Video 2.1.1: How did the Moon form? - Dr. Katherine Joy
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来自 曼彻斯特大学 的课程
探索过去的45亿年 更好地理解空气，水，土壤和生命如何形成 及相互影响
Formation, evolution, and processes of the solid Earth
Video 2.1.1: How did the Moon form? - Dr. Katherine Joy
Video 2.1.2: Why is the Moon important to life on earth? - Dr. Katherine Joy
Video 2.2: What came before plate tectonics?
Video 2.3: How did plate tectonics get discovered?
Video 2.4.1: The Earth's magnetic field
Video 2.4.2: The magnetic poles flip? You're kidding me, right?
Video 2.5: How earthquakes happen
Video 2.6.1: What's inside the Earth?
Video 2.6.2: How do we know about the insides of the Earth?
Video 2.7: How do the plates move?
Video 2.8: How does magma form?
Video 2.9: How were the Himalaya formed?
Video 2.10: Supercontinent Pangaea
Video 2.11: The supercontinent cycle
Prof. David M. Schultz
Professor of Synoptic Meteorology
School of Earth, Atmospheric and Environmental Sciences
Dr Rochelle Taylor
Postdoctoral Research Associate
School of Earth, Atmospheric & Environmental Sciences
Dr Jonathan Fairman
Postdoctoral Research Associate
School of Earth, Atmospheric and Environmental Sciences
So I'm here in the Manchester Museum, with Dr. Katherine Joy, who's a research fellow
in Lunar and Planetary Science here at the University of Manchester.
And we're here to learn about the formation of the solar system,
earth, and the moon.
So, Katherine, tell me a little bit about how this came about?
>> So the solar system formed four and a half billion years ago, and
we know that, because we've age dated the very earliest remnants of those types of
early formation processes in the form of meteorites.
>> Hm. >> Which I'll talk a little bit
about later on.
But we think the events that occurred to form the solar system,
is when you had a very dense cloud of dust and gas which is called a solar nebula,
if you can see spectacular images in some of the Hobble space telescope Imagery of
solar nebula out there in the rest of the galaxy at the moment.
>> Mm-hm. >> But, this dust and gas collapsed on
each other, possibly during a shockwave episode from an exploding star.
And as this dust and
gas kind of collapsed gravitationally, they formed a circulating ball of dust and
gas disks, which again started to collapse under their own gravitational weight.
We found a proto sun, so our very early sun and this sun was made of helium and
hydrogen and all of the other elements, and it slowly started to
burn hat hydrogen and that helium, to for the sun that we see today.
Now, not only did we form the sun, but at that time, the dust and
gas that was circulating around started to clump together as well.
And those calpins which we call proto-planets, or very early remnants of
planets help to form the types of planets that we see in the solar system's state.
So we have the rocky planets closer to the sun, like Mercury, Venus, Earth, Mars.
And then outside of that we have the gas giants, this is
where the volatile elements were stored, the cold material that formed gassy
planets like Jupiter; Saturn, Neptune, Uranus and then we have the, the debris.
[LAUGH] Further there out so that they're the cold remnants of comets and
small planetary bodies like Pluto and the other, the other small planets as well.
So, it formed a complicated solar system that we see today.
>> Mm-hm. Great, so you mentioned meteorites.
What are, what are meteorites, and
what can you tell from them about, how the solar system got put together?
>> So meteorites are the remnants of this early kind of planet formation process and
they are basically the products of asteroids, which are remnant planetary
bodies that mostly are now found between the planet Mars and the planet Jupiter.
And these are small early types of planets that,
that were being colliding with each other and smashed up.
And have since then kind of got knocked out of their orbit, travel through
space and then they become on Earth crossing orbits, they've come down
through the Earth's atmosphere we see them as shooting stars and fireball events.
And then we can go out and
we can find meteorites that have fallen as part of this process.
So we find them in places like hot deserts such as Africa, or
even we go down to Antarctica which is really neat,
and we find them sitting on the ice in Antarctica.
>> Hm. >> But meteorites are really special,
this is a particular type called Allende,
which forms the earliest type of material we have in our solar system, so
these are what the earliest planetary bodies would've looked like.
And what we can do is we can extract the small components that you see within it,
date them using very high precision dating techniques in the lab to figure out
how old the solar system was, and
the types of processes that are effected asteroids such as this early on.
So I can tell you that some of the things in this such as this little white object
here is 4.56 billion years old,
it's the oldest thing that we're ever going to find on Earth.
>> Hm. >> Which is why it's really special.
And the variability between different asteroid types,
tells us that the Solar System is a very complicated place early on.
It has different oxygenation conditions it had different rates of clumping and
collapse in formation as well, so it was a very, very diverse.
So after we started forming an early planetary material like this,
from the, the dust and, and, and gasses that kind of collapsed in on each other.
Planetary bodies grew, and
they grew, and they grew by collision, by attracting more and more material.
>> And when they got to a certain size, roughly about 100 kilometers or
so, the amount of material that accreted together had a lot of
radioactive elements specifically, isotopes like aluminium 26,
which induced melting and caused these early planetary bodies to differentiate.
