| Planetary Structure: The Earth.
The Challenge of Remote Sensing.
You have learned about the
and I have pointed out how this is understood and explained by the current theory of formation. Let us now turn, however, to a discussion of the best-studied planet we know: the Earth. I hope that it will give you some insight into the often indirect ways in which we have to deduce important physical facts (and perhaps leave you a little dubious about the precision with which we can claim to 'know' them!).
Geophysics is like astronomy in some respects: both face the challenge of remote sensing. Although the interior of the Earth is only a few thousand kilometers beneath our feet, as compared to the many trillions of kilometers separating us from even the nearest stars, the Earth's core can not be directly sampled in any way, and the difficulty is to extract information in some indirect fashion. The fact that so many imaginative ways have been found is a great testament to our ingenuity and resourcefulness, and we must also acknowledge that we are helped by certain accidents of Nature (such as the fact that the Earth is seismically active). But I begin, as I so often do, with an everyday analogy.
A Medical Analogy.Suppose you are a doctor anxious to know about the internal state of one of your patients. What can you do? There are a variety of possible approaches: You can rely on external evidence - the appearance of the person. Are they flushed or jaundiced? Are they covered with chicken pox? Does the skin have a good tone? You could study the patient's emissions. Does the breath have an acetone smell? (I am told that this is significant for diabetics.) What does urinalysis tell you? You could get an X-ray image of the patient to look for evidence of broken bones, tumours, and what have you. You could physically probe the interior. This might involve rather superficial sampling, such as withdrawing a bit of blood, but could be much more invasive. You might, for instance, do a spinal tap, insert an arthroscopic fibre-optic imaging tool (which can even travel along arteries right to the heart), or carry out deep exploratory surgery. If you were a believer in holistic or new age medicine, you might advocate studying a person's 'aura' to gauge whether or not there were any troubling signs of ill-health. You could carry out ultrasound imaging. This entails putting a source of ultrasound (a sound of inaudibly high frequency) onto the patient and determining how the sound waves propagate through the body. New parents know about such techniques: ultrasound images of babies in the womb are now almost a standard obstetric tool. There are other things you could do, such as Positron Emission Tomography (PET scans, which I will describe in Phys 016 when we talk about the nuclear reactions within the sun), Computer-Aided Tomography (CAT scans), and Magnetic Resonance Imaging (MRI), but I don't want to belabour the analogy. What I would like to do now is show how planetologists have similar lines of approach, of greater or lesser usefulness, in studying the interior of the Earth.
The Interior of the Earth: Varied Approaches.How do we know anything about the interior of the Earth: its composition, its structure, the sources of heat and energy within it? There are several answers, and I believe that your understanding will be aided by a discussion which parallels the analogy just explored. External Signs: One simplistic approach would be to rely on superficial evidence to deduce the global properties of the Earth. For instance, we know that the outer parts of the Earth are rich in silicate rocks. Do we dare assume that this is true throughout? (If the first lick of ice cream is chocolate, does that mean that the whole scoop is chocolate?) For the Earth, this is clearly untenable. For one thing, the overall density of the Earth is significantly greater than that of the surface rocks. This might simply be because the central materials are more densely compressed, but it could also be evidence of some compositional stratification. Emissions: The Earth provides us with material from a depth of a couple of hundred kilometers: the output of volcanoes. As impressive as that sounds, the information we get is from a depth which is still very small compared to the six thousand kilometer radius of the Earth. On the other hand, it does provide us with necessary information from geologically active parts of the Earth, so is welcome. Clearly, though, we need to do considerably better than this. Probing the Interior: If necessary, a surgeon can explore any part of a patient's body. The very deepest mines on Earth, however, reach scarcely a few kilometers into the crust. We can drill narrow holes somewhat deeper than that, but even they reach in only a small way. The information we derive this way is useful but clearly insufficient, and indeed less far-reaching than the information provided by volcanoes (although we can drill in regions where volcanoes are not to be found). It is worth recalling why we will never be able to dig mines or drill holes much deeper. Imagine trying to dig a hole in the ocean: it cannot be done because the surrounding water, being fluid, will flow sideways into the gap, impelled by the water pressure. This is exactly what happens when we try to drill a very deep hole on solid land. The rocks surrounding the hole are under enormous pressure from the weight of kilometers of overlying material, and they will simply flow into the gap. It is all a question of the as I noted in a very early lecture. X-Rays: A doctor can have a patient stand in front of a source of X-rays and record, on photographic emulsion, what comes through and what is shadowed by the bones and other material in the path. It is not possible to do anything equivalent for the Earth, although the concept of `shadow zones' returns in a slightly different guise in seismological studies of the Earth. The Aura: There are ways of studying the Earth which have no immediate medical analogue, one of which is to infer something about the deep interior from the presence, strength, and behaviour of the Earth's magnetic field. Although the analogy is imperfect, this might best be seen as comparable to the study of a person's aura. (The analogy is weak in various ways, one of which is that there is considerable skepticism - shared by me! - about the reality of personal auras.) As we will learn, the Earth's magnetic field has an importance which far transcends the merely diagnostic, however: the magnetic field, and especially the way in which it periodically changes, turns out to provide a critical link in developing our understanding of the way in which Continental Drift works. (See pages 400-404 of your text.) Ultrasound: The very best diagnostic tool for geophysical studies turns out to be one which is analogous to the use of ultrasound on the human body. But this is so important that it deserves a section on its own.
