Mantle

Below the crust is the mantle, which makes up over 80% of the earth's volume. It extends from the base of the crust to the outer core, about 2900 kilometers below the surface of the earth.

*Composition: The mantle consists of ultramafic silicate rock, rich in iron and magnesium. The chemical composition of the mantle remains relatively constant throughout, but it is so thick that it is subjected to a wide range of temperature and pressure. Although the composition by element of the mantle is nearly constant, the actual minerals that make up mantle rock change to denser forms as depth (along with temperature and pressure) increases.

Behavior: As depth increases, the physical properties of the mantle change, and so does its behavior. The mantle is divided into several layers by behavior. The mantle goes from rigid in the uppermost mantle (down to 100 km) to plastic and partially molten (only a very small percentage is actually molten) in the upper part of the lower mantle and back to being fairly rigid (but still plastic) in the lower mantle. The crust and rigid upper layer of the mantle together are called the lithosphere and it ranges in thickness from about 10 to 200 km. Rigid lithospheric plates "float" on a layer called the asthenosphere that flows like a very viscous fluid, like Silly Putty®. It is important to note that although the asthenosphere can flow, it is not a liquid, and thus both S- and P-waves can travel through it. At a depth of around 660 km, the pressure becomes so great that the mantle can no longer flow, and this solid part of the mantle is called the mesosphere. The lithospheric mantle, asthenosphere, and mesosphere all share the same composition (that of peridotite), but their mechanical properties are significantly different.

*How do we know? Information on the composition of the mantle comes from many sources:

1) Volcanic rocks: Rising magma commonly brings pieces of the mantle up to the surface without melting them.

2) Faults: In a handful of places around the world, large faults from mountain building brought upper mantle rocks up to the surface where we can examine them.

3) Seismic waves: Sensitive seismometers can tell us the velocity at which P-waves and S-waves travel through different parts of the crust. Because different rocks have different seismic properties, this information helps us confirm that the mantle varies little in its seismic properties and hence is likely to have few variations in composition.

4) Experimental rocks: Geologists using powerful anvils and ovens can create laboratory conditions approximating the high temperatures and pressures found in the deep mantle and core. By examining the seismic properties of suspected mantle compositions under these high temperatures and pressures, geologists have found that a mantle of nearly uniform composition provides a best fit to the seismic velocity data derived from earthquake studies.

Outer Core

At the core-mantle boundary, composition changes. Seismic waves suggest that the material making up the core has a very high density (10-13 g/cm3), which can only correspond to a composition of metals rather than rock. The presence of a magnetic field around Earth also indicates a molten metallic core. Unlike the crust and the mantle, we don't have any samples of the core to look at, and thus there is some controversy about its exact composition. Most scientists, however, believe that iron is the main component. The inner core is a crystalline solid with this composition, whereas the outer core is a liquid. Scientists think that movements within the liquid outer core cause the magnetic field of the Earth.

How do we know? We can't visit the center of the earth, so we have to find other ways to tell us what the core is really like. But don't worry; scientists are a creative bunch. The composition of the core is strongly based in both theory and observation.

1) Mass balancing: Astronomers have long known the total mass of the earth from calculations of the earth's orbit around the sun and its orbital interaction with the moon. Knowing also the size of the earth, simple division yields the average density of the earth's materials. Because we have observations and experimental laboratory data that tell us the average density of crust and mantle rocks, we can compare these with the whole earth density and find that the earth is too light! There must be a very heavy component deep inside the earth, and a core made of iron and nickel is just right for balancing the mass of the earth.

2) Seismic waves: Earthquake data regarding the velocity of seismic waves inside the core fit nicely with experimental laboratory data regarding the velocity of seismic waves in iron-nickel alloys under extremely high temperatures and pressures. Seismic waves also tell us that the core exists in the first place (because of the very strong change in seismic wave velocity that occurs at the core/mantle boundary).

3) Meteorites, Part I: Because we have much data suggesting that our solar system and the meteorites, asteroids, and planets within it all formed at the same time (about 4.55 billion years ago), we can get a good idea of the average composition of the entire earth by careful analysis of the sun (from a distance) and of meteorites (up close). Data from many meteorites suggest that the earth should be significantly richer in iron than the composition of our crust and mantle allow. Therefore, there should be a very iron-rich component deep inside the earth, and a core made of iron and nickel is just right for balancing the iron composition of the earth.

