Goldilocks and the three planets
Life and times of a yellow dwarf
The fire within
The observer that was lost and found
A planet in the balance
No life without light?
One of the favourite walks I tend to take on Saturday afternoons with my children leads us across the green fields and into the science area of Oxford University. We end up in front of a lawn with fresh dinosaur footprints. No, I'm not joking. In this very moment, as I'm writing these lines, they are only a couple of weeks old. But I'll admit that they are fakes. They are in fact concrete casts of the petrified traces that a lorry-sized, carnivorous Megalosaurus left in the northern parts of Oxfordshire some 168 million years ago. I totally admire the idea to put the casts there, even though it makes the sign saying "no games allowed" look a little bit ridiculous. It's just so typical -- letting the dinosaurs run free but banning the kids from playing on the grass. But what did you expect? After all, the dinosaur-haunted lawn is the front garden of the superb Victorian building which was the first science department at Oxford University -- the University Museum of Natural History.
The museum was built in 1855-1860 as a three-winged neogothic structure, reminiscent of the French medieval cloth houses. The fourth wing was left out on purpose to allow for a possible extension, which was indeed built later on and now houses the Pitt-Rivers Museum. The newly built museum became the arena of a major piece of science history within less than a year after its opening. It was here that the British Association for the Advancement of Science debated Darwin's theory of evolution in June 1860. A book entitled "On the Origin of Species" had just come out in November 1859, and was causing a bit of a stir. Darwin's disciples, Thomas Huxley and Joseph Hooker, apparently won the battle of words against the bishop of Oxford, Samuel Wilberforce, in front of some 700 people. Science had for the first time in history found a home in Oxford, and with that, it also found a new degree of confidence in the debates with religious opponents.
The building also features prominently among the places where Lewis Carroll found inspiration for his Alice books. Most famously, the museum's dodo specimen, now derelict, found its way from these remises into the pages of Alice in Wonderland. Twentieth century residents of the museum include the Nobel prizewinning crystallographer, Dorothy Crowfoot Hodgkin, and the biologist and author Richard Dawkins.
Before we walk in , though, let us have a look at the impressive facade of its front wing, with its dormers and first floor windows arranged in immaculate symmetry around the central tower, while the ground floor windows seem to be obeying more subtle rules, possibly derived from chaos theory. It only adds to the confusion that the carving of the window-jambs has remained unfinished after a dispute between the builders and architects concerning the finances of the work. Twenty metres to the right of the front wing, and connected by a narrow passage, we see a chapel-like structure known as the Abbot's kitchen (with reference to the building at Glastonbury that served as its model). Built as a chemical laboratory that was part of the original museum plan, it related to the main building like the loo at the back of the garden to the Victorian terraced house. In order to prevent any noxious smells from entering the main museum building, the planners banished chemistry to this ghetto. Maybe they were worried about explosions and fires, too. Little did they know that this little appendix would proliferate into half a dozen major buildings over time. In a sense, the museum can be called the mother of all science departments at Oxford. As new disciplines have come of age, they have one by one left this cosy nest and set up their own homes alongside South Parks Road.
Walking in through the heavy oak door feels very much like entering an old church, and even the noticeboard in the porch would still be compatible with this impression. Having scrambled up the stairs and entered a second, equally heavy oak door, one finally realizes that it's not a church, because churches don't normally have big skeletons in them. After a few more steps straight ahead, we're finding ourselves in a square courtyard under a glass roof supported by ornamented cast iron pillars. They are thickly painted in a stone-like colour, but when you knock on them, the sound gives the metal core away. The whole structure is strangely reminiscent of a well-preserved early railway station although the hushed atmosphere and the heated floor contradict this impression.
This courtyard, enclosed by the three main wings of the original building, and by the Pitt Rivers Museum at the back side, contains the larger exhibits and showcases, while smaller items are also displayed under the arcades around it. The items on show look very traditional at the first glance -- of course there is the usual mix of dinosaur skeletons, amethysts, and stuffed birds you expect to see in such a place, but there are also a few jewels to be discovered. Walking round the quadrangle you may notice that the columns supporting the arcades are all made from different kinds of stone -- a geology course incorporated into the very substance of the building. And if you let your gaze follow the iron pillars towards the glass roof, you will notice that they grow leaves at the top, like a forest of metal trees growing towards the light.
