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What I'm going to attempt to do in the next two videos is really just give an overview

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of everything that's happened to Earth since it came into existence.

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We're going start really at the formation of Earth or the formation of our Solar system or the formation of

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the Sun, and our best sense of what actually happened is that

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there was a supernova in our vicinity of the galaxy,

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and this right here is a picture of a supernova remnant,

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actually, the remnant for Kepler's supernova.

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The supernova in this picture actually happened four hundred years ago in 1604,

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so right at the center a star essentially exploded

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and for a few weeks was the brightest object in the night sky,

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and it was observed by Kepler and other people in 1604,

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and this is what it looks like now.

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What we see is kinda the shockwave that's been traveling

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out for the past 400 years, so now it must be many

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light years across. It wasn't, obviously, matter wasn't

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traveling at the speed of light, but it must've been traveling

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pretty, pretty fast, at least relativistic speeds, a reasonable

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fraction of the speed of light.

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This has traveled a good bit out now, but what you can

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imagine is when you have the shockwave traveling out from

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a supernova, let's say you had a cloud of molecules,

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a cloud of gas, that before the shockwave came by just

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wasn't dense enough for gravity to take over,

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and for it to accrete, essentially, into a solar system.

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When the shockwave passes by it compresses all of this gas

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and all of this material and all of these molecules, so

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it now does have that critical density to form, to accrete

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into a star and a solar system.

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We think that's what's happened, and the reason why we

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feel pretty strongly that it must've been caused by a supernova

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is that the only way that the really heavy elements can form,

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or the only way we know that they can form is in kind of

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the heat of a supernova, and our uranium, the uranium that seems

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to be in our solar system on Earth, seems to have formed

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roughly at the time of the formation of Earth, at about

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four and a half billion years ago, and we'll talk

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in a little bit more depth in future videos on exactly how

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people figure that out, but since the uranium seems about

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the same age as our solar system, it must've been formed

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at around the same time, and it must've been formed by a supernova,

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and it must be coming from a supernova, so a supernova

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shockwave must've passed through our part of the universe,

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and that's a good reason for gas to get compressed and begin to accrete.

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So you fast-forward a few million years.

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That gas would've accreted into something like this.

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It would've reached the critical temperature, critical density

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and pressure at the center for ignition to occur, for fusion

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to start to happen, for hydrogen to start fusing into helium,

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and this right here is our early sun.

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Around the sun you have all of the gases and particles

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and molecules that had enough angular velocity to not fall into the sun,

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to go into orbit around the sun.

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They were actually supported by a little bit of pressure, too,

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because you can kinda view this as kind of a big cloud of gas,

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so they're always bumping into each other, but for the most part

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it was their angular velocity, and over the next tens of millions of years

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they'll slowly bump into each other and clump into each other.

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Even small particles have gravity, and they're gonna slowly

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become rocks and asteroids and, eventually, what we would call

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"planetesimals," which are, kinda view them as seeds

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of planets or early planets, and then those would have a reasonable amount

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of gravity and other things would be attracted to them

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and slowly clump up to them.

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This wasn't like a simple process, you know, you could imagine

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you might have one planetesimal form, and then there's another

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planetesimal formed, and instead of having a nice, gentle

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those two guys accreting into each other, they might have

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huge relative velocities and ram into each other, and then just,

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you know, shatter, so this wasn't just a nice, gentle process of constant accretion.

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It would actually have been a very violent process,

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actually happened early in Earth's history, and we actually think

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this is why the Moon formed, so at some point

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you fast-forward a little bit from this, Earth would have formed,

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I should say, the mass that eventually becomes our modern Earth

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would have been forming. Let me draw it over here.

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So, let's say that that is our modern Earth, and what we think

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happened is that another proto-planet or another,

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it was actually a planet because it was roughly the size of Mars,

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ran into our, what it is eventually going to become our Earth.

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This is actually a picture of it.

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This is an artist's depiction of that collision, where this planet

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right here is the size of Mars, and it ran into what would eventually become Earth.

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This we call Theia. This is Theia,

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and what we believe happened, and if you look up,

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if you go onto the Internet, you'll see some simulations

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that talk about this, is that we think it was a glancing blow.

