2012: Advent Atmosphere: In The Beginning

by on December 1, 2012

I dithered for ages about what to do for this Advent series. I considered teaching you about carbonyl compounds and their reactions, but really, though that’s useful chemistry, it’s not very exciting to look at in depth unless you actually intend to use the knowledge. As topics for an advent calendar go, it’s a bit technical.
So I changed my mind about that, and decided to go with something easier to tell interesting stories about: the history of the atmosphere.
This may sound deathly dull, but bear with me. It’s not all about complex molecular cycles, though we might look at those too. It’s mostly a series of stories and the evidence behind them: How The Earth Got Its Air, Why It Rains, The Day The Ice Melted, and so on.

So, let’s start at the very beginning. Once upon a time, the Earth was part of a massive gas-cloud. Somehow (nobody is quite sure of the details) that gas cloud condensed into a sun, but left a field of gas and debris still around itself, which condensed into planets.

How big a planet is depends mainly on how much starting material it has. The bigger you are, the bigger you can get, because of gravity. It feeds back on itself. The rocky planets – Mercury, Venus, Earth, and Mars – are too small to retain really light elements, so they stayed small.

The rocky planets had another disadvantage: temperature. Back when the Sun was first forming, the whole gas cloud was much, much hotter than it is now, and the further in you went the hotter it was. Where we are it was about 1000 Kelvin. At 1000 Kelvin, it’s too hot for anything but rock to be solid. Things like methane and water were superheated vapours, far too energetic and fast-moving for anything as small as the Earth to catch, so rocks were all we got. Further out, outside what’s now the asteroid belt, it was cooler. Cold enough for water to freeze, into solid lumps of ice that, just like rock, collected together under the force of gravity. So there was more solid matter to collect, and the planets got bigger. Jupiter is as large as it is because it was sitting in the first orbit where ice could exist.

The outer planets got big enough to capture hydrogen and helium without those very light gases flying back off into space, and then they got huge. Most of the mass of the gas giants is made up of elements that Earth is just too small to hold on to. When you reach ten times the mass of Earth, your escape velocity is high enough that you can capture and retain hydrogen, and then everything in your part of space ends up a part of you.

So the basic rules about what elements make a planet up are set by its gravity, which in turn is dependent on its total mass, and the bigger you are the bigger you can get, until the gas cloud that the planets formed from has all been swallowed up into planets and everyone stops growing. This concentration of mass into the planets is still going on at a very slow rate – every time an asteroid hits a planet, it gets bigger by one more lump of rock.

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