This is Geology Bites with Oliver Strimpel.
Imagine forcing a piece of material no bigger than a pencil
eraser into a vice that generates pressures and
temperatures equivalent to those at the centre of the earth.
This is what happens at the Z Machine, a facility at Sandia
National Laboratories, and my guest today uses it to
understand what materials are really like deep inside planets.
Most of the material inside the Earth and other rocky bodies
exists under conditions utterly unlike anything on our surface.
Descend just a short distance below the surface, and pressures
and temperatures quickly reach values orders of magnitude
beyond our everyday experience. To understand how different
planets work, why some are geologically dynamic and others
are static, and how their interiors have evolved since
they formed, we really want to replicate those conditions in
the lab and see how familiar materials respond.
And that is the focus of Steve Jacobson's research.
He is a mineralogist and material scientist and a
professor in the Department of Geological Sciences at the
University of Colorado Boulder. What makes his work especially
striking is the range of extremes he tackles.
In this episode, we'll explore three very different
environments he has sought to replicate.
The crushing pressures at the core mantle boundary, nearly
3000 kilometres beneath our feet.
The mantle transition zone, where minerals can lock away
vast quantities of water. And a completely different kind
of extreme The harsh vacuum and radiation of the lunar surface
where NASA's Artemis program hopes to build the permanent
base. Steve Jacobson, welcome to
Geology Bites. It's great to be here, Oliver.
Thank you for inviting me. Let's start at the very bottom
of the mantle, the core mantle boundary.
Why is it important to study this region and how can we try
to comprehend the pressures we're dealing with down there?
The reason why these are interesting environments to
study materials in is because most of the universe, those are
the conditions under which materials persist, including
planets and stars. Now, when we look at our own
planet, the Earth, the biggest boundary really below the crust
is the core mantle boundary. And as you mentioned in the
introduction, it's around 3000 kilometers depth, like flying
from Chicago to Hawaii, but straight down.
The unit of pressure that we use is called a gigapascal, so a
billion pascals. And to put that into
perspective, one giga Pascal translates to about 10,000 bars,
and a bar is about an atmosphere.
So you could think of 1 giga Pascal as being around 10,000
times the squeezing force that the air is putting off your
forearm. Then pressure is force over
area. And so we'll be looking at how
using just the very tiny tip of a diamond with a tip about the
diameter of a human hair that we're able to generate these
pressures and static compression experiments.
The pressure at the core mantle boundary is around 120
gigapascals. It's quite a lot.
And temperatures we know much less well.
That is to say, we can calculate the variation of density or
pressure within the Earth pretty easily just using mechanics, but
the temperature is unknown. We have estimates based on
experiments and theory. For example, if you were to be
able to determine the melting temperature of iron at 120
gigapascals, you'd have a pretty good idea of what the
temperature might be. We estimate the temperatures at
the core mantle boundary to be somewhere between 3 and 4000°C.
So, as I mentioned, you use the Z machine at Sandia National
Laboratories, a facility originally designed to simulate
thermonuclear detonations. Can you walk us through what
actually happens during an experiment?
What it does is it's a very large storage device of
electricity, and it uses a concept called pulse power,
which is to say that if we could compress electrical energy in
space and time to the point where there's a very high
density of energy, we could transfer that energy to a
material and basically blow it up.
And by doing so, understand or measure in situ during a very
brief experiment what some of the thermodynamic properties of
those materials are. So it's plugged into the wall
and it charges up current. The capacitors are able to store
around 20 mega amps of electricity.
That's 20 million amps and the target in the centre of the
machine is meant to be a short, an electrical short.
So imagine if you were to short out two wires, you would see a
spark. That's what the Z machine does.
But along the way it's compressing those 20 mega amps
into a very, very short pulse, just a few nanoseconds in
duration. And by shorting out that current
at around 100,000 volts, we're able to push on the sample to
many, in fact thousands, of giga Pascals.
So traditional shock compression follows something that's known
as the Hugoniot curve, where both pressure and temperature
shoot up together. But I gather the temperatures
that you get that way are much higher than what we'd actually
expect inside the Earth. So you've found a way around
this. Can you explain how you do that?
