The Scoop: It's thousands of miles underground, totally beyond our reach. But somehow geoscientists are unlocking the mysteries of Earth's core. Discovery News' Larry O'Hanlon chats with Arizona State University seismologist Ed Garnero about what we've learned and what we have yet to discovery about the core.
eddiegarnero: buenos dias. I guess [the chat service] works!
me: Yup. You can hit the diagonal arrow above the chat box to expand the box.
eddiegarnero: Thanks. Done. I have a 10 AM meeting, which is my only boundary condition.
me: Okay. We'll let's get started. You're in Tempe, AZ, right? How's the weather there today?
eddiegarnero: Yes, in Tempe at ASU's School of Earth and Space Exploration. The weather is sublime: I had to take my sweater off today walking to school.
me: Excellent. Not bad here in Albuquerque either. Great fall day. So you know a thing or two about the Earth's interior, right?
eddiegarnero: To be honest, I like to answer that question on two levels. One is: yes, it is what I do for my day job. I use seismology to interrogate Earth's inner workings. On the other hand, the interior is largely inaccessible, so we have to use tools to formulate best guesses. So, in fact, what I "know" may not be correct.
me: Well, that's a good scientific answer! As I told you when I asked for this interview, I'd really like to explore what we have learned about the Earth's core in the last 10 or 20 years. Any major paradigm shifts?
eddiegarnero: No, and maybe. We've know for a long time that the outer core is fluid, and the inner core is solid. The two shells combine to the things we call "the core". The properties of the core from seismic methods haven't changed too much. Our understanding of the actual chemistry of the core continues to be viewed as predominantly iron plus 10% something else. How the fluid part convects to make our magnetic field has progressed as computers (and laboratories) permit better predictions. But all in all, the advances are incremental. That said, there are some interesting thoughts brewing about how the core and mantle may chemically react at the core mantle boundary (CMB). This process has interesting predictions about how a fine layer of reaction products might form on either side of the CMB (which might relate to how Earth evolved over time). There is also a lot of work trying to better understand the inner core...
me: Was there any seismic data about the core derived from the 2004 Sumatra-Andaman quake?
eddiegarnero: That earthquake was so large (magnitude ~ 9.2) that it puts Earth into these wobbling motions (called "free oscillations" & "normal modes") where the whole entire planet oscillates. These data can indeed be used to improve our understanding of core structure. They are very long wavelength motions, and are best at describing the general properties. For super fine scale phenomena, e.g., scum at the top of bottom of the fluid shell, one must turn towards "body waves". These are seismic waves that travel down there and bounce off of or vibrate through things. The Sumatra-Andaman earthquake was actually too big for that type of analysis.
me: I had a feeling that might be the case. It was pretty darned huge. I do see a lot of CMB papers out there. That's the hot spot for research then?
eddiegarnero: Statistically speaking, the number of researchers for Earth science things is proportional to Earth's radius. So there are orders of magnitude more researchers studying things at Earth's surface (earthquakes, volcanoes, etc.). So, to call Earth interior research a 'hot spot' makes me smile (since there are not a lot of us looking inwards). That said, I would claim the field is growing much faster than surface science because we are better understanding the connection between internal and surface processes, and surface manifestations of these things. As far as Earth interior research, there are a number of hot topics (admitting some subjectivity...). In my opinion, the hottest topics are those that ultimately tell us how Earth works, how it came to be. You might be surprised to know the things we do not know (the "known unknowns").
me: Okay. So what DON'T we know? 2 or 3 big questions.
eddiegarnero: What drives plate tectonics? [what's the relationship between the patterns in the slowly convecting mantle and the tectonic plates that sit on top of that convection, and even instigate the patterns as dense things fall inward?]. Where does the lava that erupts from many volcanoes, like Hawaii or some of the South Pacific "hot spot" volcanoes, come from? [the depth of their origin, and the inference for mantle chemistry and evolution, are in question]. Third and last: Earth's water (perhaps the biggest and farthest reaching question): where did it come from and why does Earth appear to uniquely have it (compared to our neighbors)? [Did it come from the mantle rock that released the H and O's early on in Earth formation? Or, was Earth blanketed in a much thicker ocean that slowly was devoured by the process of subduction -- where ocean crust and sediments, saturated in water, are convected into the planet, where mantle rock can "store" some amount of water? Or was it some late add on? Like a meteorite pummels Jupiter and we got some H from it (this has been proposed)?] All of these have an obvious connection to the interior. Without understanding the interior, we cannot answer these questions...
me: Wow. Those are some pretty fundamental questions. How about 2 or 3 major misconceptions people have about the Earth's interior?
eddiegarnero: This biggest misconception is that Earth's interior is this lovely collection of colored homogeneous shells (just google "Earth's interior" and look at the images). Yes, there are dominantly stratified shells, which is the first order feature of all planetary interiors. But on Earth, we know it is heterogeneous, convecting, exchanging heat, mass, and energy at many scales. It is really dynamic, and controlling what we end up with (rock formations, continents, ...) and feel (earthquakes, volcanoes, ...) at the surface. I think there are many other misconceptions regarding the sizes and dimensions of things. Ask a collection of folks how deep the Earth is to its center, or how hot the center is. You might be able to multiply their answer by 10. (6371 km deep and about 6000 degrees hot, same as surface of the Sun, at the center)
me: How do we know that?
eddiegarnero: Dangit! I knew you were going to ask that... The radius is easy, from our planet's orbit and shape. The temperature is a calculation based on a number of things (many of which may be in error). Laboratory experiments have estimated temperatures at which iron melts, pressures at which the melt solidifies again, and so on. These pressures correspond to specific depths in the Earth, then an extrapolation calculation must be done (to Earth's center). So, I wouldn't bet you dinner on that 6000 deg number. I'd bet on a range, though. Like 5000 to 6500.
me: Okay. That makes sense.
I know we're running low on time, but can you say there is anything about the Earth's magnetic field and how this sort of research relates to it?
eddiegarnero: I often tell people that the Earth's core is the last frontier in Earth science. There is so much that we do not know well. Or, rather, that we have best guesses but they are not constrained well. The magnetic field is one of these. We depend on it for so many things (navigation, keep the Sun's deadly energy at bay, etc...). But how is it generated? There are numerical models out there that help us understand the type of convection currents and patterns in the outer core that might give rise to what we observe. But there is presently no computer on this planet that comes close to the capability needed to really do that calculation (some suggest we are decades away...). I think the next discoveries in research on Earth's core will be from seismic imaging of the fine scale layering (if present) right at the top and bottom of the liquid core. These have important evolutionary pathways that will tell us about how the core formed, what it's made of, and hence how Earth formed.
(gotta go in 2-3 minutes)
me: Gotcha. Thanks for your time. I really appreciate it! Such a huge topic and so little time!
eddiegarnero: Yes, so little time. But, really so much time (encoded in these structures in the interior) and so much space (the volume of the interior is HUGE!)
thanks, was a pleasure
Interview posted on Nov. 9, 2009