摘录

Chapter One Energy balance sheets        Without energy, nothing happens. There is no motion and so no change. Not only is a complete absence of energy impossible to imagine, but so far as we can tell there is nowhere in the Universe that is totally unmoving and unchanging. At the most fundamental level, matter that possesses energy has atoms and molecules, and indeed lesser constituents, that vibrate. This vibration constitutes temperature. Matter signals its presence and that of its energy and temperature by emitting electromagnetic radiation, which moves at the speed of light. The wavelength of this radiation can range from extremely short--close to the dimensions of the smallest particles--as in gamma rays and X-rays, through the narrow visible spectrum to radio waves, whose wavelength is measured in kilometres or more. Matter that is devoid of motion emits no signal and does absolutely nothing. In terms of every conceivable thing, it is at absolute zero. Even if such a state existed, we would not know, simply because there would be no signal of any kind. All the laws of physics and chemistry would cease to have any meaning. Complex cooling technologies take temperature towards absolute zero, when many bizarre properties of matter appear, such as superconductivity. Though we can approach the state of nothing, it can never be reached; it is akin to infinity.     Matter that has a temperature, and therefore energy and motion, emits radiation in a range of wavelengths. It loses energy. The rate at which energy flows away as radiation is the body's power. We detect that by the amount of work done in some kind of detecting system, simplest to understand from the movement of a needle on a dial. A body's total power--its rate of energy emission--is proportional to its temperature above absolute zero on the Kelvin scale (0 K = -273.15°C) times itself four times, or to the power 4. One wavelength always carries more of this power than others, and is inversely proportional to absolute temperature. These two laws, the Stefan-Boltzmann law and Wien's law respectively, underpin much of astronomy and cosmology. The weak background of long-wave radiation from all directions in the Universe, as well as the radiation from stars and galaxies, analysed through these laws confirms that even intergalactic space contains matter. It is above absolute zero by a few degrees and is the signal from the `Big Bang' when the Universe formed between 12 and 20 billion years ago, or so most modern cosmologists reckon.     Energy, power and work are by no means restricted to processes at the atomic or molecular levels, bound up with heat and its transfer, but they pervade every process. Physical movement of tangible matter involves this triumvirate too. The energy of motion (the kinetic energy) is proportional to the mass involved and half the square of its speed or velocity. How quickly it is delivered is a measure of the power of a physical process, and this is bound up with the amount of physical work it can do. Just by virtue of its relative position within a force field (gravitational, electromagnetic or those involved in binding matter at the atomic and sub-atomic levels), matter has an energy potential that can release power and do work. All the forms are related, for one can be transformed to the other and radiation interacts with matter at all levels. And even matter itself is not separate from the scheme of things, as Albert Einstein predicted. Mass has an energy and radiation equivalent, expressed by his famous relationship ( E = mc² ). That is at the heart of processes in stars, and bound up with the generation of matter in the form of different chemical elements, as you will see in Part III. Most of the processes involved in the evolution of planets and the rocks that comprise them involve physical movement. But they also interweave with energy-governed chemistry, as do those at the base of life. The energy-power-work relationship with matter, as expressed by radiation, underlies the processes that take place at the Earth's surface. That is as good a starting point as any. Imported energy and the surface budget In relation to the Sun, position in the Solar System governs how much energy planets receive from outside. Mercury is over-indulged, while Pluto orbits with a tiny supply. For us and the rest of terrestrial life, as Goldilocks found with the wee bear's bowl of porridge, it's just right. Mars and Venus come close, but, for other reasons too, not close enough.     The share of energy provided by the Sun to each planet is simple to work out; it depends on the radius of their orbit. The Sun radiates energy, in equal amounts in all directions, so the total solar output E at a particular radius R metres is shared over a spherical shell. The shell's area is 4[Pi] R² , giving an energy for each square metre of E/4[Pi] R² . The amount of energy received per square metre each second, i.e. joules per square metre per second (J [m.sup.-2] [s.sup.-l]), is a measure of power given in watts per square metre (W [m.sup.-2]). Solar power falls off with the square of distance, so that a planet 10 times further than Earth from the Sun receives not 10 but 100 times less solar power. At the Earth's average distance from the Sun, the unit of solar power is 1370W [m.sup.