The Science of Discworld Revised Edition

by Terry Pratchett

  It contained crabs.

  The Senior Wrangler had to admit that crabs looked a good contender for Highest Lifeform status. HEX had located some on the other side of the world that were moving along very well indeed, with small underwater cities guarded by carefully transplanted sea-anemones and what appeared to be shellfish farms. They had even invented a primitive form of warfare and had built statues, of sand and spit, apparently to famous crabs who had fallen in the struggle.

  The wizards went and had another look fifty thousand years later, after coffee. To the Dean’s glee, population pressure had forced the crabs on to the land as well. The architecture hadn’t improved, but there were now seaweed farms in the lagoons, and some apparently more stupid crabs had been enslaved for transport purposes and use in inter-clan campaigns. Several large rafts with crudely woven sails were moored in one lagoon, and swarming with crabs. It seemed that crabkind was planning a Great Leap Sideways.

  ‘Not quite there yet,’ said Ridcully. ‘But definitely very promising, Dean.’

  ‘You see, water’s too easy,’ said the Dean. ‘Your food floats by, there’s not much in the way of weather, there’s nothing to kick against … mark my words, the land is the place for building a bit of backbone …’

  There was a clatter from HEX, and the field of vision of the omniscope was pulled back rapidly until the world was just a marble floating in space.

  ‘Oh dear,’ said the Archchancellor, pointing to a trail of gas, ‘Incoming.’

  The wizards watched gloomily as a large part of one hemisphere became a cauldron of steam and fire.

  ‘Is this going to happen every time?’ said the Dean, as the smoke died away and spread out across the seas.

  ‘I blame the over-large sun and all those planets,’ said Ridcully. ‘And you fellows should have cleared out the snowballs. Sooner or later, they fall in.’

  ‘It’d just be nice for a species to make a go of things for five minutes without being frozen solid or broiled,’ said the Senior Wrangler.

  ‘That’s life,’ said Ridcully.

  ‘But not for long,’ said the Senior Wrangler.

  There was a whimper from behind them.

  Rincewind hung in the air, the outline of the virtually-there suit shimmering around him.

  ‘What’s up with him?’ said Ridcully.

  ‘Er … I asked him to investigate the crab civilization, sir.’

  ‘The one the comet just landed on?’

  ‘Yes, sir. A billion tons of rock have just evaporated around him, sir.’

  ‘It couldn’t have hurt him, though, could it?’

  ‘Probably made him jump, sir.’

  THIRTY-TWO

  DON’T LOOK UP

  THE WIZARDS HAVE been convinced all along that a planet is not really a good place to put living creatures. A nice, flat disc, with an attendant turtle who can deal with any inbound rocks before they can wreak havoc, makes much more sense.

  It looks increasingly as if they’re right. The more we learn about the history of our planet, and the greater universe in which it resides, the more we have to admit that the wizards have a point. Not about the shape of our world, but about how dangerously exposed it is without a turtle. The universe is riddled with flying rocks and awash with radiation; most of it is either close to absolute zero or hot enough to make a hydrogen bomb seem like a nice, comfortable bonfire. Yet, somehow, life managed to gain a hold on at least one planet, and to retain that hold for four billion years – despite everything that the universe threw at it. (Often literally.) And despite every kind of nasty that the planet itself managed to concoct.

  There are two ways to interpret this.

  One is that life is incredibly fragile, and that Earth is one of the few places where the conditions necessary for life managed to hold together long enough for life to develop, diversify, and thrive. At any moment some disaster could undo all that good work and wipe the face of the planet clean of living creatures. The crab civilisation is fictitious, of course, but it’s in our story to make two important points. First, that there has been plenty of time for lifeforms at least as intelligent as us to evolve on Earth; second, that if they had done, they could easily have left no trace of their existence. Oh, and third … that there are plenty of ways in which they could have come to a sticky end. So we’ve been incredibly lucky to avoid going the way of the crab civilisation. On millions of other apparently suitable worlds, life was not so lucky; it either never got started, or something wiped it out. Life is a rarity; Earth may be the only place in the entire universe where that fragile miracle happened.

