Windswept_ The Story of Wind and Weather - Marq de Villiers [13]
These were a small boy's ponderings and soon faded. But behind Colin's simple question, I know now, was something much more profound. That we are indeed living at the bottom of a towering, restless sea of air goes some way to explaining the nature and persistence (but also the fragility) of life, the biosphere that inhabits the troposphere, here in the unsettlingly narrow layer between the bleakness of outer space and the unrelenting pressure of the boiling rock deep beneath the mantle.
We experience little of this directly, except as wind and weather. But where does wind begin? With the pressure differentials between highs and lows? With the solar energy that causes those differentials? With the nuclear fires in the sun that cause the solar energy that causes the pressure differentials? Or merely with that substance that was so invisible—and then later so mysterious—to our ancestors, the thing called "air"?
Wind, after all, is just air in motion.
Was there ever a time when there was no air? Very likely not. It may have been poisonous to us—but there was something . . . What exactly it was is still subject to speculation, which is the scientific word for guesswork. However, it is probably true that Earth has had three successive atmospheres with varying degrees of stability.
The most common assumption is that Earth itself is around 4.6 to 5 billion years old, formed by gravity from cosmic junk, clouds of ionized particles around the sun, and debris left over from the somewhere-sometime explosion called the big bang. This cosmic tip-heap coalesced to form a protoplanet, which grew by the gravitational attraction of even more junk, what the cosmologists call particulates. This was the so-called Hadean Eon: a sort-of Earth existed, but there were no continents, and no oceans—and most definitely no life. Just boiling clouds of gas.
At first, this not-quite planet was almost certainly too hot to retain much of the primitive atmosphere it was born with. Based on what we know of free gases in the universe, this first atmosphere would probably have consisted of helium and hydrogen. Until recently, the scientific consensus was that both these gases would have boiled off into space, to be replaced as the planet cooled with the products of volcanic outgassing—water vapor, carbon dioxide, carbon monoxide, sulfur dioxide, nitrogen, ammonia, methane, hydrogen sulfite, and chlorine. A new study, however, has found evidence that hydrogen persisted in the atmosphere, escaping into space much more slowly than earlier thought. It's possible this secondary atmosphere would have contained almost 30 percent hydrogen. There would have been no oxygen at all.1
The massive amounts of water vapor expelled from the condensing Earth would have formed a dense cloud layer, which then precipitated out as pure water.
Which raises the questions: Where did the water vapor in the atmosphere and in the condensing Earth come from in the first place?
What was it in the volcanism that caused our first weather to produce HO? Very likely, it was already present in all that cosmic junk—comets are sometimes little more than frozen lakes of water— and it came from space, an alien and infinitely curious little molecule. For water is curious, much more curious than it might at first appear, and is actually little understood. Why is it, for instance, that water is the only substance whose solid form is less dense than its liquid one, a phenomenon that has profound implications for aquatic life? "As a liquid, water has special thermal features that minimize temperature fluctuations. First among these features is its high specific heat—that is, a relatively large amount of heat is required to