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It wasn't a direct hit that would've just kinda shattered

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each of them and turned into one big molten ball.

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We think it was a glancing blow, something like this.

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This was essentially Earth. Obviously, Earth got changed

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dramatically once Theia ran into it, but Theia is

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right over here, and we think it was a glancing blow.

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It came and it hit Earth at kind of an angle, and then it

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obviously the combined energies from that interaction

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would've made both of them molten, and frankly

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they probably already were molten because you had

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a bunch of smaller collisions and accretion events and

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little things hitting the surface, so probably both of them

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during this entire period, but this would've had a

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glancing blow on Earth and essentially splashed a bunch

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of molten material out into orbit.

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It would've just come in, had a glancing blow on Earth,

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and then splashed a bunch of molten material,

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some of it would've been captured by Earth, so this is

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the before and the after, you can imagine, Earth is

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kind of this molten, super hot ball, and some of it

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just gets splashed into orbit from the collision.

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Let me just see if I can draw Theia here, so

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Theia has collided, and it is also molten now because

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huge energies, and it splashes some of it into orbit.

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If we fast-forward a little bit, this stuff that got splashed

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into orbit, it's going in that direction, that becomes

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our Moon, and then the rest of this material eventually

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kind of condenses back into a spherical shape and is what

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we now call our Earth.

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So that's how we actually think right now that the Moon

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actually formed.

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Even after this happened, the Earth still had a lot more,

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I guess, violence to experience.

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Just to get a sense of where we are in the history of Earth,

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we're going to refer to this time clock a lot over the next few videos,

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this time clock starts right here at the formation of our solar system,

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4.6 billion years ago, probably coinciding with

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some type of supernova,

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and as we go clockwise on this diagram,

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we're moving forward in time, and we're gonna go

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all the way forward to the present period,

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and just so you understand some of the terminology,

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"Ga" means "billions of years ago"

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'G' for "Giga-"

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"Ma" means "millions of years ago"

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'M' for "Mega-"

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So where we are right now, the Moon has formed,

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and we're in what we call the Hadean period

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or actually I shouldn't say "period."

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It's the Hadean eon of Earth.

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"Period" is actually another time period,

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so let me make this very clear. It's the Hadean,

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we are in the Hadean eon, and an eon is kind of

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the largest period of time that we talk about, especially

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relative to Earth, and it's roughly 500 million to a billion years

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is an eon, and what makes the Hadean eon distinctive,

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well, from a geological point of view what makes it

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distinctive is really we don't have any rocks from the

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Hadean period. We don't have any kind of macroscopic-scale

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rocks from the Hadean period, and that's because

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at that time, we believe, the Earth was just this molten

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ball of kind of magma and lava, and it was molten

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because it was a product of all of these accretion events

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and all of these collisions and all this kinetic energy turning into heat.

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If you were to look at the surface of the Earth,

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if you were to be on the surface of the Earth during

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the Hadean eon, which you probably wouldn't want to be

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because you might get hit by a falling meteorite

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or probably burned by some magma, whatever,

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it would look like this, and you wouldn't be able to breathe anyway;

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this is what the surface of the Earth would look like.

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It would look like a big magma pool, and that's why we

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don't have any rocks from there because the rocks were

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just constantly being recycled, being dissolved and churned

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inside of this giant molten ball, and frankly

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the Earth still is a giant molten ball, it's just

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we live on the super-thin, cooled crust of that molten ball.

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If you go right below that crust, and we'll talk a little bit more

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about that in future videos,

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you will get magma, and if you go dig deeper,

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you'll have liquid iron.

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I mean, it still is a molten ball.

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And this whole period is just a violent,

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not only was Earth itself a volcanic, molten ball,

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it began to harden as you get into the late Hadean eon,

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but we also had stuff falling from the sky and constantly

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colliding with Earth, and really just continuing to add

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to the heat of this molten ball.

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Anyway, I'll leave you there,

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and, as you can imagine,

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at this point there was no, as far as we can tell,

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there was no life on Earth.

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Some people believe that maybe some life could've formed

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in the late Hadean eon, but for the most part

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this was just completely inhospitable for any life forming.

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I'll leave you there, and where we take up the next video,

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we'll talk a little bit about the Archean eon.