4) Meteorites, Part II: One common type of meteorite is the "iron" meteorite, which consists of metallic iron, with a minor amount of nickel and other trace elements. A picture of an iron meteorite with its distinctive crystal pattern is shown on the right side of the accompanying diagram. These crystals do not form in our laboratory experiments, and careful analysis and comparison with other crystals suggests that such crystals can only occur when the iron is very slowly cooled, under tremendous pressures, to the point at which these slow-growing crystals can appear. Geologists

hypothesize that iron meteorites represent rocks from the solid core of a (now missing) planet.

5) Orbital considerations: Also from astronomers come measurements that tell of earth's angular momentum. In a nutshell, these measurements tell us that the earth cannot consist of rocks that have a homogeneous density throughout - rather, the earth must have lighter rocks in the mantle and core and much more dense materials in the core. Experimental work with iron-nickel alloys shows that this iron-rich composition is just right for balancing earth's angular momentum.

How do we know the outer core is liquid?

Early seismologists discovered that seismic waves point unambiguously toward the presence of a liquid core inside the earth. But how did they figure that out?

*Shadow zones - S-waves do not pass through the core. Instead, S-waves are absorbed by liquid in the outer core. We know this because seismometers on the far side of the earth from an earthquake typically record only a P-wave and no corresponding S-wave. We call the far side of the earth from any large earthquake the "S-wave shadow zone" S-waves cannot travel through liquids or gases; thus the simplest interpretation is that the core is a liquid. Experimental data regarding the properties of a liquid iron core at those depths confirm that the iron-nickel mixture should indeed be molten at the core. Thus the absence of S-wave data on the far side of the earth gives us strong evidence for a liquid core in the middle.

*Shadow zones - P-waves: There is also a P-wave shadow zone, with a different shape from the S-wave shadow. Because the liquid outer core has a slower seismic velocity than the mantle above it, P-waves are bent, or refracted, as they pass into the core. When they come out again, the ray paths follow a different course, and this leaves a shadow zone where no P-waves reach the earth's surface. If the outer core were solid, a P-wave shadow zone (if there were one at all) would have a very different shape.

Inner Core

At the core-mantle boundary, composition changes. Seismic waves suggest that the material making up the core has a very high density (10-13 g/cm3), which can only correspond to a composition of metals rather than rock. The presence of a magnetic field around Earth also indicates a molten metallic core. Unlike the crust and the mantle, we don't have any samples of the core to look at, and thus there is some controversy about its exact composition. Most scientists, however, believe that iron is the main component. The inner core is a crystalline solid with this composition, whereas the outer core is a liquid. Scientists think that movements within the liquid outer core cause the magnetic field of the Earth.

How do we know? We can't visit the center of the earth, so we have to find other ways to tell us what the core is really like. But don't worry; scientists are a creative bunch. The composition of the core is strongly based in both theory and observation.

1) Mass balancing: Astronomers have long known the total mass of the earth from calculations of the earth's orbit around the sun and its orbital interaction with the moon. Knowing also the size of the earth, simple division yields the average density of the earth's materials. Because we have observations and experimental laboratory data that tell us the average density of crust and mantle rocks, we can compare these with the whole earth density and find that the earth is too light! There must be a very heavy component deep inside the earth, and a core made of iron and nickel is just right for balancing the mass of the earth.

2) Seismic waves: Earthquake data regarding the velocity of seismic waves inside the core fit nicely with experimental laboratory data regarding the velocity of seismic waves in iron-nickel alloys under extremely high temperatures and pressures. Seismic waves also tell us that the core exists in the first place (because of the very strong change in seismic wave velocity that occurs at the core/mantle boundary).

3) Meteorites, Part I: Because we have much data suggesting that our solar system and the meteorites, asteroids, and planets within it all formed at the same time (about 4.55 billion years ago), we can get a good idea of the average composition of the entire earth by careful analysis of the sun (from a distance) and of meteorites (up close). Data from many meteorites suggest that the earth should be significantly richer in iron than the composition of our crust and mantle allow. Therefore, there should be a very iron-rich component deep inside the earth, and a core made of iron and nickel is just right for balancing the iron composition of the earth.

4) Meteorites, Part II: One common type of meteorite is the "iron" meteorite, which consists of metallic iron, with a minor amount of nickel and other trace elements. A picture of an iron meteorite with its distinctive crystal pattern is shown on the right side of the accompanying diagram. These crystals do not form in our laboratory experiments, and careful analysis and comparison with other crystals suggests that such crystals can only occur when the iron is very slowly cooled, under tremendous pressures, to the point at which these slow-growing crystals can appear. Geologists

hypothesize that iron meteorites represent rocks from the solid core of a (now missing) planet.

