The Earth: Crash Course Astronomy #11

Summary

This script delves into Earth's complex structure, from its core to the atmosphere, highlighting its uniqueness with liquid water and life-sustaining conditions. It explains the role of Earth's magnetic field in protecting the atmosphere from solar wind, the significance of plate tectonics in shaping the planet's geography, and the impact of human activities on climate change. The episode underscores the importance of preserving Earth as the only known habitable planet.

Takeaways

• 🌏 The Earth, once thought of as unique in the universe, is now understood to be one of many planets.
• 🔭 The invention of the telescope and spacecraft transformed our perception of planets from distant lights to tangible worlds.
• 💧 Earth is distinguished by the presence of liquid water on its surface, which is essential for life and geological processes.
• 🌐 Earth's magnetic field, generated by its liquid outer core, protects its atmosphere from solar wind and helps sustain life.
• 🌀 The Earth's core, composed of an inner solid core and an outer liquid core, is a critical heat source for the planet's geological activity.
• 🌋 Volcanic activity, driven by the movement of tectonic plates, is a result of the Earth's internal heat and contributes to the formation of new land and the composition of the atmosphere.
• 🌈 The Earth's atmosphere, primarily nitrogen and oxygen, protects life by absorbing UV light and creating weather patterns.
• 🌡️ The greenhouse effect, involving gases like carbon dioxide, is necessary for Earth's warmth but is being disrupted by human activities.
• 🌌 The aurora, a natural light display, is caused by the interaction of solar particles with the Earth's magnetic field.
• 🏞️ Earth's surface is predominantly water-covered, with implications for the planet's climate and life-supporting capabilities.
• ⚠️ Human-induced climate change is causing significant environmental shifts, including ice melting and ocean acidification.

Q & A

What significant change in perspective did the invention of the telescope bring about regarding planets?

The invention of the telescope transformed planets from one-dimensional points in the sky to worlds that could be observed in greater detail, making it evident that Earth could be one of them.

Why is Earth considered the largest of the terrestrial planets?

Earth is considered the largest of the terrestrial planets because it is about 13,000 kilometers across, larger than the other three smaller, denser, rocky worlds that orbit close to the Sun.

What unique feature does Earth have that the other terrestrial planets lack?

Unlike the other terrestrial planets, Earth has liquid water on its surface, which is essential for supporting life and facilitating various chemical processes.

How does the presence of water on Earth's surface contribute to the support of life?

The presence of liquid water on Earth's surface allows it to flow, evaporate, form clouds, and rain down, mixing chemicals in a way that supports life by enabling complex biochemical reactions.

What are the two layers of Earth's core, and what are their primary compositions?

Earth's core consists of two layers: the inner core, which is solid and primarily composed of iron and nickel, and the outer core, which is liquid and also predominantly made of iron and nickel.

Why is the inner core of Earth solid despite the high temperatures?

The inner core remains solid due to the extremely high pressure exerted by the weight of the entire planet, which allows iron to stay solid even at high temperatures.

What is the role of the mantle in the movement of Earth's crustal plates?

The mantle's convection, driven by heat from the core, causes the solid rock to flow very slowly over geologic time, which in turn powers the movement of Earth's crustal plates.

How does the process of convection in Earth's mantle compare to the movement of continents?

The rate of convection in the mantle is very slow, with a flow rate of a couple of centimeters per year, similar to the rate at which human fingernails grow, which is also the approximate rate of continental movement.

What is the primary source of heat within Earth's core?

The primary source of heat within Earth's core is leftover from its formation over 4.5 billion years ago, with additional heat coming from the gravitational contraction of the Earth and radioactive decay of elements like uranium.

How does the Earth's magnetic field protect its atmosphere from solar wind?

The Earth's magnetic field deflects most of the charged particles from the solar wind and traps some, preventing them from directly hitting the atmosphere, which would otherwise erode it over billions of years.

What is the significance of the ozone layer in Earth's atmosphere?

The ozone layer, located about 25 kilometers above the Earth's surface, is critical for absorbing solar ultraviolet light, which can be harmful to biological molecules, thus protecting life on Earth.

What is the role of carbon dioxide in Earth's atmosphere and why are climate scientists concerned about it?

Carbon dioxide in the atmosphere contributes to the greenhouse effect, which warms the Earth. While necessary for maintaining temperatures above freezing, an excess of carbon dioxide due to human activities is trapping more heat, leading to global warming and climate change.