Yeah, you brought up the Hugoniot.
That's a very important concept in shock physics.
But as you mentioned, the states of pressure density temperature
along the Hugoniot don't quite match what we find to be in the
Earth. The pressure temperature that we
experience when we go down in the Earth is called the
geotherm, and that's basically a measured temperature in a
borehole, for example. But once you go below the crust,
the temperature pressure path follows what we call an adiabat.
An adiabatic compression means there's no flow of heat in or
out during compression. OK, So what we want to try to do
is mimic an adiabat, not the Hugonio.
So how does the experimental setup enable you to depart from
what you get if you just whack the sample with the hammer?
Yeah. There's actually 2 stages of
compression that we're using in our pulse power experiments on
the Z machine. The first step is sort of like a
conventional shock experiment where an impactor reaching many
kilometers per second impacts the sample to produce a
shockwave. So you want to think of Stage 1
as a shock, and that brings the pressure temperature state in
the sample up to some point on the Hugonio.
What's unique about the ZE machine is that we can control
the release of current in a way that leaks out the energy.
Instead of it producing a single sharp pulse, which one might
want to use for a fusion experiment, we can pulse it in,
but then let the rest of the energy slowly creep into the
sample. So what we're going to do is
shock up to some state close to the core mantle boundary
condition and then what we call ramp compress the rest of the
way. It's the ramp compression that's
actually following a pressure temperature curve that's very
close to the Earth's Adia bat. And in order to do that, it's
not only the design of the target, which is the piece of
metal that the sample is sitting on, it's actually V short that
produces the lightning bolt that shocks everything.
So the design of the target itself is what allows us to ramp
compress following the shock. So we call this technique shock
ramp compression. And that enables the pressure to
go up without the temperature shooting up beyond what we
expect to prevail down there. Yeah, that's right.
You also use a completely different approach, diamond
anvil cells, where you squeeze tiny samples between diamonds
under static compression. How does that compare with what
you do on the Z machine, and when would you use one technique
rather than the other? Yeah, the diamond anvil cell is
a very versatile tool. And these static compression
experiments are different principally by the fact that
they are isothermal. And that is to say, if I put a
sample in the diamond anvil cell and I increase the pressure, the
temperature remains room temperature.
And that's because the heat due to compression, unlike an
idiabatic compression, does whisk away into the environment.
So you might have a sample sitting at core mantle boundary
pressures in a diamond anvil cell, but it's at room
temperature. Now, the advantage of the
diamond anvil cell is that there's a lot of different
measurements we can make during compression.
For instance, we might use synchrotron X-ray diffraction to
determine the structure of the material at high pressure.
But the diamonds are also transparent to most of the
electromagnetic spectrum. So we might use optical
techniques, lasers, UV, visible light, infrared light.
There's a lot of different tools that we can perform in situ
experiments on materials under static compression.
Therein lies the real power of the diamond anvil cell.
Now, it's not to say that you can't heat the sample.
Many of my colleagues do heat samples in diamond anvil cells
with lasers or even with resistors.
You know, you can wrap the diamond anvil cell in a little
tungsten wire and heat up the whole thing.
Of course, you can't go to very high temperature that way,
because if you're not careful, the diamonds will burn.
But what happens is that the quality of the physical
properties measurements and the types of physical properties
measurements you can make while heating a diamond anvil cell
become very limited. So we'll go back to the Z
machine for a minute. The advantage of that is that we
can simultaneously achieve a pressure temperature equivalent
to that found at the centre of the Earth while making a density
measurement. And that's one of the key
parameters that we want to know about materials that conditions
of planetary interiors, because what do we know about the core
mantle boundary? Well, there's really only a
couple of things that we know about it based on the
propagation of seismic waves through the Earth.
We know about what compression velocity, the seismic P wave
velocity is, what the seismic S wave velocity is, and we know
what the density is. Those are the three principal
things that we know about the interior of the Earth from
seismology. So that's kind of what we're
after, the sound velocities and the density.
So that reminds me of a previous episode of Geology Bites I did
with Barbara Romanowitz, who talked about how seismic
tomography has revealed a couple of enormous regions near the
core mantle boundary in which seismic waves travel anomalously
slowly, the so-called large low shear velocity provinces.