Coming back to the front wing of the quad, we walk up the staircase on the right hand side corner (seen from the entrance). At the turn of the stairs we pass the museum's very own bee colony, buzzing behind acryl windows. On the first floor, we turn right and walk down along the right wing, passing millions of insects of all shapes and sizes, until we bump into the Snow White style glass shrine of a Roman inhabitant of the Science Area who probably would not have predicted that people would still stare at his skeleton some 1800 years after his death. Here we turn left into the back arcades, but stop short after a third of the length. On the balustrade to our left, throning high above the dinosaurs and other treasures of the courtyard, is a shiny brass globe slightly bigger than a basketball, with a little label underneath. Reading the label we learn that this is part of an accurately scaled model of Sun, Earth and Moon with the appropriate distances between them, and that the rest of the model is on the balustrade on the opposite side of the court. Of course, as the court measures some 33 metres across, we can't see any of it from where we are, so we obediently trace back our steps to the other side and find there a little showcase which we would certainly have overlooked had it not been for the brass ball and its label.
Well, of course you know that our planet is a lot smaller than the Sun, and quite far away from it, but both these concepts are difficult to imagine on their own, and even more so in combination. The drawings in books tend to either show the right sizes, or the right distances between the Sun and its planets, never both. So I am warning you that you will probably be shocked when you look into the box on the other side, only to find a pinhead of less than 3 mm diameter representing Earth, with an even smaller pinhead (less than 1 mm) for the Moon just eight centimetres away, on the backdrop of an irritatingly featureless black cardboard square. Looking back across the gallery you still see the brass Sun, but the space between the two balustrades suddenly appears vast and empty.
On the scale of this model, the neigbouring stars might be in some other museum in New York or Tokyo. Just imagine the task of seeing from Oxford whether they have habitable pinheads in Tokyo! Of the other planets in our system, Mars would scrape round the corners of the museum building, while Pluto would be some 1.3 km away. Incidentally, the average distance between Neptune and Sun misses my house by 30 metres. Taking the elliptical shape of Neptune's orbit into account, I could probably build an extension of the model by installing an 11 mm marble in my front porch for casual visitors ...
So, right now, we are sitting on this pinhead and we ask ourselves what the shiny ball on the other side is made of, why it came into being, why it shines, and why there are these little specks circling around it which we call planets and consider to be important, although between them they make up less than one percent of the mass of the solar system. We will see that we are incredibly lucky to have our Earth speck in the place where it is.
Goldilocks and the three planets
Our blue planet Earth is an oasis of life in the cold immensity of the universe. Just remember the pinhead in the museum gallery, and then think that the nearest other star and hence the nearest place with even a small chance of harbouring diverse life, would on this scale be a football somewhere near New York. We're on the only habitable pinhead in a sphere of at least the 1.5 fold diameter of the Earth. Even if there are many other Earth-like planets outside this sphere, life will still be a very small exception in a largely empty and lifeless Universe. This remarkable circumstance needs an explanation. Why Earth, rather than Mars, Venus or Europa?
It is true that in comparison with all other celestial bodies we know, Earth appears as just the perfect place. The wide variety of chemical elements, the abundant supply of liquid water, the atmosphere, the protective ozone layer, the slowly changing continents, all the major features of our planet appear to be ideally suited for the task of developing and sustaining a rich and diverse community of living species like the one we observe today.
However, this should not fool us into thinking that Earth had always been a greenhouse, just waiting for seeds to be placed in it so it could be filled with life. Conditions have not always been as gentle as they are today, and when the planets formed it was not obvious that this one was special. Like its immediate neighbours, Mars and Venus, early Earth was an inferno of volcanoes and battered by meteorites. When the crust had solidified and the meteorite bombardment ceased, adolescent Earth was probably still quite similar to the neighbours. All three may have come up with primitive life at that time. But while life on Earth took over the planet and drastically changed the composition of its oceans, sediments and atmosphere, our two neighbours went different ways. Mars lost its atmosphere and therefore became too cold to sustain a substantial biosphere, and Venus had too much of a good thing, with the greenhouse atmosphere leading to unbearably high temperatures.
There are many reasons why things turned out "just right" for Earth, but not for any other planet in the solar system. Apart from the chemical composition and the size of our planet, the distance from the Sun is one of the most important criteria. We are within this narrow band of possible paths around a star in a distance which allow for the presence of liquid water and a biosphere like ours, and which is called the Goldilocks zone (after the Victorian fairytale "Goldilocks and the Three Bears"), because it is "just right". When astronomers search for planets in other solar systems (which can only be detected by indirect means so far), they are most interested in objects that are likely to be within the Goldilocks zone of their stars. As the gravitational pull a planet effects on its star is so far the only clue that we can get about the existence of other planetary systems, most of the few dozen planets discovered since 1995 are Jupiter-sized giants whizzing round their stars on a Mercury-style orbit in a matter of days. This doesn't exclude the possibility that each of these systems could have additional Goldilocks planets, but so far we just cannot tell.
Being at the right distance from a suitable star is what can make a habitable planet a living planet. It appears that those cultures that identified the Sun with a life-spending deity did have a valid point (see chapter 6).