What is the concept of terraforming and how does it relate to current changes on Earth?

Terraforming is the science fiction concept of transforming an uninhabitable alien planet to be more Earth-like. The opposite process, which could be termed 'de-terraforming', is what humans are inadvertently causing on Earth by altering its climate and environment in ways that could make it less habitable.

Outlines

00:00

🌏 The Earth's Composition and Life-Sustaining Features

This paragraph delves into the Earth's planetary status and its evolution from a celestial body to a known world. It emphasizes Earth as the largest terrestrial planet with a unique characteristic: liquid water on its surface, which is essential for life. The Earth's layers are described, starting with the solid inner core made of iron and nickel, followed by the liquid outer core. Above the core lies the mantle, a thick, plastic-like layer that behaves as a solid but can flow over geologic time. The crust, Earth's outermost layer, is divided into oceanic and continental types and is broken into tectonic plates. The movement of these plates, driven by the mantle's convection, shapes the Earth's geography over millions of years. Volcanoes, both at plate boundaries and hot spots, are highlighted as creators of new land and contributors to the atmosphere. The paragraph concludes by discussing the sources of Earth's internal heat, including residual heat from formation, gravitational contraction, radioactive decay, and friction from dense material sinking to the core.

05:03

🌌 Earth's Protective Magnetic Field and Atmospheric Composition

The second paragraph focuses on the Earth's outer core as a source of its magnetic field, which is crucial for deflecting solar wind and protecting the atmosphere. The geomagnetic field is likened to a bar magnet, with poles near the Earth's rotational axis. The absence of a strong magnetic field on Mars is contrasted with Earth's, suggesting it as a reason for Mars' lost atmosphere. The Earth's atmosphere is described in detail, with its composition and the Kármán line marking the boundary with space. The role of the atmosphere in weather, the ozone layer's protection against ultraviolet light, and the aurora phenomenon are also covered. The paragraph touches on the atmospheric pressure exerted on Earth's surface and the significance of liquid water coverage. It discusses the origin of Earth's water and the role of carbon dioxide in the greenhouse effect, highlighting the impact of human activities on climate change. The script ends with a reflection on the uniqueness of Earth as a habitable planet and a call to preserve its conditions, mentioning the production team behind the educational content.

Keywords

💡Terrestrial Planets

Terrestrial planets refer to the four solid, rocky planets in our solar system, including Earth, Mercury, Venus, and Mars, which are located relatively close to the Sun. These planets are characterized by their dense composition and relatively small size compared to the gas giants. In the script, Earth is described as the largest of the terrestrial planets, emphasizing its size and unique features among its peers.

💡Magnetic Field

A magnetic field is a region around a planet or star where its magnetic force affects the surrounding space. Earth's magnetic field, generated by the motion of molten iron in its outer core, is crucial for deflecting harmful solar wind particles and protecting the atmosphere. The script highlights the importance of Earth's magnetic field in maintaining the conditions necessary for life by preventing the solar wind from stripping away the atmosphere.

💡Convection

Convection is the process by which heat is transferred in fluids (liquids and gases) through the movement of mass. In the context of the Earth, convection occurs in the mantle, where heat from the core causes the semi-solid rock to slowly rise and then sink, driving the movement of tectonic plates. The script explains that this slow process shapes the Earth's surface over geological time scales.

💡Tectonic Plates

Tectonic plates are large slabs of solid rock that make up the Earth's lithosphere and float on the semi-fluid asthenosphere below. The movement of these plates is responsible for the creation of mountains, earthquakes, and volcanoes. The script mentions that the crust is broken into plates driven by the flow of rock in the mantle, which is powered by heat from the Earth's core.

💡Volcanoes

Volcanoes are openings in the Earth's crust through which molten rock (magma), hot rock fragments, and gases are expelled. They are a result of tectonic plate movement and the upwelling of magma from the mantle. The script describes how volcanoes not only create new land but also contribute to the Earth's atmosphere by releasing gases.

💡Atmosphere

The atmosphere is a layer of gases surrounding a planet, held in place by gravity. Earth's atmosphere is composed mainly of nitrogen and oxygen, with trace amounts of other gases, and it plays a critical role in supporting life by providing air to breathe and protecting the surface from harmful solar radiation. The script discusses the composition and importance of Earth's atmosphere, as well as its current challenges due to human activities.