-2]. But the total power input here is distributed as if the Earth was sliced through its centre to give a flat, circular cross-section. This cross-sectional area is [Pi] r² , with r being the Earth's average radius. Because the Earth is roughly a sphere, the surface that receives the radiation has an area of 4[Pi] r² . So, our planet's average import of power is a quarter of that available in space, 343 W [m.sup.-2]. Solar power does in fact change dramatically from place to place. Between the tropics the Sun can shine at noon from directly overhead, giving full-power conditions. At increasing latitudes the midday Sun illuminates the ground at a decreasing angle. The solar power unit spreads over a larger area, and its effect falls off towards the poles. How much power a planet receives is by no means the only factor that governs how it responds. This is a fairly complex issue that needs several steps to understand.     Henry Ford was once asked by a potential customer what colour Model-T could be ordered. `Any colour, so long as it's black' was the laconic reply. Sitting in a black car on a sunny summer's day is far more uncomfortable than sitting in a white one. Today's range of paint jobs has little to do with the important physical principle involved. The darker a surface appears to us, the greater the proportion of light radiation it absorbs and converts to other forms of energy. This conversion means that work is done by the absorbed radiation. At the simplest level it sets atoms and molecules in vibration, thereby raising the body's temperature. The lighter the surface, the more power carried by radiation is reflected away unchanged; in other words the more power that does no work. Nor is this process restricted to visible light; it applies to radiation of all kinds.     How well a planet uses incoming radiation to do work at or near its surface is strongly affected by how reflective it is. This ability to reflect radiation, known as albedo, depends very much on the materials that make up the planet's surface. Rocks and soils have a wide range of albedo, from the almost white of some sand to the near black of basalt lavas. Vegetation varies too, but of course ice and snow are highly reflective. More than 70 per cent of the surface is water, and the oceans are efficient absorbers of radiation from directly above. But oblique illumination can be reflected strongly, depending on how calm the surface is. Of the Sun's radiation that reaches the surface today, about 12 per cent is reflected back to space. So, without an atmosphere, we can say that Earth's albedo is about 0.12. This is low compared with most other planets and their moons, except for the Moon and Mercury (0.07 and 0.06 respectively). Mars with its rocky and sandy surface has an albedo of 0.16, but all the rest are higher than 0.70. This is because they possess dense atmospheres, and there is much to say about that shortly.     For an airless world, absorbed radiation has only a simple job to perform. It sets the atoms and molecules in the surface in vibration. This work raises the temperature of the surface, and any body with a temperature emits radiation as well as absorbing it. Since the power radiated by anything with vibrating molecules is proportional to the fourth power of its absolute temperature, a doubling of temperature means a 16-fold increase in power output. Heating means rapidly increasing the power output of the warmed body, so a balance is soon reached, where power in equals power out. Temperature becomes more or less fixed. Using the Stefan-Boltzmann law and the Earth's albedo allows us to judge the temperature that it would achieve by solar heating in a naked state. It comes to 255 K or--18°C. An airless Earth would be icebound, and probably colder still because of the high albedo of ice. On average our home planet is 33 degrees warmer at 15°C. Clearly air has an extremely important role to play in retaining enough solar energy to keep water in liquid form and thereby to have allowed living things to form, develop and survive.     A clear night is generally colder than when the sky is overcast. You might well think that this is because the Earth's outgoing energy is reflected back to some extent by clouds. That is not true, and to understand why means first a brief look at the nature of radiation, more specifically its wavelength.     The Earth does not `glow' as the Sun does, yet both emit radiation in the general, literally broad, sense. Digging out an explanation requires some fundamental physics connected with radiation. One view sees radiation as vibrations or waves in electrical and magnetic fields, and this is supported by experiments first devised by James Clerk Maxwell. The distance between adjacent `ups' in the fluctuations is the radiation's wavelength. All radiation travels at the `speed of light', about 300 thousand kilometres per second. So the number of waves that pass in a second--the radiation's frequency in hertz (Hz)--is light speed divided by wavelength. There is no practical limit to how long or how short such simple waves can be, and there is a very wide spectrum of radiation that is known and in some cases used. X-rays are less than a billionth of a metre in wavelength while radio waves can far exceed a kilometre; both are fundamentally the same but are generated by different processes.     