We are very lucky in that we are in the right place, and the time is right, too. The early Earth was permanently battered by meteorite impacts, while the late Earth, in a few billion years from now will lose its atmosphere and water as a consequence of the imminent death of our star. In about five billion years from now, the Sun will begin to run out of fuel which means that it will first expand into a red giant roasting our planet to death, and then the fire will eventually go out and the solar system will cool down to the temperature of the surrounding universe, just a couple of degrees above absolute zero. In between, there is a window of opportunity of ca. eight billion years in which a diverse biosphere can flourish on our planet.
Life and times of a yellow dwarf
Some five billion years ago, somewhere in the Milky Way, a loose cloud of gas particles, possibly the scattered leftovers of a supernova explosion, collapsed into a ball with a thin flat rotating disk around it. The ball contained enough mass for the gravitational energy to fire up the atomic reactions which make stars shine, and thus a quite normal yellow star was born -- the one we call Sun. During the first few million years of its new life as a star, the mass compacted further, and the star shone several times brighter than in the very beginning. The mass contained in the rotating disk also collapsed into lumps which eventually became the planets we know today. For the overall book keeping of the mass contained in the solar system, however, the planets are mere peanuts: 99.9 % of the mass are contained in the central star.
Since then, the disk surrounding the young star has consolidated somewhat, forming nine planets with their thirty-odd moons. Most of the stray bodies that made life dangerous in the first billion years or so have either hit a planet, found a safe orbit, or left the solar system. The energy output of the sun has risen by another 25 %, but life on Earth has found ways of maintaining its overall environment in a life-enhancing shape. Although catastrophic impacts can still happen and although the celestial mechanics is not quite as predictable as Newton would have predicted, our planet has a fair chance of sustaining diverse life for another four billion years, as long as we humans don't do anything too silly. Provided that life and some kind of civilisation do survive through to the end of the sun's fuel reserves, an interstellar ark will then be required to transfer our biosphere to a younger star.
But let us leave this problem for science fiction authors and have a look at the processes which keep the solar fire burning.
The fire within
Atoms, those allegedly indivisible (in Greek: "a-tomos") fundamental building blocks of matter, have soon after the discovery of their physical reality been proven to be a lot more divisible than their name suggests. Firstly, they are made of negatively charged electrons and a positively charged nucleus. Electrons can be removed from most atoms quite easily -- that is what chemical reactions are all about. Atomic nuclei have two kinds of building blocks: the positively charged protons, and the uncharged neutrons. Certain kinds of nuclei, especially those which are quite heavy and rich in neutrons, have been shown to be unstable -- they decay by forming lighter atoms and emitting radioactivity. In 1938, Otto Hahn, Lise Meitner, and Fritz Straßmann found that one can split heavy atoms by bombarding them with nuclear particles.
If you list atomic nuclei by increasing weight, the most stable ones will be in the middle, around iron and nickel. For this reason, energy can not only be freed by splitting atoms heavier than iron, but also by merging atoms substantially lighter than it. While the former process can be technically mastered (at least in some places, and most of the time), the latter, known as nuclear fusion, has been used in hydrogen bombs, but not yet tamed in a way as to provide energy in a controlled reaction and in a process that is economical overall. And still, this is exactly the kind of energy we are getting from the sun. Solar energy is exclusively produced by nuclear fusion.
Hydrogen is the most abundant element in the universe, and it is also the lightest element, as far away as possible from the very stable iron nucleus. Its neighbour in the periodic table, helium, has an unusually high energy of binding keeping its nucleus together. Therefore, the merger of two hydrogen atoms to form helium can liberate an enormous amount of energy, as it does in the inner core of the sun. The catch is that the atomic nuclei which are to be merged are positively charged, and hence repel eachother, like all bodies of equal charge would. Therefore, energy has to be invested into overcoming this repulsion at the beginning, before the much larger energy yield of the fusion reaction can be harvested. This is the reason why nuclear fusion is not yet being used for energy production here on Earth, and why it works so nicely in the sun and in other stars, at extremely high temperatures (up to 16 million degrees C).
The fusion reactor which provides us with heat and light is in the core of our Sun, a ball within the ball. On the scale of the museum model, it would be a little bit smaller than a tennis ball. Because the nuclear reaction is very rich in energy and occurs at very high temperatures, the energy liberated by it does not take the shape of heat and light as in an ordinary fire or light bulb. It is mainly radiated off as extremely short-waved electromagnetic radiation, known as gamma rays. (In this respect, fusion resembles nuclear fission.) A further product of the fusion reaction is a stream of strange elementary particles, the neutrinos, which have very little mass (or perhaps no mass at all, this is still controversial -- see flashback).