💡Ozone Layer

The ozone layer is a region of the Earth's stratosphere that contains a high concentration of ozone (O3) molecules, which absorb the majority of the Sun's harmful ultraviolet radiation. The script points out the ozone layer's importance in protecting life on Earth from the damaging effects of UV light.

💡Aurora

Auroras, also known as the Northern and Southern Lights, are natural light displays caused by the collision of solar wind particles with the Earth's magnetic field. The script describes how the Earth's magnetic field channels these particles into the atmosphere, where they interact with air molecules to produce the beautiful glow of auroras near the geomagnetic poles.

💡Greenhouse Effect

The greenhouse effect is the process by which certain gases in a planet's atmosphere trap heat, preventing it from escaping into space and thereby warming the planet. Carbon dioxide is a significant greenhouse gas, and the script discusses how an increase in its concentration due to human activities is causing global warming and climate change.

💡Carbon Dioxide

Carbon dioxide (CO2) is a greenhouse gas that plays a crucial role in the Earth's carbon cycle and climate system. While a small amount is necessary for the greenhouse effect that keeps the planet warm, excessive amounts can lead to global warming. The script highlights the impact of increased CO2 levels on the Earth's climate and ecosystems.

💡Core

The core of the Earth is its central region, consisting of two layers: the solid inner core and the liquid outer core. It is primarily composed of iron and nickel and is extremely hot, with temperatures comparable to the Sun's surface. The script explains that the heat in the core is a result of the planet's formation, gravitational contraction, radioactive decay, and the sinking of dense materials, which together drive the geodynamo that creates Earth's magnetic field.

Highlights

The Earth was once thought to be unique in the Universe, but with the invention of the telescope and spacecraft, it is recognized as one of many planets.

Earth is the largest terrestrial planet with a single, large Moon.

Earth's surface features liquid water, which is crucial for supporting life and mixing chemicals.

The Earth's ability to sustain life is also dependent on its atmosphere and magnetic field.

The Earth's core is composed of two layers: a solid inner core and a liquid outer core, primarily made of iron and nickel.

The Earth's core has extremely high temperatures and pressures, with the inner core remaining solid despite the heat.

The mantle, above the core, is thick and behaves like very thick, hot plastic, capable of flowing over geologic time.

The crust floats on the mantle and is divided into two types: oceanic and continental, with significant differences in thickness.

The movement of tectonic plates is driven by the flow of mantle rock, powered by heat from the Earth's core.

Volcanoes form as magma pushes through weaker crust areas, and can create linear chains over millions of years.

The Earth's core is a source of heat, leftover from its formation and maintained by radioactive decay and gravitational contraction.

The Earth's liquid outer core generates a magnetic field through convection, similar to processes in the Sun.

The Earth's magnetic field protects the atmosphere from solar wind, preventing the loss of air like on Mars.

The Earth's atmosphere is defined as extending 100 kilometers up, known as the Kármán line.

The atmosphere is composed mainly of nitrogen and oxygen, with trace gases including water vapor and carbon dioxide.

The greenhouse effect, caused by gases like carbon dioxide, is essential for Earth's warmth but can be dangerous in excess.

Human activities have increased atmospheric carbon dioxide levels, leading to climate change and its associated impacts.

The Earth is the only known habitable planet in our solar system, emphasizing the importance of preserving its conditions.

Transcripts

The Earth is a planet.

That’s a profound statement, and one that’s not really all that obvious. For thousands

of years, planets were just bright lights in the sky, one-dimensional points that wandered

among the fixed stars. How could the Earth be one of them?

With the invention of the telescope those dots became worlds, and with spacecraft they became

places. The Earth went from being our unique home in the Universe to one of many such…well, planets.

The Earth is the largest of the terrestrial planets, the four smaller, denser, rocky worlds

orbiting close in to the Sun. It’s about 13,000 kilometers across, and has a single,

large Moon which we’ll learn a lot more about next week.

Unlike the other three terrestrial planets, Earth has something very important: Water.

Or, more specifically, liquid water on its surface, where it can flow around, evaporate,

become clouds, rain down, and then mix up chemicals so they can do interesting, complex

things—like support life.

Earth’s ability to sustain life depends on that water. It also depends on Earth’s

atmosphere, of course—breathing has its advantages—and both, weirdly enough, depend

on Earth’s magnetic field to exist. And that, in turn, depends on what’s going on

deep inside our planet. So, let’s take a look.

