This is geology, B. I'm all of us trampled.
People who study the Earth, have an enormous advantage over those
who study other celestial bodies such as other planets and the
Stars. That's because we live on the
earth. So we can explore its surface at
will sample and analyzes rocks. Listen to its earthquakes and
drill holes into it, to get at least a few miles below the
surface. So why would we want to look to
another planet or even to another solar system to help us
understand the Earth down? Mackenzie is Professor of Earth
Sciences at University, the best known for discovering the theory
of plate. Tectonics in the late 1960s.
He is also among many other subjects, made fundamental
contributions to our understanding of the Moon, Mars,
and Venus Dan. Mackenzie welcome to geology B.
Thank you. I know one shouldn't really
attach labels to people but I think of you as an earth
scientist. So when you first turn your
attention to Venus, did you do that?
Because you are anyway, expanding your research beyond
the Earth. Or was it because you actually
thought Venus might offer us some Clues as to how things work
here on Earth. My interest is being in
essentially how the rocky planets behave and if you work
on the earth who got one, you've no idea how special and what
actually governs things here. And whether if you had another
rocky planet, whether it would look the same, it's always a
help to look at more than one thing that was really why I got
interested. I'd always been interested in
Venus, but the problem with Venus is that it's covered with
clouds made out of sulfuric acid and these never clear.
So nobody had the slightest idea before radar as to what was
underneath the clouds. So yes, we knew how big it was.
It's more or less the same size as the Earth, more or less, the
same mass of the Earth. And if the next plan is in, from
the earth, towards the Sun. So it seemed really quite likely
that We would learn something by looking at Venus and that was
really why I applied to be an investigational Magellan
program, which was a spacecraft, which would being sent to Venus.
So, with Magellan, we finally could look under the clouds as
you say with radar and it was very successful at doing that,
the main purpose was to map the surface, but you had an idea
that we could do something else with the data that was sent back
from the spacecraft. Once it was in orbit around
Venus, I was more interested In the interior of the planet.
Because on Earth, the plates are moved by convective forces.
And what I wanted to know was whether the same things
happening on Venus and if so, we could learn a lot about it
because we didn't really understand what actually
controlled the plate motions that we can see it all going on,
but it was really quite strange notion that cats.
So Caps of the earth whose distance across is perhaps
10,000 kilometers like the Pacific.
Just to be clear. These spherical caps are the
plates of plate tectonics. Yes.
And the elastic part of these things is about 30 kilometers
thick, that these should actually move as rigid caps as
most extraordinary notion. We're all quite used to it now
because, you know, we can actually see that motion with
GPS with earthquakes and things, but before we actually could see
it. I mean, nobody really would have
taken that seriously. So what actually controls what's
going on that was what interested me is where the Venus
was doing the same thing and if so could we learn about what was
going on from essentially looking at the gravity field,
which is something that you get for free.
There goes base. Craft going around in orbit
around the planet, Venus undergoes the acceleration due
to gravity. But the gravity field of planets
Venus of the earth is not just that of a point Mass, it
actually all the things like mountains and stuff or produce
gravity Fields. So you can learn the very great
deal about what's going on inside the planet by looking at
the gravity field, which is controlled by essentially
density variations inside the planet and that was what I hope
to be able to do. But the mountains on Earth and
on Venus are pretty small compared to the planets
themselves. So these must have been pretty A
small fluctuations or they're not very big on Earth the few
parts in 100,000, there's a method by which you can actually
track the spacecraft using Doppler and you can measure the
velocities of the spacecraft at the other side of the solar
system to a few tens of microns. The second set is perhaps a foot
an hour is about the source of be that you've measure.
And if you take the change with time, that tells you the
acceleration, In the direction which are looking so by using
this, which is actually how NASA navigates it spacecraft.
So I didn't have to do anything special.
They were doing this anyway. You can actually see the gravity
field of the planet directly as a sort of map from the way that
you're looking. And this was what really
interested me. But the problem was that when
they spacecraft was closest to Venus.
And so the most sensitive to its gravity, it was making images of
the surface only when it was far away.