((insert Flashback: neutrinos approximately here))
While the neutrinos, which are hardly ever thrown off their track no matter what obstacle arises, escape through the outer shells of the sun within seconds and just keep going straight ahead, the gamma rays will on their path interact with countless hydrogen and helium atoms, thereby losing their direction and part of their energy. Thus, the energy emitted as gamma radiation in the core of the sun need thousands of years to penetrate to the surface, from where it is sent out into space as a much less energetic mix of visible light, heat, and ultraviolet light. The layer which sends out this radiation, which is essentially our sunlight (except for those parts which our atmosphere filters out), is called the photosphere. It is much colder than the core, at only around 6000 °C and reaches from a depth of 1.8 to 1.9 mm in the model (see legend to Figure 1 for the "real" numbers). Between photosphere and core there is also a zone where energy is mainly transported by convection, that is by hot gases rising and cold gases sinking as in our atmosphere or in a pot of water heated from below.
The photosphere is in turn surrounded by a thin gas layer known as the chromosphere (the outer 1.8 mm of the brass ball). This layer produces the turbulences of the solar surface which we can sometimes observe. While the photosphere and chromosphere determine our visual image of the sun, they are by no means the end of the story. The visible ball is surrounded by an invisible wrapping of gas, the corona, which could be described as the sun's atmosphere. It expands all the time and in all directions (away from the sun), a phenomenon which is also known as the "solar wind". This wind is causing the tails of comets, always pointing away from the sun. The corona becomes thinner as the gas particles available spread out over a bigger sphere, but it is hard to define an outer limit for it. Probably the solar wind ends only when it collides with the corresponding winds of other stars.
Now that we have seen how the sun works, we can come back to the fate it faces in the future. The solar fusion reactor has still got enough hydrogen atoms to keep burning for a few billion years longer, but it it will be getting a serious problem of waste disposal. The helium atoms formed as a product of the fusion reaction are heavier than hydrogen and hence they accumulate close to the centre of gravity, in the middle of the core. There is a "nuclear waste dump" at the centre of our sun, which grows steadily, and is more dense than the surrounding hydrogen. Because of its higher density and bigger gravitational pull, this helium core within the core will collapse further. When it reaches a critical mass and density, helium itself will become nuclear fuel, with carbon atoms as the waste product (which will again form a core within the core). The two step fusion reactor (hydrogen to helium to carbon) will on the outside be a red giant putting our Earth in a rather uncomfortable position.
When enough carbon has accumulated to fuel higher fusion reactions (leading to oxygen and neon), the end is nigh. Hydrogen is running out, and the nuclear fusion of the heavier elements does not liberate enough energy to counteract the gravitational forces which make the red giant collapse into a white dwarf. The dwarf casts off its gas layers and keeps glowing at a lower intensity for a few billion years. When the fusion reactions eventually die down, the sun will turn into a black dwarf and gradually cool down to the temperature of the surrounding empty space.
The observer that was lost and found
All we know about the Sun has been deduced from the detailed analysis of the radiation it sends out. Much of it is outside the visible spectrum, in the high energy wavelength bands of the ultraviolet and beyond, which our atmosphere thankfully filters out. If solar physicists want to get the full picture, they need to place their instruments in space. And thats exactly what they have been doing over the last few decades. The most recent and advanced satellite to be dedicated to studies of the Sun is the SOlar and Heliospheric Observatory, SOHO, which the American and European Space agencies, NASA and ESA, launched in December 1995.
On February 14th, 1996, SOHO arrived at its destination, the privileged point on the Sun-Earth axis where the gravitational forces of both bodies are exactly balanced. Due to the inequality of the masses involved, this so-called inner Lagrangian point is only 30 cm away from our pinhead (ca. 1 % of the total distance to the Sun). But then, unlike any satellite orbiting the Earth, SOHO sees the Sun 24 hours per day, and remains completely unaffected by any terrestrial problems (other than controllers errors). Six of its 12 onboard instruments are designed to probe the Suns atmosphere, three look at its interior, and three at the solar wind. Its location also allows it to serve as an early warning system to report magnetic storms coming our way.
SOHO started its mission at the lowest point in the magnetic activity of the Sun, which changes in a 11-year cycle that is thus far poorly understood. The spacecraft carries enough fuel to keep on watching the Sun throughout one complete cycle, including the 2000/2001 solar maximum and the decrease of the activity towards the following minimum.
The hopes for such a long-term observation were very nearly shattered on June 24th, 1998, when a routine maintenance operation left one of the gyroscopes on the wrong setting. The onboard computer wrongly assumed the probe was spinning around and fired the thrusters to counteract the movement. Control staff responded swiftly by switching off the gyroscope, only they picked the one which was still working ok, leaving the probe tumbling out of control. With its solar panels turned away from the Sun its energy supply was off. A few minutes later, SOHO was frozen in a coma, unable to move or respond to commands from Earth.
