Terry Pratchett - Science of Discworld

It must have absolutely bucketed down.

When the rain hit the hot rocks beneath, it promptly evaporated back into vapour, but as it did so it cooled the rocks. Heat and tem­perature are not the same. Heat is equivalent to energy: when you heat something, you input extra energy. Temperature is one of the ways in which that energy can be expressed: it is the vibration of molecules. The faster those vibrations are, the higher the tempera­ture. Ordinarily, the temperature of a substance goes Up if you heat it: all the extra heat is expressed as more vibration of the molecules. However, at transitions from solid to liquid, or liquid to vapour or gas, the extra heat goes into changing the state of the substance, not into making its temperature higher. So you can throw in a lot of heat and instead of the stuff getting hotter, it changes state, a so-called phase transition. Conversely, when a substance cools through a phase transition, it gives off a lot of heat. So the cooling water vapour put more heat back into the upper atmosphere, from which it could be radiated away into space and lost. When the hot rocks turned the water back into vapour, the rocks got a lot cooler very suddenly. In a geologically short space of time, the rocks had cooled below the boiling point of water, and now the falling rain no longer got turned back into vapour, at least, not much of it did.

It may well have rained for a million years. So it's not surprising that Rincewind noticed that it was a bit wet.

Thanks to gravity, water goes downhill, so all that rain accumu­lated in the lowest depressions in the Earth's irregular surface. Because the atmosphere had a lot of carbon dioxide in it, those early oceans contained a lot of dissolved carbon dioxide, making the water slightly acidic. There may have been hydrochloric and sul­phuric acids too. The acid ate away at the surface rocks, causing minerals to dissolve in the oceans; the sea began to get salty.

At first the amount of oxygen in the atmosphere increased slowly, because the effect of incoming sunlight isn't particularly dramatic. But now life got in on the act, bubbling off oxygen as a by­product of photosynthesis. The oxygen combined with any remaining hydrogen in the atmosphere, whether on its own or com­bined inside methane, to produce more water. This also fell as rain, and increased the amount of ocean, leading to more bacteria, more oxygen, and so it continued until the available hydrogen pretty much ran out.

Originally it used to be thought that the oceans just kept dis­solving the rocks of the continents, accumulating more and more minerals, getting saltier and saltier until the amount of salt reached its current value of about 3.5%. The evidence for this is the per­centage of salt in the blood of fishes and mammals, which is about 1%. In effect, it was believed that fish and mammal blood were 'fos­silized' ocean. Today we are still often told that we have ancient seas in our blood. This is probably wrong, but the argument is far from settled. It is true that our blood is salty, and so is the sea, but there are plenty of ways for biology to adjust salt content. That 1% may just be whatever level of salt makes best sense for the creature whose blood it is. Salt, more properly, the ions of sodium and chlorine into which it decomposes, have many biological uses: our nervous systems, for instance, wouldn't work without them. So while it is entirely believable that evolution took advantage of the existence of salt in the sea, it need not be stuck with the same proportion. On the other hand, there is good reason to think that cells first evolved as tiny free-floating organisms in the oceans, and those early cells weren't sophisticated enough to fight against a difference in salt concentration between their insides and their outsides, so they may well have settled on the same concentration because that was all they could initially manage, and having done so, they were rather stuck with it.

Can we decide by taking a more careful look at the oceans? Oceans have ways to lose salt as well as gaining it. Seas can dry out; the Dead Sea in Israel is a famous example. There are salt mines all over the place, relics of ancient dried-up seas. And just as living creatures, bacteria, took out carbon dioxide, turning it into oxy­gen and sugar, so they can take out other dissolved minerals too. Calcium, carbon and oxygen go into shells, for instance, which fall to the ocean floor when their owner dies. The clincher is ... time. The oceans are thought to have reached their current composition, and in particular their current degree of saltiness, about 2 to 1.5 bil­lion years ago. The evidence is the chemical composition of sedimentary rocks, rocks formed from deposits of shells and other hard parts of organisms, which seems not to have changed much in the interim. (Though in 1998 Paul Knauth presented evidence that the early ocean may have been more salty than it is now, with some­where between 1.5 to 2 times as much salt. His calculations indicate that salt could not have been deposited on the continents until about 2.5 billion years ago.) Simple calculations based on how much mate­rial dissolves in rivers and how fast rivers flow show that the entire salt content of the oceans can be supplied from dissolved continen­tal rocks in twelve million years, the twinkling of a geological eye. If salt had just built up steadily, the oceans would now be far more salt than water So the oceans are not simply sinks for dissolved min­erals, one-way streets into which minerals flow and get trapped. They are mineral-processing machines. The geological evidence of the similarity of ancient and modern sedimentary rocks suggests that the inflow and the outflow pretty much balance each other.

So do we have ancient seas in our blood? In a way. The propor­tions of magnesium, calcium, potassium, and sodium are exactly the same as they were in the ancient seas from which our blood may have evolved, but cells seem to prefer a salt concentration of 1%, not 3%.

'HE'S RIGHT ABOUT THE RAIN,' said the Senior Wrangler, who was at the omniscope. 'You've got clouds again. And there's lots of volcanoes.'

'I'mmoving him on further ... Oh. Now he says it's dark and cold and he's got a headache ...' 'Not very graphic, is it?' said the Dean. 'He says it's a splitting headache.' HEX wrote something.

'Oh,' said Ponder 'He's under water. I'm sorry about that, I'm afraid he's a little hard to position accurately. We're still not sure what size he should be. How's this?'

The trumpet rattled, 'He's still under water, but he says he can see the surface. I think that's as good as we're going to get. Just walk forward.'

As one wizard, they turned to watch the suit. It hung in the air, a few inches above the floor. As they watched, the figure inside made hesitant walking motions.

It was not a nice day.

It was still raining, although it had slackened off recently, with sporadic outbreaks during the early part of the millennium and scattered showers during the last couple of decades. Now ten thou­sand rivers were finding their way to the sea. The light was grey and gave the beach a flat, monochrome, and certainly very damp look.

Whole religions have been inspired by the sight of a figure emerging, miraculously, from the sea. It would be hard to guess at what strange cult might be inspired by the thing now trudging out of the waves, although avoidance of strong drink and certainly of seafood would probably be high on its list of 'don'ts'.

Rincewind looked around.

There was no sand underfoot. The water sucked at an expanse of rough lava. There was no seaweed, no seabirds, no little crabs -nothing potentially dangerous at all.

'There's not a lot going on,' he said. 'It's all rather dull.'