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

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

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

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

  • there was a supernova in our vicinity of the galaxy,

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

  • actually, the remnant for Kepler's supernova.

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

  • so right at the center a star essentially exploded

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

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

  • and this is what it looks like now.

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

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

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

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

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

  • fraction of the speed of light.

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

  • imagine is when you have the shockwave traveling out from

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

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

  • wasn't dense enough for gravity to take over,

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

  • When the shockwave passes by it compresses all of this gas

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

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

  • into a star and a solar system.

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

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

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

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

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

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

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

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

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

  • people figure that out, but since the uranium seems about

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

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

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

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

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

  • So you fast-forward a few million years.

  • That gas would've accreted into something like this.

  • It would've reached the critical temperature, critical density

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

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

  • and this right here is our early sun.

  • Around the sun you have all of the gases and particles

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

  • to go into orbit around the sun.

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

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

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

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

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

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

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

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

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

  • of gravity and other things would be attracted to them

  • and slowly clump up to them.

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

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

  • planetesimal formed, and instead of having a nice, gentle

  • those two guys accreting into each other, they might have

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

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

  • It would actually have been a very violent process,

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

  • this is why the Moon formed, so at some point

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

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

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

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

  • happened is that another proto-planet or another,

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

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

  • This is actually a picture of it.

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

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

  • This we call Theia. This is Theia,

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

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

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

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

  • each of them and turned into one big molten ball.

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

  • This was essentially Earth. Obviously, Earth got changed

  • dramatically once Theia ran into it, but Theia is

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

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

  • obviously the combined energies from that interaction

  • would've made both of them molten, and frankly

  • they probably already were molten because you had

  • a bunch of smaller collisions and accretion events and

  • little things hitting the surface, so probably both of them

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

  • glancing blow on Earth and essentially splashed a bunch

  • of molten material out into orbit.

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

  • and then splashed a bunch of molten material,

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

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

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

  • just gets splashed into orbit from the collision.

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

  • Theia has collided, and it is also molten now because

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

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

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

  • our Moon, and then the rest of this material eventually

  • kind of condenses back into a spherical shape and is what

  • we now call our Earth.

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

  • actually formed.

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

  • I guess, violence to experience.

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

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

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

  • 4.6 billion years ago, probably coinciding with

  • some type of supernova,

  • and as we go clockwise on this diagram,

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

  • all the way forward to the present period,

  • and just so you understand some of the terminology,

  • "Ga" means "billions of years ago"

  • 'G' for "Giga-"

  • "Ma" means "millions of years ago"

  • 'M' for "Mega-"

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

  • and we're in what we call the Hadean period

  • or actually I shouldn't say "period."

  • It's the Hadean eon of Earth.

  • "Period" is actually another time period,

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

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

  • the largest period of time that we talk about, especially

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

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

  • well, from a geological point of view what makes it

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

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

  • rocks from the Hadean period, and that's because

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

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

  • because it was a product of all of these accretion events

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

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

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

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

  • because you might get hit by a falling meteorite

  • or probably burned by some magma, whatever,

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

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

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

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

  • just constantly being recycled, being dissolved and churned

  • inside of this giant molten ball, and frankly

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

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

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

  • about that in future videos,

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

  • you'll have liquid iron.