This article is a small sample from
DISCOVER's special issue Extreme Earth.
Planetary scientist David Stevenson has spent three decades studying the gigantic collisions and geologic cataclysms that created the planet we call home. His discoveries have helped answer some of the biggest questions about Earth’s formation, structure, and evolution. We know that the planet has a partly solid, partly liquid core, composed largely of iron, surrounded by a thick, flowing mantle, topped by a thin layer of crust. Still, we are surprisingly ignorant about our planet’s deep structure, says the 63-year-old New Zealand native. Hoping to learn more, he contributed ideas for NASA’s Juno mission, which blasted off in August. The spacecraft will orbit Jupiter to study its interior, which may indirectly reveal insights about Earth as well—such as how the core was formed, where our planet’s magnetic field came from, and why we have so much water. From his office at Caltech in Pasadena, Stevenson talked with DISCOVER about the still-simmering controversy regarding the moon’s formation, his wild proposal to send a probe into Earth’s fiery depths, and why we remain largely in the dark about what lies right beneath our feet.
Earth apparently had a harrowing birth. What was it like?You start with the formation of the sun and a disc of material around it, which contained both gas and small bodies. Those small bodies progressively built up into bigger bodies, which collided with each other and then collapsed together because of the action of gravity. The end stages of making the Earth involved very large bodies hitting each other, releasing a lot of heat. These events happened 4.6 billion years ago. This gives us a picture of our planet’s birth as a very traumatic, high-energy process involving very big things colliding and Earth’s being completely molten and very hot—a very nasty place.
What about the other planets—what were they doing at the time? The architecture of the solar system is dominated by the giant planets, in particular Jupiter, because it’s more massive than all the others combined and so it has such a big gravitational influence. Many of the smaller bodies that crashed into Earth were put on their collision course by Jupiter. Jupiter is also responsible for so much water being here. The most commonly accepted idea about the origin of the oceans is that the water came from icy bodies out near Jupiter; the planet sent those bodies to collide with Earth. Some of Jupiter’s satellites, such as Ganymede and Callisto,are 50 percent water. Delivering objects like that was a very effective way of providing Earth with water.
And what about our moon? According to the latest thinking, it was born out of yet another enormous collision, right? The origin of the moon involves a really big thing hitting the Earth, a giant impact of something the size of Mars, which is about 10 percent of the mass of Earth. We think the collision happened 50 million years after the formation of the solar system, and it was the last big event in the formation of Earth. So you’ve got your 10 percent thing hitting Earth at at least 7 miles per second, splashing out material that goes into orbit. Most of it goes into building the Earth, but some orbiting debris accumulates and becomes the material for making the moon. While the basic idea for the origin of the moon is well established, the way it actually happened—and even the timing—is hotly debated. If anything, this is an even hotter topic than it was a decade ago. And the connections to how Earth got its iron-rich core are very much part of the story, since the impact that formed the moon set the stage for Earth’s evolution.
What about the mantle, which makes up the bulk of Earth’s interior—did that also come from whatever slammed into our planet? Everything on Earth, not just the mantle, came from elsewhere. The things that hit Earth were both the stuff we now call the mantle and the stuff we call the outer and inner core. The mantle exists because iron separated out from the mantle rocks, and then its much greater density caused it to sink to the center.
How do we know so much about long-past events and parts of the planet we’ve never seen? The chemical composition of Earth and the moon can tell you where the material came from. We also learn about how planets form by looking elsewhere in the universe, at other places where planets are forming. The third part is computer and theoretical modeling.
Earth is the only known planet with plate tectonics, in which parts of the upper mantle and crust move about, shifting continents and triggering all kinds of geologic activity. What is so special about our world?
That’s one of the big unsolved questions. The best candidate for answering that question is water. We are familiar with the idea that liquid water is vital for life, but water also changes the rocks. It changes the strength of the rocks and makes them weaker. If they’re weaker, then it’s easier to break the outer shell [Earth’s rigid lithosphere] into plates. The chemistry of water may also help the formation of rocks like granite, which are the foundation of the continents.
To get more answers, you once proposed embedding a probe in liquid iron and shooting it to the center of the Earth. What happened with that idea?
It was tongue in cheek, although it had a serious intent, which was to get people to realize that maybe there is some way of getting down inside the Earth that we haven’t tried. We know you can’t drill very far. [The longest borehole is just over 7 miles deep.] It’s just hopeless. So there has to be some other way. We have spent many billions of dollars sending spacecraft all over the solar system. We haven’t spent anything like that amount of money to look down beneath our feet. My idea was to open a crack—perhaps explosively, but not necessarily—so you excavate a wedge in the ground, and then pour liquid iron in it. Liquid iron is heavier than rock so it could push the rock aside, and the iron would keep going down. You could put a probe inside the iron that would go down to the outer core and send back information seismically—communicate by sending sound waves.
How do we study the core now? One of the most important methods is seismology. When you have a big earthquake, sound waves go through the inside of the Earth, and you can detect those at the surface and notice how long it took for them to get there. That tells you about the materials inside, because different materials have different sound speeds. By looking in detail at the seismic record, you can deduce that Earth has a liquid outer core and a solid inner core, and that both are mostly iron.
The Earth also has a magnetic field that is generated in the outer core, in the liquid part. So if you want to know more than just composition, you look at Earth’s magnetic field. It’s a little bit like studying weather, because the field is dynamic, undergoing reversals and fluctuations in strength. Right now, the main part of the magnetic field is declining quite rapidly. But that’s not particularly unusual.
If you could dig down two thousand miles and retrieve a piece of Earth’s core, what would it be like? If I had a piece of Earth’s core in my hand? It’s mostly iron, but there are other things in there. And those other things are a memory of how the core formed and how it evolved. That’s exciting because it’s like opening a book that tells us the story of Earth. We have good reason to think the core formed early, so it is perhaps the best place for learning about the early history of Earth. And we could learn more about how the magnetic field is produced. Although we have a “dynamo theory” for how the field is produced, we can’t actually see the dynamo. Dynamos are possible because mechanical energy—in this case the movement of the fluid in the outer core—can generate a magnetic field. This is called magnetic induction, first explained by English physicist Michael Faraday over 150 years ago. Turbines that create electricity run on a similar principle.
What about the things that we really can touch—what can we learn from the rocks on the surface? The oldest Earth rocks, in the sense of something you can put in the palm of your hand, are about 4 billion years old. But there’s an enormous difference between 4 billion and 4.6 billion years, so those rocks tell you nothing about how Earth formed, although they do tell you about how continents formed and stabilized.
The oldest rocks happen to be in Canada, but there are rocks nearly as old in Australia, in Greenland, and perhaps South Africa. Tiny minerals called zircons have also survived from the early Earth. Zircons are highly resistant to weathering and some, particularly in western Australia, have been dated all the way back to maybe 4.4 billion years. Some people are hoping we might find stuff that dates all the way back to the time when Earth formed.
After three decades of study, do you feel as if you finally understand the inner construction of our planet?Not as much as I would like. Yes, we know the core is mostly iron. Yes, the outer part is liquid. Yes, the mantle is silicates and we even know approximately which silicates. But a lot of the information about how Earth formed and how it evolved is in the details. In that sense, we still don’t understand it because we haven’t had the opportunity to look more carefully.