Was it actually pointing back to the Earth.
And so, I had this frustrating business for the first,
something like two years and spacecraft.
You didn't measure any gravity at all and that was very
frustrating. I just had to live with this
hoping that the spacecraft would not fail before.
Actually, it could do something, which was my principal motive.
For being involved, unfortunately, the Imaging
failed, but we still have communications with it, so I
was. Alright.
So here we had a failure on the spacecraft.
That actually was a good thing for you, save me, but then
something else happened. It's of course, it costs 40
million dollars a year to keep the spacecraft running.
And once all the people who were making images of the surface
when they're longer interested, then NASA of course its initial
reaction was To shut the spacecraft down but we persuaded
them not to at least for a couple of years and then they
got finally. So frustrated, they used little
bit of fuel that was left to actually put the spacecraft into
Venus, right? Destroy it and that stopped all
argument. How long did you actually have
then to make these measurements it takes in ass about 250 days
to rotate and the spacecraft will be.
It doesn't rotate. So you get one.
Look at Hope. About once every 250 days.
So the mapping took or I think about two years and that after
that we then got three Cycles looking at the gravity so it was
a very slow business. So you said, you could measure
the speed of the spacecraft to enormous accuracy, using the
Doppler effect on the radio signal coming from the
spacecraft. But how does that tell you what?
The gravity field is does it speed up a bit when the gravity
is a little bit stronger and slow down a bit when the gravity
is a bit weak. Aker yes.
The gravity field is just the accelerations and that's the
gravity field at the satellite. So that's a straight measurement
at the satellite. You just have to look at the
rate at which the velocity changes and that's the gravity
in the direction. You're looking.
It's not the gravity at the surface.
That's a more complicated thing is because it's a very
straightforward measurement. Trust direct from the Doppler
shift and that's what I actually use the Doppler shift itself.
Okay, so you've got the gravity fluctuations.
Now, what did that tell you? What I did was to look at the
relationship between the gravity and the shape of the plant,
that's about your fee. And what I was interested in was
essentially, the thickness. What on Earth is the thickness
of the plates in Venus, there's a layer which essentially quite
strong and can support short, horizontal wavelength features.
So small mountains, there are completely supported by this
elastic layer. And when you look at the
gravity, there's things you Just looking at the mass essentially
that would be short wavelength features just to interjection
explain a couple of things here. Short wavelength, now means
special wavelength in other words, distance scales on the
surface. Nothing to do with the
wavelength of the Doppler signal that you're listening to and by
elastic layer. You mean, even though it bends,
it's a rigid layer as opposed to a mushy ductile layer.
That wouldn't hold anything up at all.
Yes, but Anyway, what we knew was that long wavelength
features, like for instance, too bad, these are not supported
elastically except at their edges May float.
They're like icebergs floating in the mantle, the crust of the
earth that partner we're used to has a lower density than the
interior. So, if you have a thick layer of
the crust, it essentially, floats in the material below.
The gravity field from a floating object, It is nothing.
So, the topography then, is what we call compensated, which means
that it doesn't produce a gravity field and a changeover
from things which do produce gravity field to things, which
don't obviously tells you just like, you know, walking on ice.
You can tell how thick the ice is by how much it bends that
tells you the thickness of the elastic.
Learn this is what we wanted to get at.
What I wanted to look at was the relationship of the How do you
feel to essentially the surface Topography of the planet?
Also an iceberg. Marooned on a totally rigid
plate and reaching the same height as the floating one.
In other words, with identical, topography will increase the
gravity because the whole of the iceberg is so to speak extra
mass without any low density. Compensation below it and
Venus's plate will be somewhere in between and we're in between
it is Is what you're finding out.
Yes, it's not quite as simple as that because there's also
another effect, which is that on Earth, the mantle is is actually
convicting, it's sufficiently fluid.
Its viscosity is low enough. That actually, there are regions
where the mantle is coming up and going down just like putting
water in a saucepan and boiling, it.
And that also produces a gravity field because it pushes the
surface where it's Me up. It pushes the surface up when
it's going down, it pushes the surface down.