And at their edges, they're even more extreme, the so-called
ultra low velocity zones. How's your work shed light on
what might be going on in those regions?
Not yet, but that's what we're working on.
The goal of this study is to try to test a couple of different
theories on why those ultra low velocity zones are there, you
know, and what they are. I'll say a few more things about
them. You mentioned already that
they're slow. That is why they're called ultra
low velocity zones. But another thing about them
that I think is kind of strange, they're very dense and normally
seismic velocities follow density in a way where lower
density produces lower velocity. So this is sort of the opposite
trend that one would expect for a material.
So initially there was a couple of theories about why that might
be maybe partial melt, right? Because liquids, molten rock
might be quite slow. Of course, we don't know exactly
what silicate melt sound velocities are at those
conditions. We're working on that now.
But we estimate that they're similar to melts in the Earth's
crust near the Earth's surface, and that they're they're
relatively slow compared to rock.
But the problem with that is that melts are also typically
lower density. So unless something weird
happens at very high pressures and melts become denser than
rock, which might be true, that theory has probably become a
little less intriguing than another which we're testing on
Z. And that is to modify the
composition of the rock in a way that is so extreme it produces
very low velocities but very high density.
And there's one element on the periodic table very, very common
in the Earth, which tends to do that, and that's iron.
Now, iron exists in most minerals.
For example, everyone might know the mineral olivine or purexene
garnets. These are all sort of common
mantle minerals and they all contain iron.
What we have found in laboratory experiments is that when you
increase the fraction of iron relative to magnesium and a
silicate, the density goes up, but the sheer velocities tend to
go down. So the material that we're
putting on the Z machine is called magnesiowustite or
feroparaclase, and it's just an iron magnesium oxide.
So what we want to do is crank up the iron content in the
feroparaclase and measure the speed of sound and the density
on Z as a function of iron content to see if we can line up
the physical properties of feroparaclase with what
seismologists see in the ultra low velocity zones.
That's interesting. So density and seismic wave
velocity usually go hand in hand, but you're suggesting that
these ultra low velocity zones could be not melted but iron
riched. Yes, the theory that we're
testing on iron enrichment really goes back to plate
tectonics itself. We know that the core is full of
iron, mostly iron, and the mantle, the lower most mantle is
made of iron, magnesium silicates.
So this boundary is very interesting.
You know, are they reacting with one another?
Are they in equilibrium? And another thing that could be
going on at the core mantle boundary is a piling up of
subducted oceanic crust where the oceans form crusts.
They are also destroyed at the subduction zones along
convergent pipe boundaries and over the course of geologic
time, we speculate that some of these slabs go all the way to
the core mantle boundary. We can see that in seismic
tomography. And it's possible that once
subducted slabs reach the core mantle boundary, what do they
do? Do they just kind of pile up or
does that material get mixed and come back up in convective
cycles? Well, it's possible that the
basaltic piece of the subducted oceanic crust, if that were to
pile up at the large low shear velocity provinces, we would
have a mechanism for creating iron enrichment in the ultra low
velocity zones. Some people like to call this
theory the slab graveyard theory.
It's where maybe old slabs go to be buried.
Maybe that's what we're seeing. Let's move up to another extreme
environment, the Mantle transition zone.
Can you remind us where the zone sits and what defines it?
The transition zone of the mantle, so-called because
there's a steep change in gradient of seismic velocities,
is what we consider to be the change from the upper mantle to
the lower mantle. The top of the transition zone
occurs at 410 kilometers depth, and the base of the transition
zone occurs around 660 kilometers depth.
The reason I use such specific numbers is because it's defined
by seismic discontinuities. That is to say, when seismic
waves pass through these boundaries, there are strong
reflections and there are strong changes in sound velocity, so
they're probably associated with some sort of change in the
mineralogy. Can you tell us a bit about the
minerals that we think are down there that are changing the
velocity of the seismic waves? We have a pretty good idea.
Even though it's 400 kilometers away, that's within the range of
experiments that are readily available in the laboratory. 400
kilometers to 600 kilometers depth corresponds to pressures
in the range of 12 to 16 giga pascals.
