This transcript was generated automatically and lightly edited for readability.

Oliver Strimpel: This is Geology Bites. I'm Oliver Strimpel. 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 analyze its 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?

Dan McKenzie is Professor of Earth Sciences at Cambridge University, best known for discovering the theory of plate tectonics in the late 1960s. He has also, among many other subjects, made fundamental contributions to our understanding of the Moon, Mars and Venus. Dan McKenzie, welcome to Geology Bites.

Dan McKenzie: Thank you.

Oliver Strimpel: I know one shouldn't really attach labels to people, but I think of you as an Earth scientist. So when you first turned your attention to Venus, did you do that because you were 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?

Dan McKenzie: My interest has been in essentially how the rocky planets behave. If you work on the Earth, you've got one; you've no idea how special it is, what actually governs things here, and whether, if you had another rocky planet, 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 of sulfuric acid, and these never clear. So nobody had the slightest idea, before radar, as to what was underneath the clouds. Yes, we knew how big it was. It's more or less the same size as the Earth, more or less the same mass as the Earth, and it's the next planet 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 investigator on the Magellan program, which was a spacecraft being sent to Venus.

Oliver Strimpel: 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.

Dan McKenzie: I was more interested in the interior of the planet, because on Earth, the plates are moved by convective forces. What I wanted to know was whether the same thing was happening on Venus, and if so, whether we could learn a lot about it, because we didn't really understand what actually controlled the plate motions. We can see it all going on, but it was really quite a strange notion that caps, spherical caps of the Earth whose distance across is perhaps 10,000 kilometers, like the Pacific...

Oliver Strimpel: Just to be clear, these spherical caps are the plates of plate tectonics.

Dan McKenzie: Yes. And the elastic part of these things is about 30 kilometers thick. That these should actually move as rigid caps is a most extraordinary notion. We're all quite used to it now, because we can actually see that motion with GPS, with earthquakes and things, but before we could actually see it, nobody really would have taken that seriously. So what actually controls what's going on? That was what interested me: whether Venus was doing the same thing, and if so, whether we could learn about what was going on from essentially looking at the gravity field, which is something that you get for free.

As a spacecraft goes around in orbit around the planet Venus, it undergoes the acceleration due to gravity. But the gravity field of planets, Venus or the Earth, is not just that of a point mass. All the things like mountains and so on also produce gravity fields. So you can learn a very great deal about what's going on inside the planet by looking at the gravity field, which is controlled essentially by density variations inside the planet. And that was what I hoped to be able to do.

Oliver Strimpel: But the mountains on Earth and on Venus are pretty small compared to the planets themselves, so these must have been pretty small fluctuations.

Dan McKenzie: Oh, they're not very big. On Earth, a few parts in 100,000. There's a method by which you can actually track the spacecraft using Doppler, and you can measure the velocity of the spacecraft at the other side of the solar system to a few tens of microns per second. That is perhaps a foot an hour; that's about the sort of velocity that you measure. And if you take the change with time, that tells you the acceleration in the direction in which you're looking. This is actually how NASA navigates its spacecraft, so I didn't have to do anything special; they were doing this anyway. By using this, you can actually see the gravity field of the planet directly, as a sort of map. And this was what really interested me.

But the problem was that when the spacecraft was closest to Venus, and so 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 two years or so of the spacecraft: we 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 it could actually do something, which was my principal motive for being involved. Unfortunately, the imaging failed, but we still had communications with it, so I was all right.

Oliver Strimpel: So here we had a failure on the spacecraft that actually was a good thing for you.

Dan McKenzie: It saved me. But then something else happened. 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 were no longer interested, NASA's initial reaction, of course, was to shut the spacecraft down. But we persuaded them not to, at least for a couple of years. And then they finally got so frustrated, they used the little bit of fuel that was left to actually put the spacecraft into Venus, to destroy it, and that stopped all argument.

Oliver Strimpel: How long did you actually have, then, to make these measurements?

Dan McKenzie: It takes Venus about 250 days to rotate, and the spacecraft's orbit doesn't rotate. So you get one look at the whole planet about once every 250 days. So the mapping took, I think, about two years, and after that we got three cycles looking at the gravity. So it was a very slow business.

