Credit: NASA

in Cornerstone

Where Did We Come From? Earth’s History Traced

Dating back to 4.6 billion years ago. We trace the history of Earth’s formation.

The solar system was formed alongside the Earth. Back when it was just beginning to form the solar system was a cloud of dust and gas known as a solar nebula. Gravity acted on this cloud causing it to clump in some areas. Fighting for the same material some clumps grew more than others. Eventually one clump got much larger than any others. This formed a gravitational center for the rest of the dust and gas cloud which begin to orbit around it. In binary or two star systems there are two of these much larger clumps and so the solar system forms around them both.

Solar Nebula Credit: NASA/JPL-Caltech

This fledgling sun exerted force outward through a process known as solar wind. This force would have pushed many of the lighter elements like hydrogen and helium away from the solar systems center. This explains why the inner planets are terrestrial and have a much higher composition of heavy elements. The lighter elements that were pushed far away from young sun began to clump and eventually form the gas giants characterizing our outer solar system.


The layers of Earth

More clumps formed around the sun, one of which was our young Earth. Earth was greedy and accumulated every element it could. The heavier ones sank into our solid inner core giving it its iron and nickel rich composition. Surrounding that is Earth’s liquid core. It’s usually called the liquid core, but liquid makes you think of it flowing like water. It doesn’t. It flows in the same way glass flows over time, which is very slowly. This flow is why very old windows are slightly thicker at the bottom than the top. Everything around us flows all the time, it just takes a lot of time to see much of it.

After the outer core these is the mantle. Which a thick layer of silicate rock. Silicates are minerals which combine oxygen and silicon into one compound. These are less dense elements than the nickel and iron which characterizes our core, explaining why they’re farther away from the center of the Earth. The surface layer of the Earth is known as the crust. The crust is very similar to the mantle but it has more of the lighter elements than the mantle which is why the denser mantle sank below.

Earth’s Interior Credit: BBC Bitesize

The formation of the moon

During its formation process the Earth was struck by another growing planet, later named Theia. This impact coincided with the phase where the Earth was transitioning from the dust cloud of the solar nebula to the terrestrial planet we know. Theia was doing the same thing, but it was captured by the larger Earth’s gravity and so they collided. This collision gave Earth its moon. The moon was captured in Earth’s gravity, which at that distance was stronger than the suns. The moon, before the impact, would have had a very similar composition to Earth. However in the impact it gave most of its dense elements to Earth and what was ejected was mostly Earth’s crust. This explains why the moon is mostly composed of oxygen and silicon like Earth’s outer crust.

Earth-Moon Collision Credit: William K. Hartmann

Continents, magnetism and mountains

The mantle underneath us may seem solid. But like the liquid core it’s a liquid and flows over time. The flowing of the mantle beneath us causes the movement of the continents. The surface of the Earth is heavily pitted and scarred from the many collisions it underwent during its formation. The continents are just the high points on this scarred surface. If you watch the continents over time they look like lily pads drifting on top of a ponds surface.

The occasional collisions of these continents cause volcanoes and mountains. The immense force of these collisions shoots spires of rock into the air. These rock spires either become mountains or volcanoes depending on the heat and pressure of the mantle within them. Every mountain or volcano is getting shorter every year as the continents stop colliding and drift apart again. Like the continents movement these geological processes are so slow you can’t appreciate them within a human lifetime.

The Earth, including the iron in its liquid core, is constantly spinning around its solid core. This movement is known as the Coriolis effect. The flowing liquid iron causes an electric current to form which in turn creates the magnetic field surrounding us. It’s this magnetic field that repels the solar wind bombarding Earth, saving us from the solar winds deadly radiation.

Earth's Magnetic Field

Earth’s Magnetic Field Credit: Mysterious Universe

Earth’s liquid water and atmosphere

Early collisions with comets and asteroids gave the Earth its liquid water. This liquid water flowed into the nooks of the uneven Earth’s surface giving us our oceans and continents.

The Earth’s atmosphere is next lightest layer above the layer of water, or hydrosphere, on the surface. It’s gaseous which is less dense than water explaining why it doesn’t sink any deeper. Everything captured by Earth’s gravity is constantly trying to sink towards the dense inner core. The density of these things determines how far they sink before being stopped by denser things. Following this rule, atmosphere gets thinner as it gets farther away from the core until the point where elements are too far away from the core to be pulled by its gravity over the sun and so they drift towards it instead. This ongoing process gives Earth its distinct layers and behaviour and explains why we have the conditions we do for harbouring life.

Much of this is theory. It all follows known physical principles but without more data it’s tough to be absolutely certain of the Earth’s formation process. However exciting new research into exoplanets has given us treasure troves of data to work with. Which could lead to new insights into our pale blue dots origin in the coming years.

Written by Andrew Walls

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