  The other is that life is incredibly robust, and that the conditions on Earth are sufficient for life to arise, but by no means necessary. Just because things worked out in a particular way here, it would be a mistake to conclude that the same events must happen everywhere else. An important implication of evolution is that life automatically adapts itself to whichever environment happens to be available. Boiling water at the bottom of the ocean? Just what extremophile bacteria need. Two miles down in the rocks? Super – it’s nice and warm down there, and there’s plenty sulphur and iron to provide energy. Thank providence there’s none of that poisonous oxygen; terrible stuff, violently reactive, immensely destructive. Nothing could survive in an oxygen atmosphere …

  Both points of view have their advocates, and both have a certain amount going for them. Until we get to other worlds and find out what’s there, there will be plenty of room for disagreement and debate. And, perhaps, a synthesis. Already both viewpoints agree that however life arose here, Earth was no Garden of Eden. Our planet is by no means the ideal habitat for life. In order for living creatures to survive, evolution has had to solve a lot of difficult problems, and adapt to hostile conditions.

  You may not realise just how hostile. But think of the common disasters: fires, hurricanes, tornadoes, earthquakes, volcanoes, tidal waves, floods, droughts … too much rain and we’re up to our necks in water; too little and our crops won’t grow and we starve.

  But those are feeble compared to the big disasters.

  We tend to think of the history of Life on Earth as the smooth growth of a single great Evolutionary Tree. But that image is getting very long in the tooth. The history of life is more like a jungle than a tree, and most of the plants in the jungle were strangled, squashed, or suffocated before they took even the first step on the road to maturity. And however that jungle grew, there was nothing smooth about it.

  True, there was a very long time when there were only ‘blobs’ in the seas, and we might think of that period as a fairly featureless trunk of the Tree. As far as the blobs were concerned, life probably was pretty uneventful – but only because they didn’t notice what was happening to the planet. They were largely unaffected by a whole succession of events that would have been cosmic catastrophes for later, more complex life.

  There were certainly a few pretty big impacts at the beginning of life on the planet that didn’t put them out of business, such as it was. And Snowball Earth – if in fact it ever happened – can’t have been easy. But despite all these obstacles, or even because of them, life slowly changed, evolved and diversified as the eukaryotes learned to live in an oxygen atmosphere.

  That should have been a disaster. The very composition of the atmosphere changed, and all the biochemical tricks that had evolved to suit the available range of gases became obsolete. Worse, the gas polluting the air was oxygen, an appallingly reactive substance. Think of what would happen today if the atmosphere started to be taken over by fluorine. Some of the nastiest, most explosive substances are fluorine compounds. But oxygen is just as bad, if not worse; think of fires, think of rust, think of decay.

  The eukaryote cell triumphed over oxygen, and subverted it. Oxygen’s negative characteristics were turned into positive ones. So effective was this evolutionary revolution that the deadly, poisonous pollutant became essential for (most) life. Deprive a human, a dog, or a fish of its oxygen,
and it dies very quickly. Water, food … those it can do without, for a time. But oxygen? You’ll survive for a few minutes at most, maybe half an hour if you’re a whale.

  The oxygen trick was so good that it took over. Eukaryote life radiated – diversified rapidly – in the seas, inventing entire new kinds of ecologies. With that diversification as a springboard, life came out on land. The advantage of moving to the land was that it opened up an entire range of new habitats, new ways of making a living. So many new kinds of living organism could thrive. One disadvantage, though, was that living on land made life much more vulnerable to astronomical insults. Living on land produced many more complicated kinds of plants and animals, able to protect themselves against small local changes, like hot sunshine, or snow. But, ironically, that very complication made them much more vulnerable to big problems – like stones falling from the sky.

  We all know about the meteorite that killed the dinosaurs … and that fits. Dinosaurs were wonderfully effective as long as the environment remained suitable, but they were not at all well-adapted to the sudden changes that the impact created. But bacteria hardly noticed. If anything, it was a good time for them: they got a lot of extra food for a few hundred years, as the corpses decayed, and then went back to the old boring routine.