So this is whereby you have internal heating and melting and then chemical, and
density separations which form a solid core, a mantle, and
a crust, much like we have on the Earth.
>> Hm. Hm. >> Now, I have another meteorite here this is an iron meteorite.
>> Okay. >> It is a beautiful heavy sample if
you want to have a hold there you go.
>> Yeah, you can see that it's much different in well structure, and
texture to the other one.
>> Yeah, so it's basically made of iron and nickel, and this is,
this represents what would have been once the core of one of these early planets.
>> Wow. >> Which was then smashed up [LAUGH] by
collisions and geh, you know, mixed up and
kind of became a rubber pile before falling as a meteorite on earth.
>> Hm. >> You
wouldn't want something like that landing on your head,
but it represents what was once at the center of one of these planets.
So it's really special, and
it tells us about how our own planet formed its core and-
>> Right. >> And differentiated.
>> So if this came from the core, then the other meteorite you chose is that from-
>> This is from pre-core material, so
this is just from those early accreted dust particles.
>> Oh okay. >> So we call this a conjurate.
>> All right. >> Because it represents material like
this would have been, got bigger and bigger to form [CROSSTALK]-
>> Before the differentiation.
>> Before the differentiation occurred.
>> All right.
>> So this is an undifferentiated meteorite.
>> Excellent thank you.
So we've talked about the formation of these terrestrial planets in
this new part of the solar system.
>> What about the moon?
How was, what's how did that come about, and is that part of this story as well?
>> So the Earth and moon are coupled together in their history, we think that
the Earth formed like a planet, along with all the other planets, about four and
a half billion years ago, but then something really special happened.
And we know this from age dating the, the rocks of the moon and also by understand
the, the chemical signatures with, within our atmosphere at the present date.
And this special and unique event was, we refer to as the giant impact hypothesis,
and what we think happened is we formed our early earth.
And then after about, between about 60 million years after the formation of
all the other planets, we think there was a big collision event.
Whereby a planet, maybe about the size of Mars, slammed into the early Earth
causing nearly complete destruction of this, this early Earth.
And this giant impact event threw loads of material off into space,
which formed a debris disk around the early Earth.
Which slowly collapse back in on itself to form the moon.
Now, evidence for this comes from the fact that Earth and moon rocks are very
similar, in terms of their chemistry, and their isotopic signature.
And it also explains a lot of the physical effects to to with our orbit.
And to do with the way that the Earth is spinning at the present day.
>> So, angular momentum type of effect.
Now, this giant impact hypothesis is not without its critics, and
it's still being tested.
There's a lot of really impressive computer simulations to try and
show how this even might have occurred, but there's a lot of debate,
so was this early impactor the size of Mars?
Was it a lot smaller?
Did it impact straight on?
Did it have glancing blow to throw material off in to space?
We really don't know we're also testing a lot of the chemical arguments as well.
But it does solve a lot of the things that we understand about both the Earth and
the moon, and it shows that we have a coupled history with the moon,
it's our sister planetary body.
And, and without it, you know life on Earth, the,
the types of things that we see on Earth, wouldn't be here.
>> Okay. So
this early impactor hypothesis, is this something that's relatively recent,
because when I went to school this isn't something that we learned.
>> So its a hypothesis which has emerged,
since we have actually had the Apollo samples back,
since we went to the moon and we got the samples and we looked at there chemistry.
>> Hm. >> And we saw just how
similar in many respects, not all respects-
>> Yeah. >> They are to the earth, so
there's a particularly similar oxygen isotope signature so this is the chemical
similarities, between the oxygen on the moon and the oxygen on the Earth.
They, they are pretty much near identical which is very unusual.
>> Mm-hm. >> Compared to a lot of
the other bodies in the solar system, and so
ex, to explain this th, this is why this theory came about now, there are other
suggestions the moon could have been captured as a planet on its own into-
>> Mm-hm. >> Into gravity around the Earth,
very difficult to do in a lot of the computer simulations of that
particular model, the moon would just smash into the Earth anyway.
Right? >> Yeah.
>> There's a possibility that it co-ecreted, so actually the Earth and
the moon formed in the same part of the early
that planetary space around each other.
>> that, that sort of, there are chemical arguments against that backwards and
forwards so yes, there are alternatives, but
the giant impact is the one that kind of ticks the most number of boxes.
>> Mm-hm. >> As the hypothesis of
how the moon formed.
>> But the moon is kind of unique among all the different moons in the solar
system relative to, to Earth, because of its size, relative to the Earth.
Right? >> Yes, so the moon is ve,
is very large we have a very large companion.
It's been chemically differentiated, so
the moon itself has a very small core it's formed a mantle and a crust.
So when we look up at the moon and we see the white parts of its surface,
this is the products of early crust formation.
So this is about 4.5 billion years old, roughly the same age as the Earth as well.
But, then the moon is being much more geologically active, so
it's covered in impact craters.
