Science of Discworld

'Yes?'

'Can I resign?'

'No.'

'Do I have to understand anything you just told me?'

'No.'

'Are there any monsters where I'm being sent?'

'No.'

'Are you sure?' 'Yes.'

'Are you totally positive about that?'

'Yes.'

'I've just thought of another question,' said Rincewind.

'Fire away.'

'Are you really sure?'

'Yes!' snapped Ponder 'And even if there were any monsters, it wouldn't matter.'

'It'd matter to me.'

'No it wouldn't! I have explained! If some huge toothed beast came galloping towards you, it'd have no effect on you at all.'

'Another question?'

'Yes?'

'Is there a toilet in this suit?'

'No.'

'Because there will be if a huge toothed beast comes galloping towards me.'

'In that case, you just say the word and you can come back and use the privy down the hall,' said Ponder. 'Now, stop worrying, please. These gentlemen will help you, er, insert yourself into the thing, and we'll begin ...'

The Archchancellor wandered up as the reluctant professor was enveloped in the glittering, not-quite-there stuff.

'A thought occurs, Ponder,' he said.

'Yes, sir?'

'I suppose there's no chance that there is life anywhere in the Project?'

Ponder looked at him in frank astonishment.

'Absolutely not, sir! It can't happen. Simple matter is obeying a few rather odd rules. That's probably enough to get things ... spinning and exploding and so on, but there's no possibility that they could cause anything so complex as...'

'The Bursar, for example?'

'Not even the Bursar, sir.'

'He's not very complicated, though. If only we could find a parrot that was good at sums, we could pension the old chap off.'

'No, sir. There's nothing like the Bursar. Not even an ant or a blade of grass. You might as well try to tune a piano by throwing rocks at it. Life does not turn up out of nowhere, sir. Life is a lot more than just rocks moving in circles. The one thing we're not going to run into is monsters.'

Two minutes later Rincewind blinked and found, when he opened his eyes, that they were somewhere else. There was a rather grainy redness in front of them, and he felt rather warm.

'I don't think it's working,' he said.

'You should be seeing a landscape,' said Ponder, in his ear.

'It's all just red.'

There was the sound of distant whispering. Then the voice said, 'Sorry. The aim wasn't very good. Wait a moment and we'll soon have you out of that volcanic vent.'

In the HEM Ponder took the ear trumpet away from his ear. The other wizards heard it sizzling, as if a very angry insect was trapped therein.

'Curious language,' he said, in mild surprise, 'well, let's raise him somewhat and let time move on a little ...'

He put the trumpet to his ear and listened.

'He says it's pissing down,' he announced.

AIR AND WATER

IT'S CERTAINLY A SURPRISE that the rigid rules of physics permit anything as flexible as life, and the wizards can hardly be blamed for not anticipating the possibility that living creatures might come into being on the barren rocks of Roundworld. But Down Here is not as different from Up There as it seems. Before we can talk about life, though, we need to deal with a few more features of our home planet: atmosphere and oceans. Without them, life as we know it could not have arisen; without life as we know it, our oceans and atmosphere would be distinctly different. The story of the Earth's atmosphere is inextricably intertwined with that of its oceans. Indeed, the oceans can reasonably be viewed as just a rather damp, dense layer of the atmosphere. The oceans and the atmosphere evolved together, exerting strong influences on each other, and even today such an 'obviously' atmospheric phenomenon as weather turns out to be closely related to what happens in the oceans. One of the main recent breakthroughs in weather prediction has been to incorporate the oceans' ability to absorb, transport, and give off heat and moisture. To some extent, the same point can be made about the solid regions of the Earth, which also co-evolved with the air and the seas, and also interact with them. But the link between oceans and atmosphere is stronger.

The Earth and its atmosphere condensed together out of the primal gascloud that gave rise to the Sun and to the solar system. As a rough rule of thumb, the denser materials sank to the bottom of the condensing clump of matter that we now inhabit, and the lighter ones floated to the top. Of course there was, and still is, a lot more going on than that, so the Earth is not just a series of concentric shells of lighter and lighter matter, but the general distribution of solids, liquids, and gases makes sense if you think about it that way. And so, as the molten rocks of Earth began to cool and solidify, the nascent planet found itself already enveloped in a primordial atmosphere.

It was almost certainly very different from the atmosphere today, which is a mixture of gases, the main ones being the elements nitrogen, oxygen and the inert gas argon, and the compounds carbon dioxide and water (in the form of vapour). The primordial atmosphere also differed considerably from the gas cloud out of which it condensed, it wasn't just a representative sample of what was around. There are several reasons for this. One is that a solid planet and a gas cloud retain different gases. Another is that a solid planet can generate gases, by chemical or even nuclear reactions, or by other physical processes, which can escape from its interior into its atmosphere.

The early cloud was rich in hydrogen and helium, the lightest of elements. The speed with which a molecule moves becomes slower as the molecule gets heavier, a molecule with one hundred times the mass moves at about one-tenth the speed. Anything that moves faster than the Earth's escape velocity, about 7 miles per second (11 km/sec), can overcome the planet's gravity and disappear into space. Molecules in the atmosphere whose molecular weight, what you get by adding up the atomic weights of the component atoms -is less than about 10 should therefore disappear into the void. Hydrogen has molecular weight 2, helium 4, so neither of these otherwise abundant gases should be expected to hang around. The most abundant molecules in the primal gas cloud, with molecular weight greater than 10, are methane, ammonia, water, and neon. This is similar to what we find today on the gas giants Jupiter, Saturn, Uranus, and Neptune, except that they are more massive, so have a greater escape velocity, and can retain lighter gases such as hydrogen and helium as well. We can't be certain that the Earth of 4 billion years ago possessed a methane-ammonia atmosphere, because we don't know exactly how the primal gas cloud condensed, but it is clear that if the ancient Earth ever possessed such an atmosphere, it lost nearly all of it. Today there is little methane or ammonia, and what there is has a biological origin.

Shortly after the Earth was formed, the atmosphere contained very little oxygen. Around 2 billion years ago, the proportion of oxygen in the atmosphere increased to about 5%. The most likely cause of this change, though perhaps not the only one, was the evolution of photosynthesis. At some stage, probably around 2 billion years ago, bacteria in the oceans evolved the trick of using the energy of sunlight to turn water and carbon dioxide into sugar and oxygen. Plants use the same trick today, and they use the same molecules as one of the early bacteria did: chlorophyll. Animals proceed in pretty much the opposite direction: they power themselves by using oxygen to burn food, producing carbon dioxide instead of using it up. Those early photosynthesizing bacteria used the sugar for energy, and multiplied rapidly, but to them the oxygen was just a form of toxic waste, which bubbled up into the atmosphere. The oxygen level then stayed roughly constant until about 600 million years ago, when it underwent a rapid increase to the current level of 21%.