Like the Sun, the Earth is a many-layered thing. At its very center is the core, which

actually has two layers, the inner core and the outer core.

The inner core is solid, and made mostly of iron and nickel. These are heavy elements,

and sank to the center of the planet when it was forming, leaving lighter elements like

oxygen, silicon, and nitrogen to rise to the surface. The solid inner core is about 1200

kilometers in radius, or about 10% the radius of the Earth.

The outer core is also mostly iron and nickel, but it’s liquid. The material in it can

flow. It’s about 2200 kilometers thick.

The temperature in the Earth’s core is tremendously high, reaching 5500° C. The pressure is huge

as well, as you might expect with the weight of an entire planet sitting on top of it.

You might think at such a high temperature, iron would be a liquid, but iron can stay

solid if the pressure is high enough. In the inner core, the pressure is extremely high,

and even though it’s hot, iron is solid. In the outer core, where it’s still hot,

but the pressure is a little bit lower, iron is a liquid.

Above the core is the mantle.It’s about 2900 kilometers thick. The consistency of

the mantle is weird; most people think it’s like lava, but really it’s like very thick

hot plastic. It behaves more or less like a solid, but over long periods of time, geologic

periods of time, it can flow. We’ll get back to that in a sec.

On top of the mantle is the crust, a solid layer of rock. The overall density of the

rock in the crust is less than in the mantle, so in a sense it floats on the mantle. There

are two types of crust on Earth: Oceanic crust, which is about 5 kilometers thick, and continental

crust, which is a much beefier 30-50 kilometers thick. Still, the crust is very thin compared to the other layers.

The crust isn’t a solid piece, though; it’s broken up into huge plates, and these can

move. What drives the movement of these plates is the flow of the rock in the mantle, and

that, in turn, is powered by heat.

The core of the Earth heats the bottom of the mantle. This causes convection; the warmer

material rises. It’s not exactly a speed demon, though: The rate of flow is only a couple

of centimeters per year, so it takes about 50 or 60 thousand years for a blob to move a single kilometer.

The hot material rises toward the surface, but it’s blocked by the crust. The magmatic

rock pushes on the plates, causing them to slide around very slowly. Your fingernails

grow at about the same rate the continents move. Over millions of years, though, this

adds up, changing the surface geography of the Earth—where you see continents now is

not at all where they were millions of years ago.

In some places, generally where the plates come together, the crust is weaker. Magma can push

its way through, erupting onto the surface, forming volcanoes. Other volcanoes, like Hawaii

or the Canary Islands, are thought to be from a plume of hotter material punching its way

right through the middle of a continental plate. As the plate moves, the hot spot forms

a linear chain of volcanoes over millions of years.

Volcanoes create new land as material wells out, but they also pump gas out of the Earth

too. A large part of Earth’s atmosphere was supplied from volcanoes!

The interior of the Earth is hot; in the core, it’s about as hot as the surface of the

Sun! Where did that heat come from?

Most of it is leftover from the Earth’s formation, more than 4.5 billion years ago.

As rock and other junk accumulated to form the proto-Earth, their collisions heated them

up. As the Earth grew that heat built up, and it’s still toasty inside even today.

Also, as the Earth formed and gained mass it began to contract under its own gravity,

and this squeezing added heat to the material.

Another source is elements like uranium deep inside the Earth, which add heat as the atoms

radioactively decay. And a fourth source of heat is from dense material like iron and

nickel sinking to the center of the Earth, which warms things up due to friction.

All of these things add up to a lot of heat, which is why, after all these billions of

years, the Earth still has a fiery heart.

The outer core of the Earth is liquid metal, which conducts electricity. The liquid convects,

and this motion generates magnetic fields, similar to the way plasma in the Sun generates

magnetic fields. The Earth’s rotation helps organize this motion into huge cylindrical

rolls that align with the Earth’s axis. The overall effect generates a magnetic field

similar to a bar magnet, with a magnetic north pole and south pole, which lie close to the

physical spin axis poles of the Earth.

The loops of magnetism surround the Earth, and play a very important role: They deflect

most of the charged particles from the solar wind, and they trap some, too. Without the

geomagnetic field, that solar wind would hit the Earth’s atmosphere directly. Over billions

of years, that would erode the Earth’s air away, like a sand blaster stripping paint

off a wall. Mars, for example, doesn’t have a strong magnetic field, and we think that’s

why its atmosphere is mostly gone today.

