The Science of Discworld Revised Edition

announced.

EIGHTEEN
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.