Remember when we were down at the core Mantel boundary, the
pressure was 130 giga pascals. Up in the transition zone, we
can perform what are called large volume experiments that
uses a big hydraulic press, the same kind of press that you
might see, I don't know, car parts being made with.
So rather than a very tiny little diamond anvil cell,
picture now a 5000 ton hydraulic press where you're putting now
not micrometer size samples, but millimeter size samples.
So we have over time as a field, not me kind of mapped if you
will, the mineralogy, the phase diagram against what we observe
seismically. What we can tell is that
olivine, the most abundant silicate in the uppermost
mantle, transforms to a mineral we call watsleite.
It's a modified spinal structure at conditions that correspond
exactly to 410 kilometers depth. Within the transition zone,
there's another phase, another high pressure polymorph of
olivine, because wadsleite becomes unstable around 520
kilometers depth and it forms a mineral we call ringmodite,
which is one of my favorite minerals.
It's blue and it has a very large water storage capacity.
Below 660 kilometers depth, those polymorphs are gone and
every mineral becomes either what we call a bridgemenite
phase. That's like a pyroxene
composition, perovskite structure.
And what's leftover is the oxide we talked about a minute ago,
Pharaoh periclase, this MGFEO. So there's really just two
principal minerals in lowermost mantle, which means the
transition zone from the uppermost mantle to the lower
mantle is a place where there's some pretty unusual minerals,
Wosliite, ringwoodite, majoritic garnet.
Have you been able to study the physical properties of these
strange minerals, wasleite and ringridite, in experiments that
you've done in the lab? Yeah, the.
Reason why I got interested in Wasleite is because in the late
1980s, Joe Smith, who was a professor right here at the
University of Colorado Boulder, predicted that the structure
ought to incorporate large amounts of hydrogen defects.
And by that I mean normally just a magnesium silicate.
It's very likely that that structure would substitute
hydrogen for magnesium defects, and the reason why has to do
with electrostatic potentials. In other words, if we look down
in details, gory details, at the structure of wozlyite, there's
an oxygen atom which is a little bit under bonded.
It's kind of looking for a little bit more cation charge
than it has. When he realized this, that's
when Professor Smith suggested that hydrogen atoms might
balance out that sort of charge imbalance in the structure.
And it wasn't long before people actually made the stuff in the
lab very easily with water putting several thousands of
parts per million or even weight percent of H2O.
Not liquid water, but the compositional ingredients of
water were just going right into the material, sort of like it
was acting like a sponge. So these minerals can actually
effectively store large quantities of water.
In the lab. So now the question is what
about the Earth? And I want to bring us back to
the core mantle boundary. Many years ago, I think it was
Quentin Williams and Ed Garnero who noticed that there was a
very rough geospatial correlation between the ultra
low velocity zones and the ocean islands.
That is places like Hawaii and Iceland where deep sourced
magmas are coming, probably from the core mantle boundary.
So if the ultra low velocity zones are the source of deep
melts in the Earth, those must pass through the transition
zone. And this gets me back to why
water might be important in the transition zone.
Geophysical research has had the perennial problem that magmas
that come from different depths, for example mid ocean Ridge
basalts come from very shallow depths and ocean island basalts
come from very deep depths have different compositions,
especially with respect to isotopes.
Now, that might not sound like a problem, but when you remember
that convection is supposed to be occurring on a whole mantle
scale, how then do you produce different magma types in an
Earth or a planet that is mixed through convection?
It's a dichotomy that the field of geophysics and geochemistry
and mineral physics has been grappling with for decades and
more. Well, the transition zone might
fit in very nicely because if the transition zone were acting
like a sponge for incompatible elements, we call them like
hydrogen, it's possible that it might be stripping away some of
the incompatibles as melts are ascending through the entire
mantle from the core mantle boundary and almost like
trapping it there, if you will. I want to associate that idea
with Shun Corrado. And David Bercovici at Yale some
years ago wrote a paper that they called the transition zone
water filter model. And it was a very nice theory on
how the transition zone might not only be acting as a large
reservoir of H2O in the Earth, with implications for the the
origin of Earth's water, but also acting as one of these ways
in which we can produce different compositions of magma
in a whole mantle convection planet.