Let me try and explain how this affects the gravity field.
If we think about a hot spot where the mantle is coming up,
the density, there is less than its surroundings because of
thermal expansion, that would have the effect of lowering the
gravity field above it. But as you said, it also pushes
up on the plate that lies above it, bending it.
So it bulges upward and the extra material, In the Bulge
increases the gravity. It turns out that this increase
outweighs the decrease from the lower density of the material
below with the net result that gravity increases by about a
third of what we would expect from the effect of the surface
bulge alone. So you have to have enough
information to be able to distinguish between these two
things. And the way in which you do that
is that actually the signal, the amount of gravity you get Km of
elevation is different in the two cases.
So that was what I wanted to get at and that means essentially
working over a range of wavelengths and seeing where
changes from floating to being essentially supported
elastically and that we could do and what we found, which
surprised me very much actually when we found it was that the
elastic layer on Venus is 230 kilometers.
And that's the same as the thickness on Earth.
But only the thickest part of things on Earth are the thickest
part on Earth is old oceanic plates.
And where the surface rocks are very old where there are more
than a thousand million years old underneath those, the plates
of called, they've got strong, they got elastic and they are
also about 30 km thick, but the bottom of that layer, the
thickness of the ice. They're governed by the
temperature and the bottom of that left and both places is
about 450 degrees Centigrade. The surface temperature of Venus
is about 420 degrees, Centigrade so I not expected to see
anything like that. I hadn't expected Venus have any
strength at all. Expected us to be looking at the
convective signal from the mantle convection.
We could also see that clearly, but we could see both things but
the The thing that really surprised me was how thick the
elastic level. So you are thinking that because
Venus was so hot, the Rocks had to be so weak.
That really all the mountains would be sinking into the bottom
and bit more like the floating icebergs situation or any rather
than the rigid plate situation. Exactly.
We had no previous knowledge, you can already tell.
This, of course, if there are mountains on being as which is
supported by the convection, you really need both the topography
and the gravity before you can tell.
That being pushed down by convective, circulation
underneath, or whether they're supported elastic key.
And once we got the gravity, we could see that very clearly.
So, how can such incredibly hot rocks?
Be so strong on Venus, that was the great surprise.
And that course, particularly David coasted in Minnesota.
To look at the role of water. We knew that quite small amounts
of water in rocks, made them week.
But what he found was that really quite minut.
Amounts of water were important, just simply transferring the
samples. He was going to test from the
oven in which they were dried to The Apparition which was going
to test. They absorbed enough water from
the atmosphere just on that transfer.
To change their properties. And this we had really no idea
about before Venus. We know that the Earth has got
about 100 parts per million of water in the mantle and Dave
because it experiments particularly show that this is
quite enough to make it soft and really very soft whereas Venus
because it's extremely hot and also its atmosphere is extremely
dry if you would condense all the water in Venus's atmosphere,
The sphere onto the surface, you get oceans, which about 10 cm
thick. It's really as dry as a bone and
it's very hot. So it's like they've caused it's
often and that affects both the elastic properties and also the
viscosity of the correcting mantle and that then really
solve the problem. But we had previously really no
idea that is very small amount of water on Earth in the mantle
play. Important role particularly in
plate tectonics. Then when we looked at Venus
most of the planet is absolutely not like the Earth.
It is not covered with moving plates.
There are a few places on the planet, which do look quite like
some of the features on plate boundaries, but most of the
planet is not like that at all. It's just covered with lava
flows. It's essentially a one plate
planet. Now, as far as we can tell, Ow.
And for me a justification for looking at another planet, we
had no idea that water was so important.
That's fascinating that by looking at Venus and seeing how
dry it was and how different it behave from the earth that
really made us look again at the importance of water on Mars,
even though we already knew how much water there was in the
rocks on Earth, we just didn't understand its significance.
We hadn't really thought that such small amounts of water
could matter. Or that the experiments that we
were doing, which we thought were dry, not actually, as dry,
as we thought they were this mattered.
I mean, yeah, it's a very nice illustration of how you need
both the observations from The Real World.
