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Πέμπτη, 22 Νοεμβρίου 2012

How the Earth was born


In a previous post I wrote about how we discovered the age of the Earth, and I mentioned that our planet formed at the same time as the other rocky bodies in our Solar System.
I didn’t say HOW this happened though. So now I will.
Happy now?

It all begins with a cloud of dust and gas out in space that astronomers call a molecular cloud. Space is generally pretty empty. There’s the occasional star, planet or comet, but the vast voids between these objects have little in them, perhaps some molecules of hydrogen and helium gas, and maybe some “space dust”, which is mostly bits of silica, carbon, iron and some other elements (basically very small bits of rock), but mostly space is filled with empty, erm, space. In some regions inside galaxies dust and gas accumulates though, these are the molecular clouds I just mentioned.
The Horsehead Nebula is actually part of a molecular cloud. And you thought they sounded a bit boring too. Shame on you
In our Universe, everything that has mass attracts everything else that has mass, this is gravity. So all these molecules of gas and dust within the molecular clouds are gravitationally attracting the other molecules of gas and dust in the cloud. The force of this gravitational attraction is pretty weak though, as molecules have comparatively low mass and gravity weakens quickly over distance.
However, if the cloud can become large and dense enough a limit is breached and gravitational attraction begins to kick-in, pulling the molecules towards each other. As the molecules move closer to each other the force of gravity increases, and a runaway effect begins, causing the molecular cloud to start collapsing, and as it collapses it begins to spin.
What causes the cloud to reach this required density is not known. It’s thought that the dust and gas in the cloud could be compressed by passing gravity waves (a pretty cool phenomena, but one I’ll save for a later post), but no ones ever actually seen a gravity wave, so we can’t be sure.
The collapsing molecular cloud begins to spin faster and faster as it gets smaller. This happens as the cloud has momentum, and if a body with a fixed momentum becomes smaller, it spins faster, a nifty trick called the “Conservation of Angular Momentum”. You can perform a quick experiment yourself to test this. Stand up and put your arms out and begin to spin. If you continue spinning and then pull your arms in towards your body you should find that you’ll spin a little bit faster. Then vomit.
A bit like this. No vomiting though. I promise
As the molecular cloud continues to spin faster it will first form a spherical shape but then will start to flatten into a disc, as if you rotate a sphere at a uniform speed its equator will spin faster than its poles.
You can also try this at home too. Take a ball, like an orange or a tennis ball or something, and draw one dot at its equator and one near the pole. If you spin the orange so that it takes five seconds to turn full circle (with the poles facing up and down) then both dots will take five seconds to rotate, but the dot near the pole will complete a much smaller lap in terms of distance covered than the one at the orange’s equator. Thus the equator dot will have covered more distance in the same time and therefore will have moved faster. This is why a spherical piece of pizza dough becomes flat when spun by a pizza chef, the centre of the sphere spins faster and moves outwards, compressing into a flat pizza base. (I actually used to be a pizza chef for a bit, I used a spinning machine though, so I was a cheap fraud.)
At the core of the molecular cloud the contraction will be the most intense, as the core will be the densest region and will thus experience the greatest gravitational attraction. As the core contracts it becomes denser still, experiences stronger gravitational attraction, contracts some more, becomes denser, and so on and so on, I think we can all see where this is heading.
Eventually the core of the cloud will become so dense that another critical point is breached, the point at which hydrogen atoms in the cloud begin to fuse together to create larger atoms in a process called nuclear fusion. Nuclear fusion releases energy in the form of light and is the process that powers our Sun. Thus in the heart of the collapsing molecular cloud a star is born. This is how our Sun came into existence around 4.56 billion years ago.
An artist’s rendition of a protostar surrounded by its circumstellar disk of gas and dust. Yeah I know, its pretty damn cool looking
Around 99% of the mass of the molecular cloud will have ended up in the Sun, whilst the remaining 1% of the gas and dust will have continued to spin as a circumstellar disc around our young star. Over time the dust molecules in this cloud will have begun to collide with each other as they bounced around chaotically within the circumstellar disk. In most collisions the molecules will have bounced-off each other, but sometimes they will have stuck together, and over the course of hundreds of thousands of years they will have begun to form larger and larger clumps of rock. I think a similar process causes the dust under your bed to coalesce into larger and larger clumps (dust bunnies!)
Over time some of these clumps of rock will become pretty big, and after they became larger than roughly one kilometre across another trigger point will have been breached as their gravitational attraction became strong enough to begin attracting other larger clumps of rock. In this fashion larger and larger pieces of rock begin to smash into each other and are remoulded to form planetesimals, the first step in the creation of planets.