But we do have an atmosphere, and it’s more than just air blowing around. Earth’s atmosphere

is the layer of gas above the crust. Because it’s not solid, it doesn’t just stop,

it just sort of fades away with height. By accepted definition—and by that I mean it’s not

really science, it’s more of a “Eh, let’s just do it this way” kind of thing—the

line between Earth’s atmosphere and space is set at 100 kilometers up. This is what’s

called the Kármán line, and if you get above it, congratulations! You’re an astronaut.

The atmosphere is, by volume, about 78% nitrogen, 21% oxygen, 1% argon of all things, and then

an assortment of trace gases. There’s water vapor, too, almost all of it below a height

of about 8-15 kilometers. This part of the atmosphere is warmest at the bottom, which

means we get convection in the air, creating currents of rising air, which carry water

with them, forming clouds, which in turn is why we have weather.

At a height of about 25 kilometers on average is a layer of ozone, a molecule of oxygen

that’s good at absorbing solar ultraviolet light. That kind of light can break apart

biological molecules, so the ozone layer is critical for our protection.

Incidentally, the Earth’s magnetic field does more than trap solar wind particles;

it also channels some of them down into the atmosphere, where they slam into air molecules

about 150 kilometers up. This energizes the molecules, which respond by emitting light

in different colors: Nitrogen glows red and blue, oxygen red and green. We call this glow

the aurora, and it happens near the geomagnetic poles—far north and south. The lights can

form amazing ribbons and sheets, depending on the shape of the magnetic field.

I’ve never seen an aurora. Some day.

You may not be aware of the atmosphere unless the wind is blowing, but it’s there. It

exerts a pressure on the surface of the Earth of about a kilogram per square centimeter,

or nearly ten tons per cubic meter! There’s roughly ton of air pushing down on you right

now! You don’t feel it because it’s actually pushing in all directions—down, to the sides,

even up—and our bodies have an internal pressure that balances that out.

The Earth also has liquid water on its surface, unique among the planets. The continental

crust is higher than oceanic crust, so water flows down to fill those huge basins. The

Earth’s surface is about 70% covered in water. Most likely, some of this water formed

when the Earth itself formed, and some may have come from comet and asteroid impacts

billions of years ago. The exact proportion of locally sourced versus extraterrestrial

water is still a topic of argument among scientists.

Earlier, I mentioned trace molecules of gas in the atmosphere. One of these is carbon

dioxide, which only constitutes about 0.04% of the lower atmosphere. But it’s critical.

Sunlight heats the ground, which emits infrared light. If this infrared light were allowed

to radiate into space, the Earth would cool. But carbon dioxide traps that kind of light,

and the Earth doesn’t cool as efficiently.

This so-called greenhouse effect warms the Earth. Without it, the average temperature

on Earth would be below the freezing point of water! We’d be an iceball.

This is why climate scientists are concerned about carbon dioxide. A little is a good thing,

but too much can be very dangerous. Since the Industrial Revolution, we’ve added a

lot of the gas to our atmosphere, trapping more heat. By every measure available, the

heat content of the Earth is increasing, upsetting the balance. It’s melting glaciers in Antarctica

and Greenland, as well as sea ice at the north pole. Sea levels are going up, and some of

the extra CO2 in the air is absorbed by the oceans, acidifiying them.

There's an old concept in science fiction called terraforming: Going to uninhabitable alien

planet and engineering it to be more Earthlike. I don’t know what the opposite process would

be called, but it’s what we’re doing to Earth right now.

The Earth is the only habitable planet in the solar system. And you know what?

We should keep it that way.

Today you learned that the Earth is a planet, with a hot core, a thick layer of molten rock

called the mantle, and a thin crust. The outer core generates a strong magnetic field, which

protects the Earth’s atmosphere from the onslaught of the solar wind. Motion in the

mantle creates volcanoes, and the surface is mostly covered in water. The Earth’s

atmosphere is mostly nitrogen, and it’s getting warmer due to human influence.

Crash Course is produced in association with PBS Digital Studios. This episode was written by me,

Phil Plait. The script was edited by Blake de Pastino, and our consultant is Dr. Michelle Thaller. It was

co-directed by Nicholas Jenkins and Michael Aranda, and the graphics team is Thought Café.