5) Orbital considerations: Also from astronomers come measurements that tell of earth's angular momentum. In a nutshell, these measurements tell us that the earth cannot consist of rocks that have a homogeneous density throughout - rather, the earth must have lighter rocks in the mantle and core and much more dense materials in the core. Experimental work with iron-nickel alloys shows that this iron-rich composition is just right for balancing earth's angular momentum.

How do we know the inner core is solid?

Here's a puzzler! If the outer core is clearly a liquid iron-nickel mixture, then shouldn't the inner core of the earth be liquid, too? The inner core should be hotter, and therefore it should be a liquid, too, right?

*No: Actually, the pressure is so high in the inner core that an iron-nickel mixture is not stable as a liquid and instead crystallizes into a solid form, probably much like the meteorite sample with large crystals. So even though the inner core is hotter than the outer core, it is a solid, not a liquid.

*How do we know? An extra set of P-waves. Danish geophysicist Inge Lehman discovered this in the 1930s by painstaking observations of and attention to data that other geophysicists just ignored as "noise". Until this time, geophysicists thought that the entire core was a liquid. However, she found a second, very weak, P-wave following after the first one when looking at seismic waves that had passed nearly straight through the earth's center. She concluded that the core has an inner, solid part where S-waves can exist and that her weak P-waves represented a complex seismic

wave that was part P-wave and part S-wave. Her proposed ray path consists of a P-wave from the hypocenter of the earthquake that traveled first through the mantle and outer core, then traveled as a slower S-wave within the solid inner core, and then emerged as a P-wave again through the outer core, mantle, and crust to the seismometer on the other side of the earth. Some influential geophysicists thought this was too subtle a dataset to be meaningful (that's what they considered "noise" in the data, like static in a radio signal). But several others agreed with Lehman, and by the late 1930s her work defining the presence of a solid inner core was widely accepted. She published a paper with her conclusions in 1936 that was titled " P' " (pronounced, "P Prime"), which is one of the shortest titles on record for a scientific paper. (Note: P' is the geophysicists' shorthand for the seismic boundary between the outer and inner core.)

Crust

There are two major types of crust: crust that makes up the ocean floors and crust that makes up the continents. Oceanic crust is composed entirely of basalt extruded at mid-ocean ridges, resulting in a thin (~ 5 km), relatively dense (~3.0 g/cm3) crust. Continental crust, on the other hand, is made primarily of less dense rock such as granite (~2.7 g/cm3). It is much thicker than oceanic crust, ranging from 15 to 70 km. At the base of the crust is the Moho, below which is the mantle, which contains rocks made of a denser material called peridotite (~3.4 g/cm3). This compositional change is predicted by the behavior of seismic waves and it is confirmed in the few samples of rocks from the mantle that we do have.

*How do we know? Information on the composition of the deep crust comes from many sources:

1) Volcanic rocks: Rising magma commonly brings pieces of the deeper crust (and mantle) up to the surface without melting them.

2) Faults: During mountain building, faults commonly act to bring deeper portions of the crust (and, rarely, mantle) up to the surface.

3) Erosion: Old mountains, when eroded away after tens of millions of years, gradually expose rocks from deeper and deeper sections of the mountain core.

4) Seismic waves: Sensitive seismometers can tell us the velocity at which P-waves and S-waves travel through different parts of the crust. Because different rocks have different seismic properties, this information helps us confirm that the deeper continental crust is largely silica poor and metamorphosed, in contrast with the more variable upper continental crust.

The Moho

Andrija Mohorovičić was a Croatian scientist who recognized the importance of establishing a network of seismometers. He made careful observations of the arrivals of P- and S-waves at his newly-installed stations, and noticed that the P-waves  were measured at far away stations before they were measured by stations closer to the earthquake. Although these result seemed contradictory, they could be explained if the waves that arrived with faster velocities traveled through a medium that allowed them to speed up, having encountered a structural boundary at a greater depth.This recognition allowed Mohorovičić to define the first major boundary within Earth’s interior – the boundary between the crust, which forms the surface of Earth, and a denser layer below, called the mantle ). Seismic waves travel faster in the mantle than they do in the crust because it is composed of denser material. Thus, stations further away from the source of an earthquake received waves that had made part of their

journey through the denser rocks of the mantle. The waves that reached the closer stations stayed within the crust the entire time. The name of the crust-mantle boundary is the Mohorovičić discontinuity, but it is usually called the Moho (see the interactive animation below).

http://www.visionlearning.com/library/animations/Seismic_Wave/Seismic_Wave.html

https://ees.as.uky.edu/sites/default/files/elearning/module06swf.swf

Evidence from Seismic WavesAn earthquake occurs when rocks in a fault zone suddenly slip past each other, releasing stress that has built up over time. The slippage releases seismic energy, which is dissipated through two kinds of waves, P-waves and S-waves. The distinction between these two waves is easy to picture with a stretched-out Slinky®. If you push on one end, a compression wave passes through the Slinky® parallel to its length (see P-waves video). If instead you move one end up and down rapidly, a "ripple" wave moves through the Slinky® (see S-waves video). The compression waves are P-waves, and the ripple waves are S-waves.