Planetesimals drifting around a young star. Purdy.
Over the next tens of thousands of years these planetesimals will have continued to grow larger and larger by the process of gravitational focussing, with the largest planetesimals greedily cannibalising the smaller ones and growing the fastest.
Eventually the accretion of these planetesimals will have led to the formation of a small number of planetary embryos of around 1000 km in diameter, and further collisions over the next 100 to 300 thousand years will have caused these embryos to accrete into an even small number of planets around the size of the Moon and Mars. Collisions will still have occurred between these bodies, but as there were so few of them at this point the collisions would have been far less frequent, and it may have taken another hundred million years or so for the inner rocky planets of our Solar System to form, eventually leaving us with Mercury, Venus, the Earth and Mars.
A fifth rocky planet likely tried to form between the orbits of Mars and Jupiter, but Jupiter’s strong gravitational attraction will have ensured that the rocky bodies in these regions will have collided with too much force to properly coalesce, leaving a belt of asteroids rather than a fifth planet.
Nearer the centre of the disc the temperature will have been hotter than the outer regions. Thus substances with a high boiling temperature will have solidified nearer to the Sun, and substances with a low boiling temperature will have solidified further out. The inner planets and asteroid belt are therefore mostly formed of iron, nickel and silicate rocks, but further out ices of different types could also solidify, such as water, ammonia and methane ice. Planets like Jupiter, Saturn, Uranus and Neptune therefore formed initially from mixtures of metal, rock and ice.
At some point in your life someone probably confidently told you that you could drive a bus straight through one of the giant planets as they’re entirely made of gas. This is pure madness. As at the heart of each of these planets is one of these central kernels of metal, rock and ice.
The outer regions of the circumstellar disk also contained much more hydrogen and helium gas, that these icy and rocky kernels could hoover-up in vast quantities due to the power of gravity, creating the thick covering of hydrogen and helium we see on the gas giants today, hence the name.
That’s the inside of Jupiter you’re looking at. You can see the inner kernel of metal, rock and ice, then a layer of liquid hydrogen & helium (you do not want to drive a bus through that), and then an outer layer of gaseous hydrogen & helium, you could probably get a bus through that, if you didn’t mind dying
Back to the Earth.
As the Earth first formed it will originally have been a ball of molten metal and magma. The denser elements of this liquid inferno, such as iron and nickel, will have begun to solidify first and will have sunk towards the core of the Earth. As the Earth solidified further it will have differentiated by this process into a solid metallic inner-core (although it was extremely hot in this core the pressure was sufficient to cause the iron and nickel to solidify), a liquid metal outer-core where the pressure wasn’t quite so intense, surrounded by a rocky outer mantle.
The early Earth will have been a hellish place to visit, and its first half a billion years or so are referred to as the Hadean (Hell like) Eon by geologists. The intense heat inside the Earth will have caused large regions of the rocky mantle to melt and burst onto the Earth’s surface as massive volcanic eruptions. As the mantle melted, the magma it produced will have had a slightly different composition to the mantle rock, as not all of the minerals in the mantle will have melted, and as the magma erupted on the surface and cooled a differentiated crust will have began to form around the Earth. Thus the structure of the Earth we know today was formed, with a rocky crust and mantle, and a metalic inner and outer core.
The Solar System will still have been full of large chunks of rock in the Hadean, many of which will have rained down on Earth’s surface as meteorite impacts. But early in the Earth’s life, around 4.533 billion years ago, in an act of supreme planetary vandalism she was struck by a much larger object, a proto-planet the size of Mars called Theia. A bit like your house colliding with a slightly smaller house, at the speed of a bullet.
Or, a bit like this. Forget the whole house thing.
Such a monumentally enormous collision will have released a vast amount of energy, fracturing and melting large chunks of the Earth and probably completely destroying Theia. As the Earth recovered, the debris from this collision will have been gravitationally bound to the Earth, initially forming a ring around our planet similar to that of Saturn’s. But over time the chunks in the ring will have began to coalesce, accreting into a separate planetary body that orbits our planet today, thus the Moon was born. So the Theia collision was actually a blessing, if an initially very destructive one.
So there you go, the Earth, our very own planet is basically made out of a bunch of dust that stuck together, melted, cooled again, and then got hit by another planet to provide us with the Moon we see in the sky above us today. As Professor Brian Cox would say, brilliant.
 http://astrobioloblog.wordpress.com/

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