And also, then to do lab experiments properly, control
lab experiments to see what's going on.
And that was really, a very nice illustration of the importance
of both. 'The, I'm struck that since you
found out that even a tiny amount of water can make rock
softer. It's all the more surprising
that on Earth, the spherical caps, which you referred to
earlier, which can be 10,000 km wide move about as rigid plates.
Well, yeah, that was why they took such a long time for
Georgia's to recognize, but plate motions on Earth.
And I mean, this, this sort of behavior is really only true for
the As she anak awesome. The plates conscience.
Deform like entering and much more distributed.
If you look at a map of earthquakes from Northern India
2000 km North from the whole of that region is deforming.
And in the oceans that's absolutely not the case.
The plate. Boundary beneath Japan is really
sharp. It's just one big fault which
moves in the Japanese earthquake.
It moves, 70 meters. Now the really isn't anything
like that on the continents. It was really only there.
Suppression of the oceans, which showed what was going on.
And once we had a good understanding of that, we then
could go back to the contents and see what was going on.
And of course, GPS, we knew what was going on really from
earthquake and from magnetic anomalies in the oceans but any
remaining doubts were completely squashed by the GPS
measurements, much easier to do GPS on land, and it's just to do
it at Sea and that's being done all over the world's continents
and confirmed. Absolutely everything that we
thought was the case from the earthquakes for my last
question. I want to come back to your
point that we had to look at another planet to get a better
perspective on our own and the fact that we're advancing by
Leaps and Bounds. Now in our ability to look at
planets around other stars, in fact we found over 4,000 of them
and the numbers increasing rapidly.
I'm wondering if you think that's going to help us
understand more about the Earth and how it got to be where it is
in our solar system. I'm sure.
Well up until discovery of all these exoplanets, we only really
had one solar system to look at. And furthermore, the things that
happened which formed it happened.
Four and a half thousand million years ago and the events are
clearly very strange. There's a lot of very odd
things. One of the ones which is very
simple State is a lot of meteorites, the rocky
meteorites, contain these small spheres of rock which is
actually mostly Olivine, which is the same composition as the
Earth and these things have obviously been molten.
That's very cool because of surface tension, they're all
mixed up. There are some extremely high
temperature once a much lower temperature ones and the Matrix,
a lot of it is Clay. Its it contains water so So how
in the world are these things formed and how they put
together, the composition of the earth.
Looks like essentially one set of these meteorites and we have
a whole lot of really, quite speculative theories about this
but we've daily, don't know. And that the whole process is go
on as a star forms, rather hard to image.
That is a long way away in clouds of dust and they form a
great variety. Of different sizes.
And during the formation of these stars and their planetary
systems, the big ones evolved very fast explode and do a lot
of radioactive material, which we can see in the meteorites
that when they were formed, certain of the elements were
extremely radioactive and then they decayed.
We only really understood that once we saw how stars, formed in
these clouds, and I'm quite sure the same scheme to Be true of
all the rest of how the solar system was put together once we
can image these remote ones that we've now discovered.
But yeah, technology is marvelous.
It just Marches On and all kinds of things will become possible
in the not very distant future and I'm quite sure there are
lots of them. We're going to find some, which
are forming now and that will be an enormous help to
understanding how Solar system got put together because the
problem is we don't even know how common solar system like
ours is because we live on this one.
We know that we need water to live and all the rest of it.
How common is that? We don't know if it wasn't like
that, we wouldn't be here to look be looking at it so that
extremely difficult to know how to attack problems and this
especially when there is complicated as this and one of
the things which has really come from looking at the planets of
this solar system is particularly come from their
satellites and they're all different.
They just extraordinary variety and quite astonishing that they
look as different as they are. We don't know much about the
insides yet most of them anyway but they are quite different
from each other and so presumably the same is going to
be true of solar systems that they're going to be completely
variable. Not set me.
What's coming out from the little bit of knowledge that we
have of them so far. Definitely going to do another
podcast with you. When we have enough data on
these new solar systems will be dead.
Dan Mackenzie. Thank you very much.
Good.