Illustration of a P-wave/compression wave. Illustration of a S-wave/ripple wave.Both kinds of waves can reflect off of boundaries between different materials: They can also refract, or bend, when they cross a boundary into a different material. But the two types of waves behave differently depending on the composition of the material they are passing through. One of the biggest differences is that S-waves cannot travel through liquids whereas P-waves can. We feel the arrival of the P- and S-waves at a given location as a ground-shaking earthquake.

If Earth were the same composition all the way through its interior, seismic waves would radiate outward from their source (an earthquake) and behave exactly as other waves behave – taking longer to travel further and dying out in velocity and strength with distance, a process called attenuation. (See Figure 1.)

Figure 1: Seismic waves in an Earth of the same composition.

The Moho

Andrija Mohorovičić was a Croatian scientist who recognized the importance of establishing a network of seismometers. He made careful observations of the arrivals of P- and S-waves at his newly-installed stations, and noticed that the P-waves  were measured at far away stations before they were measured by stations closer to the earthquake. Although these result seemed contradictory, they could be explained if the waves that arrived with faster velocities traveled through a medium that allowed them to speed up, having encountered a structural boundary at a greater depth.This recognition allowed Mohorovičić to define the first major boundary within Earth’s interior – the boundary between the crust, which forms the surface of Earth, and a denser layer below, called the mantle ). Seismic waves travel faster in the mantle than they do in the crust because it is composed of denser material.

http://www.visionlearning.com/library/animations/Seismic_Wave/Seismic_Wave.html

https://ees.as.uky.edu/sites/default/files/elearning/module06swf.swf

Shadow zones

Another observation made by seismologists was the fact that P-waves die out about 105 degreesaway from an earthquake, and then reappear about 140 degrees away, arriving much later than expected. This region that lacks P-waves is called the P-wave shadow zone (Figure 2). S-waves, on the other hand, die out completely around 105 degrees from the earthquake (Figure 2). Remember that S-waves are unable to travel through liquid. The S-wave shadow zone indicates that there is a liquid layer deep within Earth that stops all S-waves but not the P-waves.

Figure 2:The P-wave and S-wave shadow zones.

In 1914, Beno Gutenberg, a German seismologist, used these shadow zones to calculate the size of another layer inside of the Earth, called its core. He defined a sharp core-mantle boundary at a depth of 2,900 km, where P-waves were refracted and slowed and S-waves were stopped.

The distinction between the inner and outer core was made in 1936 by Inge Lehmann, a Danish seismologist, after improvements in seismographs in the 1920s made it possible to "see" previously undetectable seismic waves within the P-wave shadow zone. These faint waves indicated that they had been refracted again within the core when they hit the boundary between the inner and outer core.

Deepest Hole ever

The world’s deepest hole tunnels miles into the Earth. However, we know more about certain distant galaxies than we do about what lies miles beneath our very own feet. For that reason, Soviet scientists in the 1970s decided to probe deeper than humanity has ever done before. For the next 24 years, they drilled on and off into the Earth’s crust. 

The result was the Kola Superdeep Borehole and a drill-depth of more than 7.5 miles (12 kilometers). To put that in perspective, Kola descends further than the deepest point of the ocean, which lies at nearly 6.8 miles (11 kilometers). The borehole is located on the Kola Peninsula of Russia. 

So did we learn anything from these decades of labor? Thankfully, yes! Scientists found microscopic fossils of single-celled organisms at 4.3 miles (7 kilometers) down. And at nearly the same depth,  they discovered water.  They also found that the temperature at the bottom of the hole reached a blistering 356°F (180°C). Too hot to continue, drilling officially halted in 1994. 

However, what’s even more impressive is that scientists estimate that the distance to the center of the Earth is nearly 4,000 miles (6,400 kilometers). Turns out, 7.5 miles barely scratches the surface. 