Ultrasonics and Seismology.Ultrasonics works because sound travels at different speeds through materials of different densities and rigidities. The same is true within the Earth: if there is a disturbance which sets the Earth jiggling at some location, the sound waves (or vibrations) will pass through the Earth itself and may be felt, and interpreted, some distance away. This is commonly used by geophysicists when they set off small explosions (sticks of dynamite buried in the ground) and use a network of nearby detectors to determine whether, say, the subterranean geology is likely to be housing an oil deposit. If we want to study the deep interior of the Earth, right down to the core, we will obviously need to give the Earth a pretty good jolt to start with, and then analyse the vibrations using an far-flung international network of detecting instruments. How will we accomplish that? One way, a socially unacceptable one, would be to detonate lots of subterranean thermonuclear bombs. But this is in fact unnecessary. The Earth itself provides the needed jolts, by virtue of its active geology. In short, there are earthquakes, each of which sets the Earth to ringing like a bell in a way which depends on the internal structure. Moreover, there are numerous seismic monitoring stations the world over, ready to accumulate and share the important data. It is through analysis of this sort that we develop the understanding summarised in the figures on pages 257-258 of your text. Much of what you need to know about the propagation of seismic waves through the Earth is well described in section 10.2 (pages 256-262 of the text), and so I will not repeat that discussion here. But I would like to clarify a few important points. We know that the Earth has a core which is molten, at least in its outer parts, because the shear (S) waves do not propagate through this region. The text notes that seismic S-waves cannot pass through liquid. You will remember this better if you think about why this might be so. The easiest way is to think first of a gas, like the air in the lecture room. If I push a molecule in the forward direction, towards you, it will bump into an adjacent one, and bounce back. The one which it hit moves forward in turn and bumps into the next one, and so on, molecule after molecule. In this way, a longitudinal (forward and backward) motion can pass through the gas, like the rattle which propagates down the length of a freight train when the engine bumps into it. But if I push an air molecule sideways, rather than towards you, the disturbance will obviously not be transmitted in your direction: the second molecule along the line between me and you is not much affected by the motion of the first because the molecules are not connected together -- instead, they are free to move independently. (This is a gas, after all.) In other words, even if a lot of molecules are made to move at once, any sideways motion is not readily transmitted to those farther along the path in a gas. It is different, of course, if the molecules and atoms are bound to one another, as in a crystalline or solid structure. Imagine a stick which lies in front of me, pointing away. By lifting the near end of it, I can move the whole stick because each atom grips the one next to it, and they move together. The stick remains intact, without deforming, in this simple example. But even solid materials can shear (i.e. deform) when a sideways force is applied. An easy way to understand this is to visualise two different objects on a table top: a brick, and a deck of cards. If you put your hand on the brick and shove it sideways, it will slide along the table as an intact unit. If you put your hand on the deck of cards and apply the same force, it will not move as a unit (unless it is still wrapped in cellophane). Instead, the bottom few cards stay in place, the top ones move with your hand, and the rest spread out to various extents. It is this spreading out which is known as a shear. The brick has so much internal rigidity that it does not shear (deform) when pushed in this way; the pile of cards does not, since the separate cards are not tightly bound to one another. A liquid is intermediate in the ability to transmit shear, depending on a property called its viscosity. ( Water is not very viscous, but molasses is, as you will learn if you try to stir it.) In general, though, and especially over the long distances being considered in the Earth's interior, a shearing (sideways) disturbance will not pass far once it encounters a liquid region. Longitudinal waves will pass through, just as sound waves pass through the atmospheric gases in the lecture room. The net result of all this is that a reasoned analysis of which kinds of seismic waves get through, and in what directions, will allow us to probe the deepest internal structures of the Earth. A related point is that the speed of transmission of a disturbance depends on various properties of the medium through which it is passing -- its density, its rigidity, and so on. You may already know, for instance, that sound passes more rapidly through rigid steel rails than through air. (You can test this by standing a kilometer or so from a friend near a railway track. Have your friend hit the rail with a hammer, and see how long afterwards you hear the sound. Now place your ear on the rail and repeat the exercise.) We have already seen, in our discussion of the nature of light, that a wave which passes from one medium (like air) into another (like glass) changes direction. (This is how lenses can be used to bring light to a focus.) As I explained, this can be readily understood in terms of the change in speed of the wave: light travels more slowly through glass than through air. So too within the Earth: at various depths, the rocks differ in composition, rigidity, density, and so forth, so the seismic waves travel at different velocities. Sometimes these properties change gradually as the waves move deeper and deeper into the interior; in other places, there is an abrupt change, as at the boundary of the outer core, where the rigidity of the solid mantle gives way to the low-rigidity molten core. The changes of direction of the waves can be inferred from the varied arrival times of the seismic waves at various stations, and out of all this information comes the presently-accepted picture of the Earth's interior -- a picture which we have developed despite the fact that we have scarcely `scratched the surface' of our planet.
