History Channel, Part 2
Wednesday, July 15th, 2009
Ironically, I had barely finished my interview with the History Channel when my favorite web-comic, Jorge Cham’s Piled Higher and Deeper, ran a series about what happens when a TV crew comes to the lab! As always, Jorge manages to skewer everybody — the vain professor, the clueless TV producer, the grad students who are thrilled just to get the back of their heads on TV. If you haven’t ever read P.H.D. before, go and check it out!
However, my experience with the History Channel has so far been much better than the fictitious film crew in Jorge’s comic. Admittedly we had some struggles with the wind (see my previous entry), but on the whole I was impressed by how hard the producer/writer, Adrian, was working to get the story right within a very limited time frame.
Since he filmed the interview, Adrian has continued to ask me some questions by e-mail and telephone. One particular point has come up over and over: Does the moon have anything to do with Earth’s geology, in particular our uneven distribution of oceans and continents?
The question is a very interesting one scientifically, and it is also interesting as an example of the difficult interaction between science and popular culture… the interaction that Jorge’s comic was all about. Let me take up the science first, and then at the end I will talk about the popular culture aspect.
I’ll start with the assumption that you know about the giant impact hypothesis of the formation of the moon. That is the central topic of my book, The Big Splat, or How Our Moon Came to Be. So if you you have not heard of the giant impact theory, please go to Amazon.com (or your library) posthaste and get yourself a copy. Once you realize that Earth ran into another planet 4.5 billion years ago, a very natural question may occur to you. In fact, this is the first question I heard from the mouth of a little girl, maybe six years old, when she saw a museum exhibit about the giant impact: “But where’s the hole?” Or to ask a somewhat more sophisticated version of the question:
Did the giant impact create a giant hole in the Earth, which later filled in with water and became an ocean?
The answer is NO! I wrote about this in Chapter 12 of my book. The giant impact left no scar that is visible on today’s Earth, for several reasons. First, it catastrophically blew off a large part of Earth’s mantle, most of which fell back to Earth and reassembled into the nice round planet we see today. That reassembly process erased any “hole” that was temporarily created. The energy of the reassembly melted Earth’s surface and created a magma ocean, which also tended to smooth out any surface scars.
[In fact, on the History Channel episode from season 1, Robin Canup, who has done extensive computer simulations of the giant impact, says that the Earth was basically back to being round again within one day of the impact. We forget how strong gravity is on a planetary scale -- that's why everything in the solar system that is larger than a certain size (about 500 miles in diameter) is round. Gravity is able to overcome the shear strength of rocks.]
The idea of the Pacific Ocean as a remnant of the moon’s formation is actually an old one that long predates the giant impact theory. When George Darwin proposed his fission theory in 1879, a geologist named Osmond Fisher suggested that the Pacific Ocean could be the place from which the moon detached from Earth. At that time the formation of oceans and continents was not understood. However, we now know that oceans and continents are formed by plate tectonics. Earth’s continents have broken up and re-assembled several times over the last 4 billion years, and the shapes of the oceans have changed along with them. The crust that lies under the Pacific Ocean today is almost all younger than 300 million years old — and thus it is certainly not the scar of an event that occurred 4.5 billion years ago.
Okay, so the oceans and continents are formed by plate tectonics. But what caused Earth’s crust to break up into pieces in the first place? Wasn’t that due to the giant impact?
Again, the answer is no! Earth’s surface wasn’t like a Christmas ornament, fracturing into pieces when it hits the floor. The giant impact liquefied Earth’s surface. Any fracturing into pieces had to occur later.
But there is also a more fundamental point. Plate tectonics is a process that is driven by energy within the Earth. The mantle is a hot, fluid layer thousands of miles deep, and the crust is a very thin, brittle layer on top of it that is only tens of miles deep. You might be surprised to hear the mantle described as a fluid, even though its composition is rock. But on the time scale of hundreds of millions of years, the rock can move around. Most geophysicists believe that it is convection within the mantle — a rolling motion, such as what you see when you heat a pan of water up to the boiling point — that drives plate tectonics. This slow churning in Earth’s interior creates stresses at the surface that the brittle crust cannot withstand. So it fractures into lithospheric plates, and then the convection causes those plates to move around.