In a painstaking search, operators were able to localize the probe using radiowaves reflected from its solar panels, and to determine its orientation and slow rotation. They worked out that there would be a window of opportunity for reviving the probe two months later, when the solar panels would again face the Sun. Sending very carefully thought-out commands at precisely the right time, they managed to gradually thaw the fuel lines, re-establish communications with the probe, and wake up the scientific instruments. Considering the harsh conditions of spending two months in space without heating, scientists were surprised to find all scientific instruments came back to life. The gyroscopes, however, were lost for good soon after regaining contact. The operators made do without them, instructing the probe to orient itself towards the Sun instead. Since then, it has carried on with its mission without major problems.
As data from the probe are still coming in and being analysed by researchers, it is not yet possible to give a final summary. Many important findings have already been reported. Seismographic measurements made by SOHO have, for instance, enabled researchers to "see" sunspots on the far side of the Sun. As the Sun turns round once in 27 days, this knowledge allows to predict the magnetic storms that are coming our way once the sunspots have arrived on our side a few weeks in advance. This is important, because such events can seriously affect satellites, spacecraft, and telecommunication systems.
Other research areas addressed by SOHO investigators include:
* the question why the corona is so much hotter than the photosphere underneath it,
* understanding the sources of the solar wind, and
* what exactly is contained in the core.
On top of that, SOHO has also become the worlds highest scoring comet finder, enabling the discovery of more than 100 new comets by February 2000. Many of these, however, are now no longer with us: they were on collision course with the Sun.
A planet in the balance
But let's leave SOHO keeping its watch and return to Earth, to see how the Sun enables our planet to grow living things. Now we have got a planet circling a suitable star at the right distance and receiving the right energy dosis. But still, these conditions, though necessary, are not sufficient to guarantee a climate in which the life-enhancing effects of light can unfold.
If, for instance, the wrapping artist, Christo, endeavoured to wrap our whole planet in white plastic foil, the majority of the solar energy coming our way would be reflected back into space directly. The small part that could penetrate or be absorbed by the wrap would not be sufficient to stop the oceans from freezing. Thus, liquid water -- a major requirement for life -- would cease to exist. In a more likely scenario without the need of an artist, a small increase in the extent of the polar ice caps could increase the fraction of the sunlight reflected directly into space gradually (scientists call this fraction the "albedo" or whiteness of the planet). The reduced energy intake would cool our climate down, which would lead to a further growth of the pole caps, and so on. According to a controversial theory proposed by Harvard geologist Paul Hoffman in 1998, a runaway glaciation period may have actually taken place between 750 and 550 million years ago. Volcanic emissions of the greenhouse gas carbon dioxide would have eventually saved "Snowball Earth" from remaining frozen for ever.
If, in contrast, the artist set his mind on wrapping the planet in clear plastic which lets the light in but stops excess heat from getting out, the majority of the solar energy would be trapped in a global greenhouse. Ice caps would melt and thereby make things worse. Eventually, the oceans would evaporate, and Earth would become a lifeless hothouse similar to Venus. Again, such a catastrophe could conceivably happen even without an artist, triggered by greenhouse gases, or a reduction of the polar ice caps for whatever reason. The large amounts of methane trapped at the bottom of the oceans in the form of methane hydrate ("burning ice") would be more than sufficient to trigger such a change.
These simple thought experiments show how sensitive the balance of our global climate is. Living beings will both contribute to this balance (on either side) and depend on its stability for their survival. Plants, for instance, remove the green house gas carbon dioxide from the atmosphere and producing oxygen instead, which animals like ourselves need for breathing. We, on the other hand, eat plants, and use the oxygen from the air to burn the carbon they have accumulated to carbon dioxide. If the plants worked a lot harder than the animals, the oxygen content of the atmosphere might increase, leading us to a situation where ordinary materials such as paper or leaves would become dangerously flammable. If, on the other hand, the animals ate up the plants faster than they can grow, there might be a shortage of oxygen and a greenhouse effect from excess carbon dioxide.
It is intriguing that in spite of the possibilities for catastrophic runaway changes, the composition of our atmosphere and our climate has (probably) remained fairly constant for half a billion years. The ice ages are in fact only a faint echo of what might have happened (and what quite possibly happened some 700 million years ago). The British chemist James Lovelock realized how much the physical conditions prevailing on our planet are influenced in an obviously positive, life-enhancing way by the activities of the lifeforms inhabiting it. He cast this observation into the hypothesis which is now known as the Gaia theory, suggesting that the whole planet, including the biosphere, acts like a self-regulating kybernetic system or living being, a being that he named after the Greek Earth goddess Gaia.
