTLE, DOWN TO THAT TRANSITION LAYER THAT'S ABOUT 400 MILES BELOW OUR FEET, ALL OF THOSE ROCKS THAT CONTAIN WATER — THE RINGWOODITE AND THE OTHER —
Laboratory-grown (synthetic) crystals of ringwoodite containing 1wt% of H2O.
Image credit Steve Jacobsen, Northwestern University.
IT WOULD BE EQUIVALENT TO TWICE THE OCEANS AND SEAS THAT WE KNOW ABOUT ON THE SURFACE IF IT ALL CAME OUT OF ALL OF THOSE ROCKS.
I'm saying that it could be. I'm not saying that it is and that's because we were only able to image using our seismic waves only one part of the mantle beneath the United States. We had seismometers spread out over the continental U. S. running from California up to about the Midwest.
Schematic representation of the Earth and location of where seismometers have
been positioned in the U.S.-Array over the past ten years. Image credit: NSF-Earthscope.
So we're sampling only a very small part of the Earth's interior. We see evidence for more large scale hydration, but we still don't know whether or not the hydrations of the transition zone extends globally. So that will have to wait for future seismic studies of the transition zone in other parts of the world. So I can't tell you if it's equal to the amount of water in the oceans — or perhaps even double. But I can tell you that if the amount we have observed is spread out over the entire Earth, there's the potential for there to be more water inside the Earth than in the oceans.
Diamond-anvil cell viewed from the side. Each diamond is around 2.5 mm in height.
By compressing samples of ringwoodite or wadsleyite between the diamond tips,
pressures equal to the base of the transition zone and upper part of the lower
mantle are achieved. Image by Steve Jacobsen, Northwestern University.
A single blue crystal of hydrous (water-bearing) ringwoodite imaged inside the
diamond-anvil cell at the pressure corresponding to about 430 miles (700 km) depth in the Earth.
The size of the blue ringwoodite crystal is 0.1 mm across. The orange spots show where
the sample was heated with a laser to temperatures of about 2,900 F (1,600 C.) to form silicate
perovskite. Within these orange spots, signatures of melting were detected using infrared
spectroscopy and transition electron microscopy. Image by Steve Jacobsen, Northwestern Univ.
IF THAT DATA COMES IN SUPPORTING WHAT YOU JUST SAID, DOES THAT MEAN THE MANTLE OF OUR PLANET AND ITS WATER CYCLE COULD EXPLAIN ALL THE SURFACE OCEANS AND SEAS?
It's possible. But we are not able to determine that for certain at this time. The research shows that it's certainly possible for the oceans to have derived from the interior. And certainly, you could have a mixture. You could have some of Earth's water coming from comets and some of it coming from the inside.
COULD YOUR WORK THEN SUPPORT THE HYPOTHESIS THAT IN THE PAST WHEN THE EARTH WAS FULL OF VOLCANISM, THAT MANTLE LAVA COMING UP IN HUGE QUANTITIES COULD ITSELF IN THE DE-GASSING PROCESS HAVE PRODUCED LOTS OF SURFACE WATER?
Volcanism is the way in which planets degass. Whether or not that's how all of Earth's water degassed, I could not say. But the process of volcanism I can tell you is how planets degass, yes.
WE'RE ON A PLANET THAT'S ABOUT 4.5 BILLION YEARS OLD.DO YOU REACH A POINT GOING TO THE PAST THAT YOU NEED COMETARY ICE HITTING THE SURFACE OF THE EARTH TO GET A WATER CYCLE GOING THAT WOULD ALMOST DEMAND BOTH COMETS AS WELL AS WHAT IS HAPPENING IN THE WATER CYCLE IN THE MANTLE TO PRODUCE ALL THIS WATER?
Yeah, it's such a great question, Linda, and I'm not really sure I know whether or not that's the case. There are two camps and I suppose a hybrid one, who believe it could have been a little bit of both. One group believes that Earth's oceans were derived from the interior — that is the hydrogen and the oxygen in the oceans are original to the Earth and came out of the mantle through melting processes. This is the process that we call degassing. Once the Earth began to cool and the crust formed and plate tectonics started up, the mantle degassed our water.
Earth Oceans from Comets?
The other group would believe that water came much later in the Earth's history in the form of comets. And there are good arguments for both sides. Our study, which suggests the Earth's interior might contain a reservoir even larger than the oceans, I think points to the idea that Earth's water came from inside and it's original to the planet.
WHAT ABOUT THE REST OF THE SOLAR SYSTEM? THE RELATIONSHIP BETWEEN ALL OF THIS WATER LOCKED UP IN THE ROCKS OF THE MANTLE VERSUS THE SURFACE OF THE EARTH AND ALL THE QUESTION ABOUT MARS ONCE UPON A TIME PERHAPS HAVING LOTS OF WATER ON ITS SURFACE, WHAT DO YOU THINK IN THE WORK THAT YOU'VE DONE THAT WOULD EXPLAIN WHAT HAPPENED TO THE WATER ON MARS AND WHY YOU THINK THAT THE WATER ON THE SURFACE OF THE EARTH MIGHT BE RELATED TO THE WATER CYCLE IN THE MANTLE AS OPPOSED TO COMETS?
I guess I would ask people to think about differences between Earth and Mars. Whereas we have late tectonics on Earth, we don't see evidence for plate tectonics on Mars.
