The Science of Discworld

by Terry Pratchett

  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 iwice 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 I he 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.

  About a thousand meteoroids bigger than a football hit Earth's atmosphere each day, together with countless millions of smaller ones. And as time passes, we receive some big and some bigger, with the occasional dinosaur-killer. How often do we expect to see such a big one? About once every hundred million years.

  There is much more of this kind of junk in the Solar system than we used to think, and it rains down on our planet constantly. Every year, we sweep up about 80,000 tones (tonnes) of it. Nearly all of the debris falling on Earth is little bits, mostly somewhat dried-out icy dirt from the tails of comets. Debris of this kind follows the comet's orbit, marking it out like a gravel path. When the Earth's orbit takes it through this cometary junkheap, some of the gravel burns up in the atmosphere, and we see spectacular light shows: meteor showers. These arrive on particular dates each year as the Earth passes through that debris. For example the Leonids can be seen in November, and the Perseids in August.

  There is a bit of a mystery about the Geminid meteor showers, which come in December, though. They seem to be associated with a (defunct) comet whose perihelion, closest approach to the Sun, is out by Pluto's orbit. And that brings us to another source of impactors: the Kuiper Belt, which is the bit of the Oort Cloud not very far outside Pluto. In fact, Pluto and its satellite Charon are now thought not to be a 'real' planet-and-moon, but only the biggest lump in the Kuiper Belt. These lumps travel in genuine quasi-elliptical orbits, and may be the source of some of the regular comets with shorter orbital periods — like Halley's comet, which returns every 76 years or so.

  As well as comets, the asteroids also send rocks our way. Jupiter's gravitational field is strong enough to disturb the asteroids, especially those in certain 'resonant' orbits, with periods that are a simple fraction of Jupiter's —one third or two fifths, say. Of the 8000 or so known asteroids, about one in twenty has an orbit that comes close to that of the Earth, or even crosses it. All of those that cross
are potential impactors. Asteroids whose orbits approach the Sun to within a distance of 1.3 times the radius of the Earth's orbit are said to be Earth-approaching asteroids, or Amors. The best known of these is Eros. Asteroids whose orbits overlap the Earth's are called Apollos. More than 400 Amors and Apollos are known. More worrying are Atens, which are Amors too small to be detected easily but still big enough to cause tremendous damage. Most of these probably started out in the main asteroid belt, but were disturbed by Jupiter so that they crossed the orbit of Mars, and were then further disturbed by Mars.

  This leaves us with two opposite ways to view Jupiter — perhaps complementary ways. This largest planet has been proposed as the saviour of Earth's life forms on countless occasions, its enormous gravity picking up nearly all of the in-falling rocks and icy lumps — as it did comet Shoemaker-Levy 9 in 1994. But it has also been shown to shake the Asteroid Belt about, possibly causing that dinosaur-killer (if it was actually an asteroid) to hit the Earth.

  The message is that a basketball left on a billiard table has a fairly interesting life. Velikovsky, who proposed a wild theory in the fifties that made the Solar System look very much like a snooker table in Biblical times, with Mars moving substantially closer to the Earth and Venus turning from a comet into a planet, wasn't very wrong in principle.

  Only in every single detail.

  Here's something else to worry about. Out there in the Milky Way galaxy there's a lot of stars. Occasionally one goes nova, rarely supernova, as they explode. There is a sphere of very active radiation leaving such stars. If one went off in our vicinity, up to twenty light years away, say, all higher forms on Earth would be sterilised, at least. The bacteria, especially those deep in the Earth's crust, would survive. They probably wouldn't notice a thing. Wait a few billion years ... higher lifeforms could exist in abundance once more.

  More worrying still are gamma-ray bursters. Gamma-rays are very short wavelength electromagnetic radiation, such as x-rays. When astronomers managed to develop instruments that could detect such radiation, and put them into satellites, they discovered that two or three times per day the Earth is illuminated by an intense burst of gamma-rays coming from somewhere out in space. These gamma ray bursts seem to be extremely energetic: there is good evidence that the source of one of them was 12 billion light years away. Even a supernova would not be visible from that distance, so gamma-ray bursts have to be caused by something really serious.

  What? That's a mystery — perhaps the biggest mystery in today's astronomy. The best bet is a collision between neutron stars. Imagine a binary star — two stars, orbiting their common centre of mass. Suppose they are both neutron stars. As time passes, they lose energy and fall in towards each other. If you wait long enough, they will come so close together that they collide. This, by the way, is likely to be a very messy business, not at all as simple as two tennis balls sticking together and rounding off. They probably break up and reform. So far, all the gamma-ray bursters we've seen are a long, long distance away. But one could light up anywhere. If a pair of neutron stars collapsed on to each other within a hundred light years of Earth, life might survive in the deep seas and the deepest rocks, but the rest of our planet would be dead.

  And we wouldn't even see it coming.

  Asteroids and comets give you a bit of notice. We have the capability, given a year's run-up time, to tackle small Earth-crossing asteroids now. We can see them coming and plot their arrival. But gamma-rays are electromagnetic: they travel at the speed of light. They could be on their way now: we couldn't know. As soon as we did know, we and our technology would be dead.