And this is where asteroids and
comets have pummeled in to this surface making giant impact bases,
we can see them up to two and a half thousand kilometers in diameter.
Now, a record of these types of impact process, that we see on the moon
happens on the earth as well, but we've just lost that record plate tectonics and
recycled that kind of crustal modification processes.
So the moon is very, very special in that it allows us to
unlock a record of impact bombardments to both the Earth and
the moon system, throughout the part four and a half billion years.
And one of the really neat things we've learned from the moon,
is that there may have been a cataclysmic bombardment of the lunar surface.
>> Hm. >> At about 4 billion years ago and
we see that, because we've age dated a lot of the moon rocks.
>> Okay. >> we,
we can, we know that a lot of the large basins, so
these are the big craters above 300 kilometers in diameter.
Were all formed by colliding asteroids and comets-
>> Mm-hm. >> In quite a short period of time
It's called the lunar or the Late Heavy Bombardment theory or the Lunar Cataclysm.
>> Mm-hm. >> And it's really important we understand
this, because whatever was hitting the moon, was also hitting the Earth.
>> Mm-hm. >> And
was recycling a modificate, and modifying the Earth's crust as well so
it's a really important hypothesis for us to test.
>> So this is also, you know, a few 100,000 year, a few
100 million years before life we have the first evidence of life on Earth as well.
So if there was this period of late, heavy bombardment around this time, it was
probably also affecting what was trying to go on on Earth the development of life.
>> Yeah, absolutely so it could have been affecting the way that life started.
But it also had [INAUDIBLE] so it could have been bringing some of
the volatile elements the water, the amino acids.
>> Mm-hm. >> The types of things that we
have in meteorites that kind of forms far out in the solar system as well.
To Earth to s, to bring some of those kind of, products for
having early oceans, and the types of habitats that may be where life started.
>> Not only that it could have created important niches,
it could have brought heats to the oceans and, and providing the kind of
kick started things that we need for the onset of life on Earth.
So there it could have caused sterilization,
may be we lost the type of life that could have formed prior to 3.9 billion years.
>> Hm. >> It might have helped the type of
life emerge, we now know at the present day.
>> Interesting, and this collision the produce the moon, or
the hypothesized collision that may have produced the moon on earth.
We see this in other planets as well I mean, they have unusual spins and
angular tilts and so on, so, it's likely that these other
planets were impacted by bodies large enough to alter their orbital dynamics.
>> Absolutely, impact bombardment is a ubiquitous process throughout the solar
system, so it could explain, like you just said,
the unusual orbits of other planetary surfaces.
And we also see evidence of impact cratering everywhere,
on the surface of Mars, on the surface of Mercury, even on the surface of Venus,
some of these asteroids leave scars from coming down through this thick atmosphere.
>> Hm. >> So we know that this is a process which
goes on on all the other planets, and not only in our solar system, but
in other solar systems out there, as well.
So collisions, although they destroy, them also help shape the types of
planetary systems >> Hm.
>> That we see today.
>> And it's only because of plate tectonics on Earth,
that we've lost a lot of these, I guess Mercury is heavily cratered.
The moon is heavily cratered, Venus is under this thick cloud, so-
>> Yeah, so it's difficult to see and it's also had a lot of volcanism.
>> Okay. >> So, volcanism when you have
a volcanic eruption it raises a lot of the scars of impact bombardment as well.
So, the geologically active planets like Venus and like the earth and
Mars to some extent as well.
Some of the records have been lost as a consequence, which is why we look to the,
the planets with less amounts of volcanism, or volcanism that died
a long time ago, like the moon and Mercury to try and understand these processes.
So, this is a very sophisticated computer simulation make by researchers in America,
whereby they're created a simulation where a Mars size body has
collided with the earth.
And you can see this dramatic event has thrown a lot of material off into space.
The material that's been orbiting around the Earth has clumped back together again,
probably very quickly simulations suggest just 10s, to 100s, to 1000s of years.
>> Wow. >> And
slowly starts to build a proto-moon, which then gets bigger and
bigger and bigger to form the size of the moon that we see today.
So this would have been very hot and temperatures many 1000s of degrees C and
you would have had chemical equilibration between the the moon and
the Earth's forming debris disk it's a really dramatic process.
>> So the chemical equilibration then, means that the Mars, I mean that the,
that the moon and the Earth then, would have the same chemical composition.
>> Yes, it would have been a mixing effect between the impacting body and the,
>> And these different colors then, represent what?
>> They could represent temperature, they could represent density.
They could represent different types of things,
depending on what the code parameter means I think this one is temperature.
>> Okay, and you can see there's this extra body that forms this
yellow body comes back around, and then it gets looks like it gets focused and then,
but it gets splayed out and stretched out as it approaches the Earth again.
>> [CROSSTALK] Collides again, again, again with other things and
is being ripped apart by the Earth's gravitational field, so
it's probably within the Roche limit which is where the Earth's gravity takes over.
>> Hm. >> From the surrounding material and
causes debris to internally fracture and split apart.
>> Excellent thank you.
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