The Earth's Active Geology.You may think that the motivation for describing the Earth's geology as being active is that it undergoes frequent earthquakes of the sort I have been alluding to. Certainly, reports of earthquakes are featured all too often on the evening news, usually accompanied by reports of serious property damage and major loss of life. But there is a more important sense in which the Earth may be said to have an active geology, a sense which introduces us to the very long-term behaviour of the slow but inexorable change associated with Continental Drift. As I noted in the very first lecture, this is one of the of this century, and represented a breakthough in the ways in which we understand and appreciate our planet. At this stage of the course, I am sometimes in the happy position of being able to turn the responsibility for the next couple of lectures over to my brother John, who is a professor in the Geology Department here at Queen's. John is often very agreeable about spending this time with you to explain the geological and geophysical concepts involved in the science of Plate Tectonics. This year, unfortunately, that has not proven possible, and I will have to rely on the textbook to present the details of the geological science which I am not expert in (and to amplify the necessarily general discussion which I presented in the lectures). Consequently, I expect you to understand the material presented in Chapter 14, Sections 1 and 2, of your textbook at the level presented there. You may also wish to reconsider my PowerPoint presentations for a reminder of the important aspects I presented when discussing Continental Drift. Before finishing, though, I would like you to consider a few random thoughts. The first remark is to remind you of the still (and perhaps forever) inferential nature of our understanding of the deep interior of the Earth. As noted, the understanding we have developed is the product of a kind of remote sensing. Our analysis consists of constructing 'models' which are consistent with everything we have learned, but these models may not be unique. Of course, some possibilities can be readily excluded. For instance, we know that the Earth cannot be made of iron throughout, because it would be more massive than it is and its magnetic properties would be quite different. But how do we know that we have considered every reasonable model? The answer is that we don't know that for certain. Whatever model we come up with has to explain all the seismic information, the shape and rigidity of the Earth (how much it distorts under the tidal influence of the moon), the presence of a magnetic field, and so forth ... a lot of information. But there is no absolute guarantee that it is right in every particular. We simply may not be clever enough to have thought of or tested every imaginable configuration and mix of compositions. A related problem is that a real understanding of the interior of the Earth requires us to know how a variety of materials behave in extreme conditions - under the fantastic pressures experienced beneath thousands of kilometers of overlying rock, for instance. Our experimental sciences have difficulty in carrying out the tests which provide the needed data. A last point stems from a remark I had from a student in the course a few years ago, one who unthinkingly remarked that ``Earthquakes are a Godsend since they allow geophysicists to study and understand the interior of the Earth.'' Considering the devastation and suffering that they can bring, this is a bit heartless. It's as if a sociologist remarked approvingly that wars are a blessing since they create situations in which we can study how humans react under extremes of privation and suffering. Instead, we should recognize that earthquakes are a manifestation of our planet's active geology, and acknowledge that they provide us with a helpful tool; but let us not forget the harm they cause. Indeed, one reason for studying them is to learn how to predict them, so that unnecessary suffering can be avoided. Previous chapter:Next chapter
0: Physics 015: The Course Notes, Fall 2004 1: Opening Remarks: Setting the Scene. 2: The Science of Astronomy: 3: The Importance of Scale: A First Conservation Law. 4: The Dominance of Gravity. 5: Looking Up: 6: The Seasons: 7: The Spin of the Earth: Another Conservation Law. 8: The Earth: Shape, Size, and State of Rotation. 9: The Moon: Shape, Size, Nature. 10: The Relative Distances and Sizes of the Sun and Moon: 11: Further Considerations: Planets and Stars. 12: The Moving Earth: 13: Stellar Parallax: The Astronomical Chicken 14: Greek Cosmology: 15: Stonehenge: 16: The Pyramids: 17: Copernicus Suggests a Heliocentric Cosmology: 18: Tycho Brahe, the Master Observer: 19: Kepler the Mystic. 20: Galileo Provides the Proof: 21: Light: Introductory Remarks. 22: Light as a Wave: 23: Light as Particles. 24: Full Spectrum of Light: 25: Interpreting the Emitted Light: 26: Kirchhoff's Laws and Stellar Spectra. 27: Understanding Kirchhoff's Laws. 28: The Doppler Effect: 29: Astronomical Telescopes: 30: The Great Observatories: 31: Making the Most of Optical Astronomy: 32: Adaptive Optics: Beating the Sky. 33: Radio Astronomy: 34: Observing at Other Wavelengths: 35: Isaac Newton's Physics: 36: Newtonian Gravity Explains It All: 37: Weight: 38: The Success of Newtonian Gravity: 39: The Ultimate Failure of Newtonian Gravity: 40: Tsunamis and Tides: 41: The Organization of the Solar System: 42: Solar System Formation: 43: The Age of the Solar System: 44: Planetary Structure: The Earth. 45: Solar System Leftovers: 46: The Vulnerability of the Earth: 47: Venus: 48: Mars: 49: The Search for Martian Life: 50: Physics 015 - Parallel Readings.
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(Friday, 15 January, 2021.)