There is a different and equally valid perspective on radiation that stems from the fact that solids can be made to emit electrons if radiation is shone on them. This photoelectric effect only happens for a particular solid when radiation with more than a specific frequency is involved, and the threshold frequency differs from solid to solid. It was from this last observation that Albert Einstein proved that radiation also travels in packets or photons, an idea conceived a little earlier by Max Planck. The energy of radiation is emitted and absorbed in distinct amounts called quanta, in direct proportion to the frequency of the radiation as expressed by its wave-like nature. Now, isn't this getting far off the point in a book about Earth science? Not at all, because quantum theory is the only means to explain some vital aspects of the Earth's climate. Climate plays a central role in processes that shape the surface of the Earth, and as you will grasp as the book unfolds, it is irretrievably linked to internal Earth processes, to astronomical forces and to life itself. Above all, it is dominated by links between solar power supply and gases in the atmosphere.     The energy of radiation across a wide spectrum relates to the fourth power of the temperature of the radiating body. That energy is carried in photons, each of which carries its own frequency-dependent quantum of energy. Consequently, temperature has some control over the quantum energy of the most common photons, and therefore over the most intense frequency of the outgoing radiation--this linkage is the essence of Planck's law. Objects at different temperatures emit radiation with different ranges of wavelengths, the peaks of which are characteristic of the temperature. Figure 1.1(a) shows theoretically how the power in the Sun's radiation spectrum rises from very short wavelengths to a peak in the range of visible light (actually around green) and then drops to insignificant levels at longer, infrared wavelengths. By comparison, the Earth emits no radiation at short wavelengths, peaks as an infrared emitter at about 15 micrometres and then tails off towards the microwave regions. The change in the quality of the energy involved, from incoming to balanced outgoing radiation, is obvious.     Reality shows important deviations from these theoretical curves. Incoming solar and outgoing terrestrial power over a broad range of wavelengths have different spectral curves above the atmosphere from those at the surface. The differences form the pattern in Fig. 1.1(b). It expresses how the atmosphere absorbs different proportions of power at different wavelengths. Over some ranges radiation passes with little absorption. These atmospheric `windows' are separated by peaks and plateaux where the atmosphere is strongly absorbent.     The atmosphere is warmed not only by some incoming solar radiation, but also by a proportion of that re-emitted by the warmed-up surface. Absorption peaks link to different gases that make up the atmosphere (Fig. 1.1b). Oxygen and ozone, together with water vapour, account for virtually all the absorbed solar radiation. Warming connected with outgoing radiation is dominated by carbon dioxide whose long-wave absorption plateau covers the very wavelengths where the Earth emits most energy. Methane, ozone and water vapour also play a role here, together with some other gases, but most of their effects are at the short-wave end of the Earth's spectrum. Nitrogen, the most abundant atmospheric gas, has no noticeable effect. Why are there sharp absorption peaks and marked distinctions between different gases? Again, we need to turn to quantum theory.     Gases occur as molecules that link their constituent atoms by chemical bonds. In a fashion similar to gongs, these molecular connections tend to vibrate at characteristic frequencies. Things are complicated, because gas bonds can stretch, bend and rotate, but the same general principle holds. Like mechanical analogues, gases exhibit several favoured vibration frequencies or harmonics, seen clearly in their absorption spectra. Gas molecules are more likely to vibrate, and so heat up, when they encounter photons with these characteristic frequencies. They also emit energy in this way as their vibration shifts abruptly from one harmonic to another. Incidentally, that means that many different gases can be detected in the radiation coming from other parts of the cosmos.     We now have enough theory to understand the rudiments of accountancy for the perpetual income and expenditure of solar energy. Figure 1.2 is the radiation accountant's ready-reckoner. As you shall see, energy budgets need as much explanation as a tax assessor might. In the same way that the Inland Revenue is rarely satisfied with a balance sheet merely showing income, outgoings and profit or loss, so for the solar radiation budget the hidden items are the ones to chase down without mercy.     If income is 100 units of solar power, there are three immediate losses. The surface, clouds and even the atmosphere itself, because of haze and dust, reflect away 6, 17 and 8 units respectively, so the Earth's true albedo is about 0.3. The remaining 69 units are absorbed: 3 units by ozone and oxygen in the thin, dry stratosphere; 20 units mainly by water vapour and oxygen, but a little by carbon dioxide, in the lower atmosphere or troposphere; and 46 units by land and sea. On the outgoing side, the first point to note is that the total emitted to space exactly balances that which is directly absorbed from the Sun. The details of what goes on beneath, however, would raise a tax inspector's eyebrows!     