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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.
I mean, it still is a molten ball.
And this whole period is just a violent,
not only was Earth itself a volcanic, molten ball,
it began to harden as you get into the late Hadean eon,
but we also had stuff falling from the sky and constantly
colliding with Earth, and really just continuing to add
to the heat of this molten ball.
Anyway, I'll leave you there,
and, as you can imagine,
at this point there was no, as far as we can tell,
there was no life on Earth.
Some people believe that maybe some life could've formed
in the late Hadean eon, but for the most part
this was just completely inhospitable for any life forming.
I'll leave you there, and where we take up the next video,
we'll talk a little bit about the Archean eon.
コツ:単語をクリックしてすぐ意味を調べられます!

読み込み中…

Earth formation | Life on earth and in the universe | Cosmology & Astronomy | Khan Academy

136 タグ追加 保存
Caterpillar 2019 年 10 月 3 日 に公開
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