  We’ll say a bit more about the 200 million year reign of the dinosaurs and their friends soon, and indeed about what killed them off. But we need to give you some context first. Simple forms of life can put up with a lot, and did. And they changed the planet, or at least its outer skin, by putting in feedback loops that made it less liable to change.

  They started Gaia. This is the name that James Lovelock gave in 1982 to the concept of the Earth as a complex living system – metaphorically, an organism in its own right. The idea has been romanticised into the Earth being a kind of Earth-mother, but what do you expect when you attach the name of a goddess to your new scientific concept? Stripped of the romantic frills, the point is that our planet acts as a single system, and it has evolved mechanisms that keep it functioning effectively. This development is a consequence of innumerable subsystems – organisms, ecologies – evolving mechanisms that keep them functioning effectively. If every member of a team gets better at playing their role within the team, then the team as a whole improves too.

  Complexity is a double-edged sword. More complex forms of life find that the ordinary problems of living on a planet are more and more under control … except for those confounded problems from outside, like meteorites, which can be disastrous.

  The Moon, Mercury, Mars, and various satellites are covered in circular craters, some large, some small. Nearly all of those craters, we now know, result from the impact of a big lump of rock or ice or a bit of both. A few are volcanic. Not so long ago most were thought to be caused by volcanoes, but that turned out to be wrong.

  Several planets, among them the Earth, do not show obvious signs of impacts. Is that because nothing hit them? No. An atmosphere helps: smaller bodies burn up before they hit the ground. It’s the closest to a protector-turtle that we get. But bigger rocks can still get through the defences. The main reason why some planets show no clear signs of impacts is because those planets have weather, like the Earth, which erodes the craters until they disappear, or episodes of massive vulcanism, like Venus, which resurfaces the planet, or are gas to begin with, like Jupiter and Saturn, and don’t show permanent marks.

  In Quebec there is a lake called Manicouagan. You can’t miss it on a map: look near 51°N, 68°W. It’s circular, and it’s big: 44 miles (71 km) in diameter. It is the weathered remains of a gigantic crater that formed 210 million years ago when a rock two or three miles (3–5km) across collided with the Earth. There is a central peak made from rock that melted in the heat that the impact generated and then solidified; more molten rock flowed across the floor of the crater and still can be found today. The lake fills a ring-shaped valley that glaciers carved out of soft rock that originally formed the crater walls, and was eroded away and collapsed.

  Also in Canada is the Sudbury impact structure, the largest on the planet. It is 190 miles (300 km) across and 1.85 billion years old, and the rock that made it was about 20 miles (30km) in diameter, and the energy released in the impact was equivalent to one quadrillion tons of TNT, or about ten million really big hydrogen bombs. In Vredefort, South Africa there is another impact structure of a similar size, formed 2.02 billion years ago. These may not remain the record-holders: an even bigger impact structure, about twice the size, is suspected to exist in the Amirante Basin of the Indian Ocean. Altogether, more than 150 impact structures – remnants of craters – have been found on the Earth’s land-masses, and many areas have not yet been thoroughly surveyed. More than half the Earth’s surface is ocean, and incoming rocks should hit pretty much at random, so the total number is probably closer to 500.

  These are all fairly ancient craters, but there is no good reason to believe that such impacts could not happen again. Big impacts are rarer than small ones, because big lumps of rock are rarer than small ones. Impacts the size of Sudbury or Vredefort should happen about once every billion years. (It should not be a surprise that when such impacts finally did arrive, about two billion years ago, two of them came along together.) Since nothing that size has happened for two billion years, it might seems that we are overdue for another one, but that kind of reasoning is a statistical fallacy. Rare, isolated events usually obey the so-called ‘Poisson distribution’ of probabilities, and one feature of this distribution is that it ‘has no memory’. At any time, whether two major impacts have just happened, or none have happened for ages, the average time to the next one is always the same – in this case, a billion years.