In one thought experiment which he used to illustrate his theory, Lovelock populated a model planet with two species of daisies, a white one and a black one, with the former thriving better at higher temperatures, the latter at lower temperatures. When the climate warms up, white daisies spread more widely, leading to an increased albedo and reversal of the warming trend. Conversely, when it gets colder, black daisies take over and help the planet to collect more solar power. The whole process is a negative feedback loop, which essentially ensures that any deviation from the "normal state" of the system triggers a response which counteracts the deviation. Similar, if vastly more complex self-regulation mechanisms may be the true explanation for the surprising fact that the Earth's climate has stayed in the optimal range in spite of changes in the solar energy output.
In these regulatory cycles, the negative feedback loops which stabilize the status quo must be stronger than any positive feedback loops (like the albedo/icecap effect) which would favour catastrophic changes. What is happening on a global scale appears to be similar to the way in which living organisms keep their internal conditions (body temperature, pH, water content) approximately constant, which is why Lovelock identified the whole Earth as a living being. It also resembles technical regulation processes, or, for instance the way you would steer a car. If you are a safe driver, you constantly counteract small deviations from the straight intended course. In fact, every vehicle you move procedes in more or less sinewave-shaped curves rather than straight lines, as small deviations from the intended direction have to be corrected for. In this comparison, the Earth could be a self-regulating kybernetic system, after the ancient Greek word for a helmsman.
Without getting too involved in the Gaia discussion, let us just have a quick look at the energy balance of our planet. All the energy available to living beings is ultimately derived from three sources: Earth, Moon and Sun. The smallest factor among these is the input of the tidal forces. Gravitational pull and centrifugal force are exactly balanced only only the the the Earths centre of gravity and in those places which are at the same distance from the Moon. In other words, the balance is right wherever you see the moon just about rising or setting. On the hemisphere closer to the Moon, the oceans are pulled towards it a bit more strongly, while those on the opposite (moonless) side are pulled a bit more weakly than the whole Earth as an idealized rigid body. Therefore, the water is a few meters higher at these two extremes, and lower in the middle between them. This is why we see high and low tide twice in 25 hours at the coasts. (If, as one could erroneously imagine, all the water was attracted to the Moon's side, one should observe high and low tide only once per day.)
This also means that, twice every day, the coasts are being pushed against these mountains of water, which results in mechanical friction. You may not have noticed this so far, but this friction actually slows down the Earth's rotation by two thousandths of a second each year, as the motion energy is turned into heat, much the same way as braking a car results in heating up the brakes and the tyres. As spinning tops go, a planet is quite a big one which can carry a lot of energy even if it only turns round once a day. Hence, even a very minor change in its speed involves a considerable amount of energy -- in this case it's 2.7 Terawatts (trillion watts or 1012), which corresponds roughly to a quarter of the total energy consumption of humanity (see Figure 2 for a graphical illustration of all the big numbers mentioned in the following text). Not that there is any chance of using a significant part of this energy.
A ten times larger flow of energy is emerging from the interior of our planet. Part of it is residual heat stored in the liquid core, another part results from natural radioactivity. Processes like hot lava flowing out of rifts or volcanoes are heating the surface with eleven Terawatt, heat simply conducted through the upper layers provides another 21 TW. Taken together, the heat leaking out of the Earth's bowels makes up three times humanity's needs.
If you think these were big numbers, think again. All the sources mentioned so far only provide a ten thousandth of the energy which the sun gives us. 173,000 TW are burning down on us, of which roughly a third is reflected straight back into space. Half of it serves for heating land, water and air, thus compensating the heat that we constantly lose to outer space. A little less than a quarter goes into vaporizing water, and thus ultimately into heating the atmosphere, when the water forms raindrops again. Plants only use a four-thousandth of the solar energy available, which is still four times more than what we use. With this energy, plants feed almost the entire biosphere, including of course ourselves.
Most of the sunlight goes out into empty space and may travel for billions of years before it hits any object. Only a little more than a billionth of it comes our way, and of this, 1/4000 is enough to feed the entire biosphere. Just think how many copies of our biosphere the Sun could keep alive ((1))!
No life without light?
The origins of life have remained a mystery. We don't really know whether the first cells drifted around in a sunny little pond or in dark, volcanically heated clefts of the sea floor. It is certain, however, that only the combination of a generous supply of solar energy with an atmosphere which can retain a major part of this energy could guarantee the relatively constant climate that we have enjoyed over the past four billion years, and which provided ideal conditions for life to spread on Earth to the point of dominating and reshaping the whole planet.