Wow, that's fascinating. So kind of dual role of this
mantle transition zone is a kind of sponge, but also as a filter,
Yeah, I think. That you can't have oceans on a
planet until the mantle is saturated.
Imagine trying to put a little lake on a sponge.
As you add water to the sponge, it would just kind of get
absorbed by the sponge until the sponge is saturated, and only
then would you be able to put a film of liquid water on the top
of it. It's possible planets might act
the same way, and that the inside of the planets that have
liquid water on their surfaces might need something like the
transition zone, and therefore planets only of a certain size
might be able to have liquid water on their surface.
That's interesting. So does that mean that a planet
really needs to have this kind of a sponge system to regulate
how much water it has, or to allow water to exist in liquid
form on the surface? I mean, are those two things
connected at all? It's it's a theory that people
are working on from modeling perspectives and experimental
perspectives. Let's look at Mars.
Mars is not as big as the Earth. Mars mantle barely gets to the
pressure high enough to support ringwoodite near the core.
So there is no transition zone in Mars.
And going one step further, the origin of Earth water was long
supposed to have come from comets after and during
accretion. But this idea that at extreme
conditions minerals can incorporate the components of
water hydrogen leads us to think about that again.
And it's quite possible that planets could retain quite a lot
of their original water. Now that we have more
appreciation for the properties of melts and minerals at
conditions of deep planetary interiors.
They seem to be quite happy holding on to the water and we
might not needed to have degassed all the hydrogen during
accretion, which is what we thought previously.
That's interesting. It reminds me of previous
episodes about the origin of the Earth's water, both the
meteorite origin. Sarah Russell talked about that
and then Lindy Elkins talked very much about the fact that
the the Earth was had its water and Anat Shahar about a way in
which it could make its water from a magma ocean and a
hydrogen atmosphere. So it seems like there are many,
many different theories really as to how the Earth got its
water and and understanding the mineral properties of the
transition zone pretty plays into that discussion.
Just to step back to the experimental side for a moment,
I'm reminded of the episode I did with David Kolstad, who also
does high pressure experiments. Can you say how your approach
differs from his? Yeah, Professor Kolstadt is a
rock mechanics expert and he studies what we call rheology.
So he studies the way solids deform plastically beyond their
elastic limit. All of the deformation that I'm
performing to the diamond anvil cell under static compression
experiments, when we squeeze at room temperature a sample, a
crystal of olivine, say to 10 gigapascals and bring it back,
that's a fully elastic process. The propagation of seismic waves
is also an elastic process in the sense that as the wave moves
through the material, there's compression and there's
dilation, but those changes are completely reversible or
elastic. And what David does is push
rocks to their elastic limit and then studies what happens
thereafter. Now, that's very important in
the context of mantle convection because there are places in the
uppermost mantle and crust where shear stresses are so high that
there's plastic deformation going on.
We can see that. And you go outside and look at
metamorphic rocks and fabric forming in very high stress
shear zones. So there's a contrast between
work in rock mechanics that deals with those rheological
properties, and mineral physics which deals with more
fundamental intensive physical chemical properties such as
density, thermal conductivity, heat capacity and so on.
So far we've been talking about the extreme pressures deep
inside planets. Let's now shift to a completely
different kind of extreme the surface of the moon.
What makes that such a challenging environment for
materials? The challenge posed by doing
geology just even on the surface of the Moon, comes mainly from
low temperatures, high radiation.
It's very far away. And one of the goals of Artemis
is to learn how to get people onto the moon and off to Mars.
And so we have to kind of practice everything on the moon.
First thing you need to be able to do is land a rocket in the
same place multiple times. We need a base and the regolith,
the, the lunar soil, if you will.
It's just basically crushed rock.
It's very harmful. It's very brittle.
I mean, it's just nasty, glassy shards of minerals that if you
were set up at a camp already and I were to arrive with the
rocket a bit later and land without a landing pad, I would
sandblast the camp. So we're working in the area of
additive manufacturing. To develop the technology for
building a landing pad or a base on the moon requires using fake
lunar soil. We can't really work with the
real thing. So one of the things we're doing
is trying to understand simulants.
