The Formation of The Earth and The Moon

The inner planets of the solar system are terrestrial planets, which means that they are rocky worlds, and the third rock from the sun is Earth, our home planet.

At the time of the writing of this series, every human being that has ever existed was born on Earth, lived on Earth, and died on Earth, never to set foot on any other planet.

When we talked about the formation of the solar system, we envisioned a protoplanetary disk, full of heavy elements ejected into the interstellar medium during the death of one or more stars.

Over hundreds of thousands of years, these combined further to form rocky objects so large that their shapes were crushed into spherical form by the immense gravitational field that had been generated.

Once most of the matter within earth's orbital radius had been collected, collisions became more rare, giving the earth the opportunity to cool down and form a layer of crust, with the last round of major collisions producing craters, and releasing huge quantities of gas trapped in the rock.

This gas remained gravitationally bound to the planet, along with other gas ejected during volcanic eruptions, thus generating the early atmosphere.

Comets eventually delivered more of the atmosphere as well as lots of water, during the late heavy bombardment we mentioned before, but we will talk about comets later in the series.

After continued cooling, water vapor condensed to form all the oceans, and thus, the primitive earth was born.

The core of the earth is made mainly of iron, with some nickel, and is divided into a solid inner core, and a liquid outer core.

This core is surrounded by a rocky mantle, comprised largely of different combinations of silicon, aluminum, and oxygen, which we call silicates, although there are lots of other metals and minerals as well.

And all the way at the surface is the crust, a thin layer of rock that we walk around on every day.

Density determines the distribution of matter, with the heaviest elements found at the center, mainly iron, which is eight grams per cubic centimeter, and lighter compounds distributed upwards towards the crust, like most rocks, which are around three grams per cubic centimeter.

Liquid water, being one gram per cubic centimeter, sits upon the surface, filling in the nooks and crannies.

Lastly, extremely light gases float above the surface in our atmosphere, warming the planet and shielding us from UV radiation.

This phenomenon regarding the separation of materials by density is called differentiation, and it happens in large bodies of molten material, like newly formed planets, just the way that substances separate by density in a centrifuge.

It was through collisions of these tectonic plates over millions of years that has produced the major mountain ranges we see today.

The circulation happening in this molten core, when combined with the rotation of the earth, allows for the alignment of unpaired electrons, which generates an incredible magnetic field.

This magnetic field shields the earth from cosmic rays, deflecting high-energy charged particles to the poles, which then collide with atmospheric particles to form the dazzling Aurora Borealis, also known as the Northern Lights.

The evidence is very clear in suggesting that the moon is the result of a collision between two huge planetesimals in the early stages of earth's formation.

One of these was to become the earth, and the other must have been a Mars sized object, and when these two objects crashed into one another, enormous amounts of rock were splashed up into space.

This debris, gravitationally bound to the earth, eventually collected to become the moon just the way all the planets initially formed.

This model also explains the composition of the moon, rich in silicates and poor in iron, since only Earth's mantle was blasted apart from the collision.

The moon was able to cool very quickly upon forming, due to its small size, resulting in a rocky crust, covered with craters as well as dark patches called maria.

There is no atmosphere, as any gas can easily escape the moon's gravity, and therefore, no wind, which is why the footprints made by the men who have been on the moon will likely remain intact for millions of years.

The moon orbits the earth once every twenty-seven days or so, and its rotational period is exactly the same as its orbital period, meaning it rotates on its own axis once in the same time that it takes to orbit once.