The inspector would leap on the 115 units emitted by the surface--a likely story! But wait, long-wave energy is efficiently absorbed by gas molecules. Only 9 units can escape directly through the atmospheric `window' on Fig. 1.1(b). The remaining 106 units are continually absorbed by air, so heating it up. Warm molecules emit long-wave radiation in all directions, but most of them are deep in the atmosphere. So what is re-emitted there largely reaches the surface by downward radiation (100 units) or is re-absorbed and re-emitted again and again up through the air column. Only 6 units escape directly to space by this process. Another 31 units are involved in processes other than throbbing molecules. The `sensible' (7 units) and `latent' (24 units) heat emissions from the surface are involved in the physical movement of air-masses and the heat they carry, one central topic in Chapter 2, and processes in clouds. They help to shift warm gases towards the thin outer atmosphere, where re-emitted radiation can easily emerge and escape.     Adding up the units at the land-ocean surface gives 115 + 7 + 24 = 146 units emitted less 100 units absorbed. Again there is a balance with absorbed solar radiation, but the 100 units of `floating' energy form the crucial issue. They are involved in a cycle that maintains the Earth's average temperature that critical 33°C above the level for an airless world. This is the so-called `greenhouse' cycle--a poor analogy since a greenhouse is heated entirely by incoming radiation and the warmed air is physically trapped.     The crux of the `greenhouse' effect is the role of carbon dioxide. Although it is present at very low concentrations (about 350 parts in every million parts of air by volume) it is a hugely efficient absorber of the Earth's emitted long-wave radiation (Fig. 1.1b). More important still, because it is present at such a low level, small exhalations or drawings off can dramatically change its concentration and thereby its heat-trapping effect. In earlier, perhaps happier, times carbon dioxide's main claim to being noticed was its role in making champagne froth out of your nose. Nowadays, it emerges as the single most important factor in allowing life to appear, cling on and evolve into more or less thoughtful beings on the home planet. One of its atoms, carbon, is the only element with general potential for the chemical complexity bound up with life. So the climatic role of this gas interweaves with life and, as will become clear later, with the Earth's innards too. Less cheerfully, in the short term, our own intervention in the chemistry of carbon is boosting its levels in the air so much that its warming effect may become unwholesome. But you can forget that until the last part of the book. Having covered the external account, our assessment turns to the internal budget. Earth's fuel The Earth is not merely a passive staging post for energy emitted by the Sun. It also produces heat in its own right because it is mildly radioactive. On the unimaginable time-scales involved in its evolution--tens, hundreds and thousands of million years--the Earth's own heat production is generally balanced by its ultimate loss by radiation to space. You will see in Part II evidence for long-term build-ups and widely separated, dramatic releases of part of the internal budget. Today the most fearsome aspects of the home planet, explosive volcanic eruptions and earthquakes, are manifestations of the transport of geothermal energy through the deep Earth. However, a great deal of it emerges quietly deep on the mid-ocean floors, or with virtually no sign of what is going on as a result of slow conduction through solid rocks in the same manner as heat from a fire passing along a steel poker.     Compared with the average 343 W [m.sup.-2] solar-power delivery to the surface, outward heat flow from the deep Earth is more than 5000 times smaller (0.06 W [m.sup.-2]). Even where volcanoes are at their most active, geothermal power is still only about a thousandth of that from the Sun. Without the Sun, the Earth's surface would be very cold indeed. Unlike instant solar power, however, the Earth's heat production is delivered very slowly upwards. Cold as its surface might be without the Sun, it would grow hotter and hotter with depth.     Apart from the power generated by tidal action resulting from the Earth's rotation and the gravity fields of the Moon and Sun, which add a mere one part in every 500 to geothermal power, the only source is radioactivity. Of the 92 chemical elements known in Nature, several have varieties with too many neutrons in their atomic nuclei to be perpetually stable. These unstable isotopes have a tendency to break apart into a range of daughter isotopes. In the process there is a deficit in the mass of the products; energy is released, explained through E = mc² . Only three elements are plentiful enough in the Earth and have unstable isotopes energetic enough to make a significant contribution. Two of them, thorium and uranium (isotopes [sup.232]Th, [sup.235]U and [sup.238]U), are familiar enough from their use as fuel for nuclear reactors and atom bombs. The third is unexpected because it is abundant in oranges and is vital in the chemical processes that send electrical signals along our nerve cells, as well as being consumed by individuals with high blood pressure in a substitute for table salt. This is potassium, specifically its rare isotope, [sup.40]K.     