  Even our own Sun is not trustworthy. The nuclear reactions that make stars burn also make them change, as elements are created or used up, or just reach some critical level that triggers new kinds of reactions. Most stars follow the same series of changes, called the main sequence.

  When the Sun first arrives on the main sequence, it is just like our Sun, with a surface temperature of about 6,000 degrees Kelvin, a light output of about 400 septillion watts, and a composition of 73% hydrogen, 25% helium, and 2% everything else. It stays on the main sequence for ten billion years, until nearly all of its hydrogen has been fused into helium. At that point, its core starts to contract, and becomes degenerate — consisting of closely packed neutrons. Outside the core there remains a shell of hydrogen, which continues to undergo nuclear reactions, which cause the outer layers of the star to expand and cool. The star becomes a red giant, between 10 and 100 times as big.

  The radius of the Sun now is roughly 450,000 miles (700,000 km). At this stage its surface will probably be somewhere between the orbits of Mercury and Venus, and the Earth will already be in serious trouble. But there is more to come. As the core heats, it ignites a nuclear reaction that turns helium into carbon — the very reaction that allegedly is responsible for the existence of carbon-based lifeforms like us. This 'helium flash' happens very quickly, on astronomical timescales, and it destroys the degeneracy of the core. Now the core can once more sustain nuclear reactions, but now it burns helium. The outer layers of the star shrink, and become hotter.

  When the helium in the core is used up, the star again burns its nuclear material in two shells: an inner one that burns helium to make carbon, and an outer one that converts hydrogen to helium. The outer layers expand again, and the star becomes a red giant for the second time. Now the outer layers start to blow away, exposing the hot core. The star loses layer upon layer of its material, and shrinks. Finally, the outer layers are all gone, and the core once more becomes degenerate. The star has become a white dwarf.

  Our Sun has about 5.7 billion years left on the main sequence; then: kerblooie! Red Giant, and Earth becomes a cinder or even gets gobbled up completely. But don't lose any sleep over it. The typical lifetime of a species is 5 million years. We'll be long gone.

  Planets are not comfortable. Even when life has made its own bed (nice oxygenated atmosphere with an ozone layer to keep out the nasty ultra-violet, nice ooze in the bottom of the oceans, nice long relaxation times for the thermal atmospheric oscillators), there are still plenty of things the Universe can throw at a planet that can still make it a bit fragile. If not kill it altogether.

  Which brings us back to our original question. Is life fragile, and have we been extraordinarily lucky? Or is it robust, and therefore common? Is life so adaptable that it can handle virtually anything that the universe sends its way?

  Until we can explore other worlds and see what kinds of life, if any, are present, anything we say here has to be speculative. The big difficulty is an 'anthropic principle' point. Suppose that life is incredibly rare, and that on most worlds it never really gets going, or doesn't last long, because of all the disasters lying in wait. Nonetheless, there's a lot of galaxies out there, each having billions or even trillions of stars. Even if the chances of survival are very, very small, occasionally one planet will get lucky. Some proportion of planets must get lucky, that's how probability works.

  Because life on this world has survived, we are therefore one of the lucky ones. It then becomes completely irrelevant how small our chances were. We are not representative. The probability that we survived is certainty, because we did. So we cannot reason, from our existence, that the chance of survival has to be fairly large. Whether it is large or small, we are here. So this is a case where the anthropist can legitimately frighten us. Perhaps all planets do get life and, if they're allowed enough time; even extelligent life on a few. But we really could be the only one who's survived to ask the question.

  On the other hand ... The very diversity of nasty things that the universe has up its sleeve argues for the adaptability and versatility of life. Life on Earth does not look like a bunch of lucky survivors. It looks like a bunch of tough guys who have overcome every obstacle put in their way. Sure, they took casualties, sometimes severe, but as long as a few survive the battle, pretty soon the planet is covered with life again, because life reproduces — fas
t. Whatever the disaster, it bounces back.

  So far, anyway.

  THIRTY-THREE

  THE FUTURE IS NEWT

  HEX WAS THINKING HARD AGAIN. Running the little universe was taking much less time than it had expected. It more or less ran itself now, in fact. The gravity operated without much attention, rainclouds formed with no major interference and rained every day. Balls went around one another.

  HEX didn't think it was a shame about the crabs going. HEX hadn't thought it was marvellous that the crabs had turned up. HEX thought about the crabs as something that had happened. But it had been interesting to eavesdrop on Crabbity — the way the crabs named themselves, thought about the universe (in terms of crabs), had legends of the Great Crab clearly visible in the Moon, passed on in curious marks the thoughts of great crabs, and wrote down poetry about the nobility and frailty of crab life, being totally accurate, as it turned out, on this last point.

  HEX wondered: if you have life, then intelligence will arise somewhere. If you have intelligence, then extelligence will arise somewhere. If it doesn't, intelligence hasn't got much to be intelligent about. It was the difference between one little oceanic crustacean and an entire wall of chalk.

  The machine also wondered if it should pass on these insights to the wizards, especially since they actually lived in one of the world's more interesting outcrops of extelligence. But HEX knew that its creators were infinitely cleverer than it was. And great masters of disguise, obviously.