I’d like to tell you a story. It’s a story of great destruction, but also of rebirth, and even features drunken debauchery. It’s thought to be a true story.
Once upon a time, there was a star. It was fairly unremarkable, forming on the edge of an average galaxy. Around it formed a handful of rocky planets, asteroids, comets and gas giants. The star has its own story, which I may tell later. This is not a story about the star. This is the story about an unlikely partnership that resulted in an equally most unlikely outcome. This is a story about the Moon.
Back when the Earth was new, it was a ball of molten burning rock. It had no satellites of its own; it was bored and alone.
As bad luck would have it, it turned out that the Earth was not actually alone. Another planet, called Theia, had veered into its path. Theia was smaller than the Earth, but was still about the size of Mars; this wasn’t some poxy little civilisation-ending asteroid, this was a bone fide proper planet, and it was on a collision course with early Earth.
The collision was a glancing blow, full on enough to blast Theia to smithereens and rearrange Earth’s face in no uncertain terms, but not so full on that the two planets annihilated each other to form an asteroid belt. The resultant spew of rock and planet innards went into orbit around the damaged Earth, and gradually coalesced into a ball of rock. The collision also knocked Earth off kilter, putting it at an angle of around 21-23 degrees; it’s still wobbling about between those values.
The Moon was very close to the Earth at this point, and the Earth was spinning like a top – a day lasted only around 5 hours. Both were still molten hot, and the huge red glowing Moon rising rapidly in the airless sky of Earth would have been remarkable. Eventually though, both Earth and Moon chilled out. They stopped burning, the Moon cooled right down as it was small enough that the heat caused by gravity wasn’t enough to keep its core burning, and the Earth formed a thin crust of cool surface over an interior still heated by the force of gravity. At this point the Earth had no atmosphere, no liquid water. Water had been brought to the planet by colliding comets, but all in all it wasn’t looking great for the ball of rock. However the collision with Theia had had a curious side-effect; the neat layers of metal and rock that made up the planet were blasted apart, and the inner layers of iron had been splattered out of the planet, onto the surface. Iron is incredibly reactive (think rust), and it reacted to form the early atmosphere of Earth. Methane and other gasses important to life were created, and the formation of air combined with the cooling Earth allowed comet water to condense. The oceans started to fill…
Meanwhile, asteroids and other bits of celestial rubbish were going on rampant joy rides around the solar system, crashing into anything in their path. They bombarded the Earth, however the Earth had a protector. For some reason the Moon had formed so that its rotation about its axis was the same as its orbit around the Earth – it only ever faced one side towards the Earth. It wasn’t until astronauts got to see the ‘dark’ side of the Moon (which isn’t necessarily dark – it’s only called that because we can’t see it from Earth) they realised it was completely pock marked with craters; every crater representing an asteroid that didn’t hit the Earth, stopping life in it’s tracks. That was rather nice of the Moon.
So things cooled and stabilised. The tilt caused by the collision with Theia was maintained by the Moon; like spinning around holding a weight, the action of the Moon orbit gyroscopically stabilises the Earth’s natural wobble. If the Moon moves further away (which it is, at a rate of about 3cm a year) the wobble will worsen. Mars doesn’t have a big Moon to stabilise its wobble, it only has two potato shaped tiny rocks orbiting it. Although Mars is currently about about 20-25 degrees like us, it has been up to 60 degrees tilted in the past. Go home, Mars, you’re drunk.
The lack of wobble is important. If Earth wobbled over to 90 degrees for example, for 3 months of the year the South Pole would point directly at the sun for 24 hours of the day, whilst the North Pole spent the time in darkness. The side facing the sun would cook for half a year, then freeze for the other half. Such extremes are not conducive to existing life (although there are things that can cope with hot and things that can cope with cold, there isn’t much on this planet that can cope with hot AND cold), and they also aren’t conducive to life starting. Evolution needs longer than a year to adapt to climate change. So – wobble bad, Moon good. Don’t worry though – the Moon will need to be a good several thousand miles further away before Earth starts to wobble, and at 3cm per year that’s going to take a while. The tilt has another effect of course – it gives us our seasons. Not required for life, but fun.
Are you wondering how an Earth with a 5 hour day and a stupidly close Moon became an Earth with a 24 hour day and a Moon at a respectable 0.25 million miles away? If not you should be. It’s fascinating. This brings us onto the gravity thing. Gravity is the attraction between two masses. It’s incredibly weak compared to other forces of attraction and repellingness – your hair will stand on end with static electricity, a repulsive force, easily combatting the effect of gravity on your hair. You can use a magnet to pick up a weight; the magnet easily overcomes the gravitational pull of an entire planet. However if you get two big things (say, oh, I don’t know, the Earth and the Moon) the forces start to mix things up. As the Moon orbits it attracts the ocean, tugging a tidal bulge up away from the Earth. The Earth is spinning fast, and the Moon is orbiting slower, so the bulge is being dragged backwards around the Earth as it spins. This friction acts as a brake on the Earth, slowing it down, so a day is getting longer and longer (though so slowly it’s been 24 hours for the entirety of human existence). However a second effect occurs; the tidal bulge is so big itself that it has its own gravity, and as it’s slightly in front of the Moon thanks to the Earth spinning forward underneath it, it pulls at the Moon, speeding it up. When orbiting things speed up, they get pulled outward, thus the moon is moving away. Picture time!
Yes, there is more to say about this picture. The distance the Moon is from the Earth results in a specific amount of tide. Any less, and we wouldn’t have tides. Any more, and at high tide every day England would disappear under a huge wave of water. This matters, and not just for those in England. The effect of tides on rock pools are thought to be responsible for life on Earth. Think primordial soup. With clams. The water rushes in, the water rushes out, the sun warms, the UV light causes chemicals to react and BOOM – amino acids. The building blocks of life, made in a pool. This is a controversial theory, but I like it, and it’s better than the one about the flying spaghetti monster. However, if tides were too high, you end up with the extremes issue again – rapidly changing extremes and life are not happy friends. So, if the theory is correct, the Moon was just at the right distance for life to occur when it did.
So there we are. We have a large Moon that promoted early life by its own creation bringing metals to the surface that in turn gave us water and air, by causing tides to allow life to begin, protecting early life by absorbing asteroid bombardment and continued protection by stabilising the tilt of our planet. The very existence of life on this planet is a massive series of coincidences, aided by the Moon. Oh, and the best bit? The Moon is 400 times smaller than the star we orbit, and it also happens to be 400 times closer to us than said star; any deviation from this and it wouldn’t work, and remember, the Moon is moving away from us all the time. This fact that this also occured just at the point we happened to evolve and look up; one of the greatest natural wonders – the solar eclipse – is a great cosmic coinscidence.