You might think from their uses that thorium and uranium generate a lot of heat for a particular weight. In fact, a one kilogram lump of the most energetic, [sup.235]U, gives out only 20 000 joules in a year. This is less than one-tenth of the energy locked in a cucumber sandwich! For [sup.238]U the figure is about as much as the cucumber filling, for [sup.232]Th somewhat less, while for a kilogram of [sup.40]K the release is about one joule--far less than is emitted by the yeast needed to raise the dough for two slices of bread. The surprise stems from our usual perception of radioactivity. Nuclear bombs and power plants generate vast amounts of power almost instantly, from only a few kilograms of [sup.235]U in the case of an atom bomb. The process there is not fission of the fuel itself but that of other, much more unstable and powerful isotopes, such as plutonium, which do not occur naturally on Earth. They are produced in a nearly instant chain reaction by uranium absorbing the neutrons that its decay produces, if more than a critical mass is assembled in a small space.     To produce the Earth's internal power requires vast amounts of weak nuclear fuels--50 thousand billion tonnes of [sup.235]U and 20 thousand times more [sup.40]K alone, but a mixed bag is involved. The masses involved pose no real problem, simply because the Earth is so large, weighing in at around 6 × [10.sup.21] tonnes. If only 10 parts in every billion were [sup.235]U, that alone would do the trick. A problem crops up when we try to assess where the heat sources are. When rocks are analysed it turns out that those building the continents contain 5 and 20 parts, in every million of U and Th and around 3 per cent of K, of which one-thousandth is the unstable isotope. The rocks of the ocean floors are very different from continental rocks, as you will see, and values there drop to 1 and 3 parts per million and 0.5 per cent respectively. In both cases, however, there is far too much heat-producing potential if the Earth was made up entirely from such materials. It cannot be, and indeed other lines of evidence show that the outer crust of the planet, sharply divided into continental and oceanic varieties, is a thin veneer sitting on a great thickness of exceedingly monotonous rock. Occasionally fragments of this true mantle turn up in volcanic lavas that have moved to the surface from great depths. They reveal about the right amounts of U, Th and K--15 and 80 parts per billion and 0.1 per cent respectively.     How the Earth's heat producers are distributed, in absolute amounts and relative to each other, is one of the thornier problems in the Earth sciences. We simply cannot analyse every rock in the crust, and very little indeed from the mantle is exposed. Apart from the little bit of chemistry from the crust, all that we have is a product; heat that leaks out at the surface. If the energy balance sheet for solar radiation might raise the eyebrows of an accountant, that for internal energy looks as suspicious as that of a Colombian drug baron. Like 'laundered' dollar bills, heat does not carry a signature of its origins. Clearly some investigation is needed, and the only evidence is how heat flow varies over the Earth's surface.     Leaving aside the local hot-spots and strangely cool areas, the power outputs from the ocean floor and from the continents are very similar. Yet ocean floor rocks have only one-tenth of the power-generating capacity of continental rocks. The thicknesses of both sorts of crust can be estimated by using earthquake waves as a kind of depth sounder, and come out on average as 30 km for continental and 10 km for oceanic crust--we can work out the contribution of each to heat flow. Both estimates fall some way short of observations; there is a contribution from the mantle beneath. The surprise is that mantle beneath the oceans releases twice as much heat as sub-continental mantle. That is something to explore later.     The Earth's internal power production breaks down as follows: 3 per cent from oceanic crust, 35 per cent from the continents and the bulk (62 per cent) from the mantle deep below and perhaps from the Earth's metallic core. That from the crust can leak to the surface quite quickly--it does not have far to go. The mantle is 3000 km deep, so heat takes longer to escape. Not only is it the Earth's main nuclear powerhouse, but the manner in which heat is moved there makes the mantle its main engine room. Before going on in Chapter 2 to look under the hood and at how external energy affects the paint job, as it were, there are two final points regarding the heat-producing elements.     The first is this: heat producers are unstable isotopes and with time they decay away at constant, measurable rates. If we peer back in time they become more abundant and heat production increases. Fresh from the factory the Earth was more than six times more powerful than it is today. The rundown of the power source and the ins and outs of the transmission system form the underpinning to the evolution of the Earth system's inner component. The second point is somewhat more complicated, and we return to it several times later. Continents contain heat producers in superabundance, but that variety of crust has grown over time as a sort of effluent of mantle processes. The mantle continually loses fuel to the outer Earth, so this is another reason to regard this powerful `motor' as one that has become less `sporty' with age.