Light is the part of the wide range of electromagnetic radiation phenomena which we can see and therefore distinguish from the infrared (heat) rays on the long-wavelength side, and also from the potentially dangerous ultraviolet radiation on the more energy-rich short wavelength side. One should bear in mind, however, that this definition is entirely artificial and based on the limits of our visual sensory apparatus (to which we shall return in chapter 6). Cats and bees, if only they could talk, would define light very differently. Three billion years ago, there was no basis whatsoever for such a differentiation. Like today, a broad range of electromagnetic waves shone down onto the young Earth from the Sun, and as there were neither eyes to see any light, nor photosynthesis to make use of its energy, this radiation was useful for life only in an unspecific sense in that it provided heat and in the UV range even energy densities which could trigger chemical reactions.
Photosynthesis, then, was a major technological revolution -- the biggest that our planet has ever seen. Apart from providing a direct link between solar energy and chemical energy that can be used in metabolic reactions, it also changed the face of the planet forever, as we will find out in due course. So big was the change brought about that nearly all lifeforms became addicted to sunlight -- either directly or via some product of photosynthesis, such as oxygen. Without light, life would be impossible now.
((Boxes for Chapter 1:))
Flashback: Neutrinos -- mysterious messengers from the Sun (899)
Wolfgang Pauli (1900-1958) was known as a genius in theoretical physics, and dreaded as a critic who could devastate a colleague with just a few words. In 1930, he found himself deeply immersed in a dilemma without a solution that would have withstood his own critical mind. A nuclear reaction known as the beta decay ((??)), in which a neutron splits up to form a proton and an electron, appeared to be in blatant violation of the physical law concerning the overall conservation of energy. Just to save the day Pauli very reluctantly postulated (physicists' lingo for "invented") a new kind of particle that would be able to carry away the excess energy, but wouldn't contribute measurably to the charge or to the mass balance.
This hypothetical particle, which his Italian colleague Enrico Fermi baptized by the Italianate name of neutrino, was postulated to engage only in one of the three fundamental forces of physics, namely the weak interaction. This meant that neutrinos would be extremely unlikely to interact with anything else, and thus extremely different to detect. Understandably, Pauli was extremely unhappy about having to postulate such an elusive thing.
But Pauli was to be confirmed, both with respect to the existence of neutrinos and also concerning the difficulty in detecting them. A quarter of a century went by before these particles could first be detected in the proximity of a nuclear reactor. Theoreticians then put forward another one of these hard-to-prove hypotheses: The Sun, being an immense nuclear reactor, should also send out unbelievable numbers of neutrinos. Calculations carried out in the 1960s suggested that 2 % of the energy created by the sun is emitted as neutrinos. This means that the area of a thumbnail is passed by 100 billion solar neutrinos every second, but without deflecting a single one of them from its path. As even big and massive bodies, like Earth, are no significant obstacle for the neutrino's progress, they even shoot through your body while you're on the night side of the planet. But how to detect particles that just go through our planet as if it was just thin air?
All first generation neutrino detectors relied on the same principle. A huge amount of a highly pure substance was placed in the neutrinos' path in the hope that they would, by a rare nuclear reaction turn some atoms of that substance into different, radioactive atoms that would be easy to detect. Thus, the Gallex experiment had 30 tons of gallium in a salt brine. Any neutrino reacting with a gallium atom would produce a radioactive Germanium atom. In order to be sure that the radioactivity measured comes from this reaction, one has to filter out the cosmic background radiation very efficiently. A one-mile layer of granite is just about acceptable for this purpose -- that's why the Gallex expreriment is based in a tunnel in the Alps, while other, similar experiments are carried out either in mines or underneath the antarctic ice shield.
This effort only yielded very little success, however. If the researchers were lucky, they caught one neutrino per week in their gallium trap. Maybe most of them would have moved on to a more rewarding research field, had it not been for one day in 1987, when a spectacular 19 neutrinos were observed in a short time. This suggested that the usual hundred billion solar neutrinos per thumbnail and second were being swamped by a thousandfold bigger number of other neutrinos from an unknown source. Hours later astronomers observed the light of one of the closest supernova explosions to be recorded in the 20th century. The neutrino shower had been the first messengers of this event. In these hours, neutrino astronomy was born. New and improved detectors were developed in the hope of tracking down events in the dark depths of space that remain hidden to optical astronomy.
The most powerful detector of the new generation is called Superkamiokande and contains some 50,000 tons of ultrapure water in a cylindrical barrel surrounded by some 11,000 light detectors. It works on the principle that in the rare event when a neutrino hits an atomic nucleus, it can create other high-energy particles, which send out a characteristic flash of light known as Tcherenkov radiation. It comes from particles moving faster than light would move in the water (but still slower than c, the velocity of light in vacuum), and is thus comparable to the bang we hear when an aircraft crosses the sound barrier.