Oliver Strimpel: 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 weaker?

Dan McKenzie: 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. But it's a very straightforward measurement, straight from the Doppler shift, and that's what I actually used, the Doppler shift itself.

Oliver Strimpel: OK, so you've got the gravity fluctuations. Now, what did that tell you?

Dan McKenzie: What I did was to look at the relationship between the gravity and the shape of the planet, its topography. And what I was interested in was essentially the thickness of the plates on Venus. There's a layer which is essentially quite strong and can support features of short horizontal wavelength. So small mountains are completely supported by this elastic layer, and when you look at the gravity of these short-wavelength features, you're essentially just looking at their mass.

Oliver Strimpel: Just to interject and explain a couple of things here. Short wavelength here means spatial 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 that even though it bends, it's a rigid layer, as opposed to a mushy, ductile layer that wouldn't hold anything up at all.

Dan McKenzie: Yes. But anyway, what we knew was that long-wavelength features, like, for instance, Tibet, are not supported elastically except at their edges. They float. They're like icebergs floating in the mantle. The crust of the Earth, the part that we're used to, has a lower density than the interior. So if you have a thick layer of crust, it essentially floats in the material below. The gravity field from a floating object is nothing. So the topography then is what we call compensated, which means that it doesn't produce a gravity field. And the changeover from things which do produce a gravity field to things which don't obviously tells you, just like walking on ice, where you can tell how thick the ice is by how much it bends, the thickness of the elastic layer. This is what we wanted to get at. What I wanted to look at was the relationship of the gravity field to, essentially, the surface topography of the planet.

Oliver Strimpel: So 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 where in between it is is what you're finding out.

Dan McKenzie: Yes. It's not quite as simple as that, because there's also another effect, which is that on Earth, the mantle is actually convecting. It's sufficiently fluid, its viscosity is low enough, that 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 where it's coming up, it pushes the surface up, and where it's going down, it pushes the surface down.

Oliver Strimpel: 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.

Dan McKenzie: So you have to have enough information to be able to distinguish between these two things. The way in which you do that is that the signal, the amount of gravity you get per kilometer 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 it changes from floating to being supported elastically. And that we could do. And what we found, which surprised me very much when we found it, was that the elastic layer on Venus is about 30 kilometers thick.

That's the same as the thickness on Earth, but only of the thickest parts on Earth. The thickest parts on Earth are old oceanic plates, and places where the surface rocks are very old, more than a thousand million years old. Underneath those, the plates have cooled; they've got strong, they've got elastic, and they are also about 30 kilometers thick. The bottom of that layer, the thickness, is governed by the temperature, and the bottom of that layer in both places is about 450 degrees centigrade. The surface temperature of Venus is about 420 degrees centigrade, so I had not expected to see anything like that. I hadn't expected Venus to have any strength at all. I expected us to be looking at the convective signal from the mantle convection. We could also see that clearly; we could see both things. But the thing that really surprised me was how thick the elastic layer was.

Oliver Strimpel: So you were thinking that because Venus was so hot, the rocks had to be so weak that really all the mountains would be sinking in, a bit more like the floating icebergs situation, rather than the rigid plate situation.

Dan McKenzie: Exactly. We had no previous knowledge. You can't really tell this, of course, without the gravity. If there are mountains on Venus which are supported by the convection, you really need both the topography and the gravity before you can tell whether they're being pushed up by convective circulation underneath, or whether they're supported elastically. And once we got the gravity, we could see that very clearly.

Oliver Strimpel: So how can such incredibly hot rocks be so strong on Venus?

Dan McKenzie: That was the great surprise. And that caused, in particular, David Kohlstedt in Minnesota to look at the role of water. We knew that quite small amounts of water in rocks made them weak. But what he found was that really quite minute 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 apparatus which was going to test them, they absorbed enough water from the atmosphere, just on that transfer, to change their properties. And this we really had no idea about before Venus.