BUT IT HAS THE LARGEST VOLCANO IN THE ENTIRE SOLAR SYSTEM. EVEN IF IT IS NOT BLOWING LAVA, IT IS STILL THE LARGEST VOLCANO IN THE SOLAR SYSTEM.
That's right. A volcano is absolutely evidence for some type of process that would be like plate tectonics. Certainly there could have been something like plate tectonics going on in the past. But today we don't see any evidence for active volcanism on Mars. So if Mars had plate tectonics, you're right, it could have.
The largest volcano in the solar system is Olympus Mons on Mars, which rises
15 miles (24 km) and is 342 miles (550 km) wide. On Earth Mount Everest is the
tallest mountain at 5.5 miles high and Mauna Loa in Hawaii is largest volcano
rising 2.6 miles high (9 km) and 75 miles (120 km) wide. It's the lower Martian
gravity and lack of surface tectonic motion that allowed the 15-mile-high
altitude of Olympus Mons. Photograph by NASA's Mariner 9 in 1972.
Martian volcanic activity apparently has shut down. So, why has it shut down? Has it shut down because there was no water? No oceans to sort of lubricate these crustal movements like subduction?
We think that the entire upper part of the Earth at one time was completely molten. We do have theories about there being a magma ocean within the first couple of hundred million years of the Earth. So during magma ocean processes, it would not only have been delivering hydrogen atoms to the surface, but it would have been very rapidly lost to space at that time because there was no atmosphere. So, sometime in between the magma ocean stage and the formation of the crust, we probably would have begun to retain water on the surface in the form of hydrous minerals and eventually the building up of an atmosphere. But I think it's feasible for the oceans to have come almost entirely out of the inside through the process known as degassing.
THAT WOULD BE VOLCANOES.
What Shut Down Mars
Volcanoes and Flowing Water?
YOUR RESEARCH DOES BRING UP ALL THESE QUESTIONS BECAUSE MARS HAS BEEN SO MYSTERIOUS ABOUT HAVING THE SURFACE CHANNELS SUGGESTING THAT THERE WAS WATER THERE. AND IF WATER ON OUR PLANET CAME FROM THE MANTLE AND A WATER CYCLE, YOU WOULD THINK MAYBE THE SAME THING WAS HAPPENING ON MARS THAT HAS THE LARGEST VOLCANO IN THE SOLAR SYSTEM. SO WHAT COULD HAVE HAPPENED TO MARS TO SHUT DOWN THE VOLCANOES AND ALL THE WATER IS GONE?
There are a couple of reasons having to do with Mars does not have this layer in the mantle that we call the transition zone. You have to go much deeper into Mars in order to stabilize the minerals that I'm studying called ringwoodite and wadsleyite. Those are the reservoirs of hydrogen in the mantle that we're studying in this work.
Perhaps the other reason has to do with the atmosphere. So in the very early formation of the planets, the mass of atoms in the atmosphere can determine the ability for things like hydrogen to escape into space. So once the Earth's atmosphere was oxygenated and became more massive, it became harder and harder for things like hydrogen to escape. So, once we had a dense atmosphere, we probably stopped losing much of our degassing water. Perhaps that's why maybe Mars lost more of its early water because the atmosphere was different.
HOW FAR BACK DO WE GO IN THE HISTORY OF OUR PLANET BEFORE WE GET TO THE FIRST OCEANS?
That's a really good question. We have evidence for oceans as far back in time as maybe 3 to 3.5 billion years ago. There are quartz pebble conglomerate rocks in Australia that are of that age — they are pre-Cambrian in age. What that means is there was flowing water because you can't round a sediment and form a sedimentary rock like that without there being flowing water.
So, it looks like the Earth's oceans formed very early on in its history and plate tectonics has also been operating for at least three billion years on Earth, if not even longer.
SO, IF YOU ARE RIGHT, WHAT YOU HAVE FOUND IN YOUR RESEARCH EXPLAINS HOW WE COULD HAVE SO MUCH WATER ON THE SURFACE IF IT COMES FROM A VERY DYNAMIC EARLY PLANET IN WHICH THERE WAS A LOT OF OUTGASSING.
This is one of the potential implications of the work. It could have doubled the size of our current understanding of the amount of water (inside Earth).”
Other Solar System Water Mysteries:
— Martian Surface Soil Contains Water
NASA's Mars rover, Curiosity, has found that surface soil on the Red Planet contains about 2% water by weight, equal to roughly 2 pints (1 liter) of water in every cubic foot (0.03 cubic meters) of Martian dirt. That research was published in the September 26, 2013, Science journal.
Using its Sample Analysis at Mars instrument, or SAM, Curiosity heated scooped up Martian dirt to a temperature of 1,535 degrees Fahrenheit (835 degrees Celsius), and then identified the gases that boiled off. SAM saw significant amounts of carbon dioxide, oxygen and sulfur compounds — and the 2% water by weight. That means the Martian soil is sucking out the thin atmosphere's moisture like a sponge. But where is all the surface water that used to flow in the river valleys photographed by orbiters?
— Europa's “Huge Ocean,” Where Did It Come From?
To test speculation that Europa's huge ocean holds life, the joint ESA and NASA Europa Jupiter System Mission is considering whether the surface ice is thin enough that a future mission could drop hydrobots to burrow into the ocean, search for life and learn more about what the source of all the Jupiter moon's water is.
Artist's concept of the cryobot and hydrobot. These robots are in the very initial stages
of design and may look very different as the robot design evolves. See NASA JPL.