Okay, so the giant impact didn’t create the oceans directly and it didn’t create the lithospheric plates directly. But didn’t all that energy from the impact heat up our mantle and start that convection process?
Now the questions are getting better! When you start talking about energy, you’re starting to think like a physicist.
But still, there is a question of time scales to think about. The energy directly deposited by the impactor, and by the rain of debris back to Earth, did not last very long. Both on Earth and on the moon, there is evidence from grains of zircon that the magma oceans must have solidified by 4.4 billion years ago — in other words, they lasted at most 100 million years after the giant impact, and probably less. So even though the giant impact did heat the surface of our planet up, that heat dissipated long ago, and it does not explain where the energy behind plate tectonics — the energy that produces earthquakes today – comes from.
So where does the energy come from?
Glad you asked! It comes from the decay of radioactive elements, principally uranium and thorium, within Earth’s mantle. That’s right, our planet is warmed by nuclear power!
The elements I’ve mentioned happen to have half-lives that are roughly on the same scale as the age of our planet. (Uranium-238 has a half-life of 4.5 billion years; thorium-232 has a half-life of 14 billion years; Earth is about 4.56 billion years old.) An element with a much shorter half-life would decay too rapidly, and there would no longer be enough of it around to heat our planet. A radioactive element with a much longer half-life would not generate enough heat to be significant.
So you can thank the elements uranium and thorium, with a nod to Mr. Einstein, for almost all of our geothermal energy — for volcanoes and earthquakes and the movement of continents, which affects the evolution of life. This is a tremendously important story, which viewers of the History Channel ought to hear. However, the moon does not have anything to do with it. Or to put it a little less categorically, I don’t see what the moon has to do with it. If a reputable geophysicist comes along with a good explanation, then I will be very happy to change my tune.
But didn’t the giant impact give Earth an extra-large core, and isn’t it energy from Earth’s core that causes plate tectonics? So perhaps we have a tectonically active planet because of our large molten iron core.
This is the best question of all, and it’s the only one for which I feel less than 100 percent confident about my answer. It is correct that Earth has an anomalously large core for a planet its size — about 3/8 of the planet’s mass. One of the key motivations for the giant impact hypothesis was to explain this anomaly. (Computer simulations show that the impactor’s core joins Earth, while the impactor’s mantle gets blasted into orbit.)
But is it correct to say that the energy for plate tectonics is generated in the core? This is the part I’m not sure of, but I think the answer is no. The reason is kind of technical. Uranium and thorium are lithophile elements, which means they “prefer” to be in rock instead of alloying with iron. When Earth (or the impactor, for that matter) differentiated into a planet with an iron core and a rocky mantle, elements (called siderophiles) that like to alloy with iron tended to migrate into the core, while the lithophiles tended to remain in the mantle. Therefore, while there is plenty of evidence that we got an extra infusion of iron and nickel from the impactor, that does not mean we have an extra-large reserve of uranium and thorium.
Less technically, here is how I think about it. The impactor gave us an extra-large radiator (that big blob of iron and nickel in the center of the planet). But installing an extra-large radiator does not make your house any warmer. To make the house warmer, you need more fuel — in this case, uranium and thorium. And I don’t think that we got extra fuel from the impactor.
So, in conclusion: As far as I know, the moon has nothing to do with plate tectonics, and therefore it has very little to do with what is going on in Earth’s lithosphere. (The hydrosphere is another question — obviously, the moon has plenty to do with tides.) My geologist colleagues whom I consulted on this question, Brian Skinner and Barb Murck, were very emphatic about this, and so I have tried to “fight the good fight” and warn Adrian not to make any direct moon-geology connections. I’m sure he has gotten similar advice from other people, too.
The fact that these questions keep coming up, seemingly from on high, makes me think that the “network execs” wish there was a sexy connection between the giant impact and Earth’s oceans, plate tectonics, etc. That would be good TV! But I hope that in the actual episode they will stick to what the scientists tell them. It will be very interesting to see how the show finally comes out!