Results from Superkamiokande and other modern neutrino detectors have tended to show smaller numbers of neutrinos, and different ratios between the three different kinds, compared with what theorists predicted. While physicists are still scratching their heads over this, one of the possible explanations suggested is that the kinds of neutrinos can interconvert en route. Only two of the three can be detected by their Tcherenkov light, so the missing neutrinos may have turned into the third kind. But this remains a hypothesis so far, and the neutrinos are still shrouded in mysteries. One of the important open questions in physics is whether the neutrino has a resting mass at all. The interconversion theory would suggest that they have one, although it would still be many orders of magnitude smaller than that of the electron and thus be zero for most intents and purposes.
Flashback: Helium, the Sun element (749)
In the mid-19th century, the existence of atoms was an unproven hypothesis. John Dalton (see profile on page xxx) and the other founding fathers of modern chemistry had shown that the assumption of atoms that combine in fixed proportions to form molecules would neatly explain the proportions observed in chemical reactions in the gas phase. Thus, one volume oxygen gas reacted with two volumes of hydrogen to form two volumes of water vapour. It was a bold step from there to the conclusion that each of these equivalent volumes corresponds to a constant number of molecules, and that, the proportions correspond to the numbers of atoms per molecule. For the time being, direct evidence of these atoms and molecules was non-existent.
But then, in 1859, the German physicists Gustav Kirchhoff and Robert Bunsen developed an instrument that allowed them to record characteristic fingerprints of atoms. In principle, it was just a Newton style prism with a camera. If you make a metal wire glow and guide the light through the prism and onto the film, you don't get a full spectrum as you would with sunlight. Instead, you get a distinct, well-defined array of light stripes like a supermarket barcode. This so-called line spectrum of a light source is in fact a fingerprint of the chemical elements contained in the source. It tells us which kinds of atoms make up the glowing wire.
And more interesting than glowing wires, the technique allows you to analyse the composition of light sources that are very far away, like distant stars, or, like our very own star, the Sun. The lines ar so sharp and precisely localized that they allow individual elements from mixtures containing up to dozens of different kinds. What's more, if you observe lines that don't fit any known element, you know you've found a new one. In 1868, Norman Lockyer and Edward Frankland took advantage of a solar eclipse to analyse a line spectrum of the light coming from the corona. Apart from the well-known and very strong signals assigned to hydrogen atoms, they also found an unknown element, which Lockyer called helium, after the Greek name for the Sun, helios.
More than 20 years went by before the Sun element was also discovered closer to home, down on Earth. In 1890, scientists analysing uranium ores by dissolving them in acid detected a gas which Sir William Ramsay identified in 1895 as a member of the newly discovered group of the noble gases (Argon had just been discovered a year before, the others were found in the following years, after Carl von Linde succeeded in cooling air to the liquid state and separate its components on the basis of their different boiling points). Like the Sun helium, the gas found in uranium ore is the product of a nuclear reaction, but this time the reaction is radioactive decay rather than fusion.
Although helium is the second most abundant element in the universe (hydrogen is the first), it is among the rarer elements on Earth. This is simply due to its low weight. As it doesn't form molecules and its atoms are as light as a hydrogen molecule and a lot lighter than all other possible molecular species, it is most likely to rise to the upper layers of the atmosphere when it's free to do so, and eventually escape to space. If you've ever let go of a helium filled balloon, you've contributed to our planet's continuous loss of this rare element. What little we have of it is in fact only passing by. The amounts produced by radioactive decay are balanced by the amounts lost to outer space.
While helium is naturally found in places that are either very hot or contaminated by radioactivity, it has ironically found its most important applications in the technologies involved with very low temperatures. Helium is the only element that cannot be frozen at atmospheric pressure, and it only liquifies at four degrees above absolute zero (four Kelvin). At two Kelvin it converts to a very strange state called a superfluid. It will then flow through tiny capillaries with now friction and conduct heat better than any metal. Extremely powerful electrical magnets, such as those needed for magnetic resonance imaging and for nuclear magnetic resonance spectroscopy commonly contain superconducting coils cooled by liquid helium.
((Footnotes Chapter 1:))
1) I am told that Freeman Dyson suggested that a much more advanced society than ours might wish to harness all that energy by taking Jupiter apart and using the material to build a sphere that encapsulates the Sun and catches all its radiation.
((Figures Chapter 1:))
Figure 1: Cross-section of the Sun according to current models. Distances from the centre to the outer limits of the inner zones are as follows (in km): core, 150,000; radiation zone, 500,000; convection zone, 690,000. The widths of the photosphere and chromosphere are estimated to be 500 and 8000 km, respectively.
Figure 2: Illustration of the energy balance of the Earth. The cube volumes are representative of the amounts of energy flow (in Terawatts): tidal energy (from the Moon) 2,7; humanity, 10,8; geological sources, 32; from the Sun, 173,000 (of which 58,000 are reflected); sunlight used by plants, 43.3.
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