  It could be a few decades, mind you. But it couldn’t be tomorrow, or even next year, because we would have spotted such a body coming by now.

  The most recent well known impact on Earth was the Tunguska meteorite, which exploded 4 miles (6km) above Siberia in 1908, causing an explosion that felled trees for more than thirty miles (50 km) around. Craters, or other evidence of even more recent impacts have been found. A double-crater in the Saudi Arabian desert may be only a few centuries old.

  Where do all these rocks (and other junk, like ice) come from? Who or what is throwing them at us?

  First, some terminology. When you look into the night sky and see a ‘shooting star’, a glowing streak, that’s a meteor. It’s not a star, of course: it’s a lump of cosmic debris that has hit the Earth’s atmosphere at high speed and is burning up because of friction. The debris itself is called a meteoroid, and any part that remains when it has hit is called a meteorite. For convenience, though, we’ll generally use the word ‘meteorite’ for all of these. But we thought we ought to show you that we could have been pedantic if we’d wanted to.

  Some of these bodies are mostly rock, and some are mostly ice. And some are a bit of both. Wherever they come from, it’s not the Earth. At least, not directly. A few may have been splashed off the Earth by a previous impact and then come back down the next time we ran into them. However they got Up There, that’s clearly where they are coming from. What’s Up There? The rest of the universe. The closest bit is our own Solar System. So that has to be the most likely culprit. And there’s no question that it has a lot of ammunition.

  Earlier, we described the Solar System as being rather chaotic: nine planets and a few moons with some quite interesting real estate. We mentioned that there was quite a lot left over after these larger bodies were accounted for. There were relatively small lumps of actual rock in the asteroid belt, but nearly all the ‘loose change’, after the Sun and planets had been paid for out of the Solar nebula, was lumps of dirty ice.

  The biggest collection of these is the Oort Cloud, a vast, very thinly spread mass that lives outside the Solar System ‘proper’ – that is, further out than Pluto (or Neptune when Pluto gets inside the orbit of Neptune, which can happen). In 1950 Jan Hendrik Oort proposed that the source of most
of the comets we see from Earth must be some such cloud, and got it named after him. The main evidence we have for its existence is that comets with very long thin orbits, which are common, must have come from somewhere. The bodies in the Oort cloud range from pebble-sized up to lumps perhaps as big as Pluto.

  These comet materials are the usual source of the meteorites we pick up and put into museums, after most of their substance has burnt up in the atmosphere. We’re beginning to get an idea of how big the Oort Cloud could be. Its mass is about a tenth that of Jupiter, and it extends way outside Pluto’s orbit, perhaps as far as 3 light years – two thirds of the way to the nearest star. This spreads the material though a volume millions of times as great the volume inside Pluto’s orbit, our actual planetary Solar System. So the ‘cloud’ is so rarefied that if you went there, you probably wouldn’t see anything.

  The gravitational pull of the Sun is tiny at those distances, and the dirty ice lumps barely move along their orbital paths, which are probably close to circles. To the extent that the ice lumps have orbits, and don’t just drift slowly about, they take millions of years to go once round the Sun. But the universe doesn’t let them keep doing that without interference. Oort called his cloud ‘A garden, gently raked by stellar perturbations’. As nearer stars, and the pull of the whole galaxy, interact with the Sun’s pull, many of these lumps are pulled away from their normal paths.

  It turns out that the disturbances need not be as gentle as Oort supposed. About once every 35 million years a star passes through the Oort cloud, and havoc ensues. Since the 1970s another source of big disturbances has been recognised: Giant Molecular Clouds. These are huge accumulations of cold hydrogen, where stars and solar systems are born. Their masses can be a million times that of the Sun. They don’t have to come anywhere near us to shake ice lumps out of their sedate near-circular orbits in the Oort cloud.

  Such disturbances can cause lumps of ice to drift in towards the Solar System. At that point, they become comets. Some probably drift outwards too, but we’re not so concerned with those. And comets are the main (but not the only) source of cosmic junk in Earth’s backyard.