Those simulants are being then melted using a laser to produce
a rock. And I'm actually holding one in
my hand right now and looking at one of these lunar bricks, if
you will. It's pretty dark and it's kind
of layered. The layers have slightly
different shades. The rock has basically the
composition of a mixture of basalt and an orthocyte.
But it's layered because the process we're using is called
powder bed fusion. So imagine a sandbox of
regolith, of lunar regolith that you then use the laser to scan
and melt. And where those melt pools form,
we can dash more material in and build up a solid material one
layer at a time. And this is how we're working
towards developing 3D printing on the Moon.
Now obviously the lunar surface is a totally different
environment. As you said, it's a vacuum.
It's subject to extreme radiation.
Does that change the material? Is it something that makes it
really different from what we can simulate here on Earth?
It does. We call that space weathering.
You know, on Earth, when we talk about weathering, we're talking
about the reaction of rocks with water and the atmosphere and the
transport of that material through gravity on the moon.
When we talk about weathering or space weathering, we're really
talking about modifications due to radiation from the sun.
Those may include bombardment from high energy particles,
hydrogen, helium ions for example.
There is also a continuous stream of micrometeorite
bombardment on the surface of the moon.
And all of these processes modify the regolith in turn
because there is no weathering in the Earth perspective.
That is to say that the regolith sitting on the surface of the
Moon has been there for 4 1/2 billion years.
So it's highly modified through ion bombardment and
micrometeorite bombardment. The regolith simulant, the fake
stuff that we're working with to the 3D print is not space
weathered and we are very concerned about what that might
mean for the engineering. One of the things we see in real
lunar regolith is little tiny particles, actually nanoscale
particles of iron on the surfaces.
This nanophase iron would certainly have a big impact on
the laser heating properties of the 3D printing process.
So we'd like to try to simulate space weathering in the lab.
We actually have built a very large vacuum chamber inside
which we're doing this laser 3D printing.
What we find, for example, is that a lot of the elements that
are volatile potassium, sodium, I mean, they immediately boil
off during this process. So there's almost like a whole
new patrology that we have to explore under vacuum in order to
work with these materials on the surface of the moon.
Some of the experiments you do to understand planetary
interiors have created materials that have never existed before.
Can you give us an example? Sure.
Let me just start with one that is in full scale production.
The field of super hard materials has to do with
abrasives or materials that might be able to withstand very
high pressure temperatures without degradation.
We think of diamond an extreme material, but in fact diamond
burns pretty easily. Diamond will start to burn in
air around 700°C even though it's the hardest known
substance. And so the field of super hard
materials pertains to it doesn't necessarily have to be harder,
it just has to have higher thermal stability.
Cubic boron nitride is a good example of a material that was
discovered through high pressure temperature synthesis.
It has a hardness very close to that of diamond, but it's stable
up to like 1300°C. Now you need 5 or 6 gigapascals
of pressure and 1200°C to make it.
And so commercially, it's not easy, but it's set a scale where
you can buy cubic boron nitride parts.
The idea that we can study materials in extreme
environments of pressure, temperature, radiation and so on
to learn about the universe can be flipped around and put in a
way that we use those extreme conditions to create new
materials with functional properties.
And there's a whole suite of ferroelectrics, thermoelectrics,
super hard materials, materials with exotic magnetic states,
superconductors and so on that are being generated in these
very high pressure temperature states.
And so there's a whole branch of material science that comes out
of materials in extreme environments.
What are you most excited about working on at the moment?
I'm excited by the fact that we continue to make breakthrough
discoveries in science. When I look back and I think
about how much has changed in our knowledge about the Earth,
the mantle, the composition of the mantle, and the technology
we've developed everything from materials discovery to stuff we
just talked about 3D printing bricks or conducting compression
experiments where we measure sound velocities directly at
core mantle boundary conditions. I mean, in hindsight, that would
have looked like science fiction to me.
And I think that that's of course going to just continue.
So graduate students today can think about that for a minute,
that that what they'll be doing maybe 2025 years from now might
sound like science fiction to us.
Now. That's what I get excited about.
Steve Jacobson, thank you very much.
Thank you, Oliver. It's been really fun to talk to
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