We know that the Earth has got about 100 parts per million of water in the mantle, and Dave Kohlstedt's experiments particularly showed that this is quite enough to make it soft, really very soft. Whereas Venus is extremely hot, and also its atmosphere is extremely dry. If you were to condense all the water in Venus's atmosphere onto the surface, you'd get oceans about 10 centimeters thick. It's really as dry as a bone, and it's very hot. So it's like Dave Kohlstedt's oven, and that affects both the elastic properties and also the viscosity of the convecting mantle. And that then really solved the problem. But we previously had really no idea that this very small amount of water in the Earth's mantle played an 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. And for me, that's a justification for looking at another planet: we had no idea that water was so important.

Oliver Strimpel: That's fascinating, that looking at Venus, and seeing how dry it was and how differently it behaved from the Earth, really made us look again at the importance of water on Earth. Even though we already knew how much water there was in the rocks on Earth, we just didn't understand its significance.

Dan McKenzie: We hadn't really thought that such small amounts of water could matter, or that the experiments we were doing, which we thought were dry, were not actually as dry as we thought they were, and that this mattered. I mean, it's a very nice illustration of how you need both the observations from the real world and properly controlled lab experiments to see what's going on. That was really a very nice illustration of the importance of both.

Oliver Strimpel: 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 kilometers wide, move about as rigid plates.

Dan McKenzie: Well, yeah, that was why it took such a long time for geologists to recognize plate motions on Earth. This sort of behavior is really only true for the oceanic plates. The continental plates deform in a much more distributed way. If you look at a map of earthquakes from northern India 2,000 kilometers north, the whole of that region is deforming. In the oceans, that's absolutely not the case. The plate boundary beneath Japan is really sharp. It's just one big fault, which moved 70 meters in the Japanese earthquake. Now, there really isn't anything like that on the continents. It was really only the exploration of the oceans which showed what was going on, and once we had a good understanding of that, we could then go back to the continents and see what was going on there. And of course, GPS... We knew what was going on, really, from earthquakes and from magnetic anomalies in the oceans, but any remaining doubts were completely squashed by the GPS measurements. It's much easier to do GPS on land than it is to do it at sea, and that's been done all over the world's continents, and it confirmed absolutely everything that we thought was the case from the earthquakes.

Oliver Strimpel: 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've found over 4,000 of them, and the number is 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.

Dan McKenzie: I'm sure. Well, up until the discovery of all these exoplanets, we only really had one solar system to look at. And furthermore, the things that formed it happened four and a half thousand million years ago, and the events are clearly very strange. There are a lot of very odd things. One of them, which is very simple to state, is that a lot of meteorites, the rocky meteorites, contain these small spheres of rock, which are actually mostly olivine, which is the same composition as the Earth, and these things have obviously been molten. They're very round because of surface tension. They're all mixed up: there are some extremely high-temperature ones and much lower-temperature ones, and a lot of the matrix is clay; it contains water. So how in the world were these things formed, and how were they put together? The composition of the Earth looks like essentially one set of these meteorites. We have a whole lot of really quite speculative theories about this, but we really don't know.

And the whole process going on as a star forms is rather hard to image. Stars form a long way away, in clouds of dust, and they form in a great variety of different sizes. During the formation of these stars and their planetary systems, the big ones evolve very fast, explode, and produce a lot of radioactive material. 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 is going 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. I'm quite sure there are lots of them, and we're going to find some which are forming now, and that will be an enormous help to understanding how the solar system got put together. Because the problem is, we don't even know how common a solar system like ours is. 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 be looking at it. So it's extremely difficult to know how to attack problems like this, especially when they're as complicated as this.

And one of the things which has really come from looking at the planets of this solar system, and particularly at their satellites, is that they're all different. There's just extraordinary variety, and it's quite astonishing that they look as different as they are. We don't know much about the insides yet, of 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: they're going to be completely variable. That seems to be what's coming out from the little bit of knowledge that we have of them so far.

Oliver Strimpel: We'll definitely have to do another podcast with you when we have enough data on these new solar systems. Dan McKenzie, thank you very much.

Dan McKenzie: Good. Thank you.