Mantle Holds Clues to Quakes and Earth Origins

Between Earth's molten core and hard, thin crust, the roughly 2,000-mile-thick (3,200-kilometer-thick) mantle contains the vast bulk of Earth's rocks. But we don't know much about them, because all we have are bits that have come to the surface via volcanoes  or been trapped in ancient mountain belts.

But all these mantle samples no longer really represent mantle conditions and makeup, since they've been altered in the long process of coming to the surface, so they providing only tantalizing glimpses of what lies below, scientists say.Drilling would tell scientists not only what the mantle is like, but also reveal the nature of the Moho layer, a shadowy transitional layer at the base of the crust."We know what the happens to seismic waves as they cross the Moho, but we don't know what it is," geologist Damon Teagle said.

Scientists would also be able to look for signs of life in the deep crustal rocks."Wherever we've looked, up to 120 °C (248 °F), we've seen evidence of microbial activity," Teagle said. "We would certainly test that on the way to the mantle." But the big prize is the mantle itself. Getting a sample, he said, would tell us much about the Earth's origins and history. Mantle rocks would also provide insight into how current mantle processes operate—highly important in understanding the plate tectonics that drive many earthquakes, tsunamis, and eruptions, he added.

Deep Ocean, Shallow CrustThe best place to drill, Teagle said, is in the mid-ocean, because that's where Earth's crust is thinnest—only about four miles (six kilometers) thick, versus tens of miles deep in continental regions. But the mid-ocean, is, of course, still deep—about 2.5 miles (4 kilometers) in the targeted areas. That's nearly twice the depth reachable by today's offshore drilling techniques, Teagle said. So far, drills have penetrated only about 1.2 miles (2 kilometers) into undersea crust.

And while the seabed is cold, the drill would have to be able to reach into a zone where temperatures would hit 570°F (300°C) and pressures would mount to 2,000 atmospheres—equivalent to more than 4 million pounds per square foot (21 million kilograms per square meter).

Composition of the Earth It may seem like the Earth is made up of one big solid rock, but it's really made up of a number of parts. Some of them constantly moving! You can think of the Earth as being made up of a number of layers, sort of like an onion. These layers get more and more dense the closer to the center of the earth you get. See the picture below to see the four main layers of the earth: the crust, mantle, outer core, and inner core. Crust The crust is the thin outer later of the Earth where we live. Well, it looks thin on the picture and it is thin relative to the other layers, but don't worry, we're not going to fall through by accident anytime soon. The crust varies from around 5km thick (in the ocean floor) to around 70km thick (on land where we live called the continental crust). The continental crust is made up of rocks that consist primarily of silica and alumina called the "sial". Mantle The next layer of the Earth is called the mantle. The mantle is much thicker than the crust at almost 3000km deep. It's made up of slightly different silicate rocks with more magnesium and iron. Tectonic plates The tectonic plates are a combination of the crust and the outer mantle, also called the lithosphere. These plates move very slowly, around a couple of inches a year. Where the plates touch each other is called a fault. When the plates move and the boundaries bump up against each other it can cause an earthquake. Outer Core The Earth's outer core is made up of iron and nickel and is very hot (4400 to 5000+ degrees C). This is so hot that the iron and nickel metals are liquid! The outer core is very important to earth as it creates something called a magnetic field. The magnetic field the outer core creates goes way out in to space and makes a protective barrier around the earth that shields us from the sun's damaging solar wind. Inner Core The Earth's inner core is made up of iron and nickel, just like the outer core, however, the inner core is different. The inner core is so deep within the earth that it's under immense pressure. So much pressure that, even though it is so hot, it is solid. The inner core is the hottest part of the Earth, and, at over 5000 degrees C, is about as hot as the surface of the sun. 

http://www.ducksters.com/science/composition_of_the_earth.php

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Structure of the Earth!Get ready to dig deep, gang, and join us on a fascinating journey to the center of the Earth!.

The secrets buried inside our planet are revealed by recording and studying things called seismic waves. Caused by things like earthquakes, explosions and the movement of our oceans, there are two types of seismic wave – a shear wave, which won't travel through liquid; and a pressure wave, which moves through both liquid and solids. These waves show that the Earth is made from five layers: the inner and outer core, the lower and upper mantle, and the crust. 

Inner CoreTemperature: 5,000°C - 6,000°CState: SolidComposition: iron and nickel

The Earth’s inner core is a huge metal ball, 2,500km wide. Made mainly of iron, the temperature of the ball is 5,000°C to 6,000°C – that’s up to 6,000 times hotter than our atmosphere and scorching enough to make metal melt! The metal at the inner core stays solid because of the incredible pressure surrounding it.

Outer CoreTemperature: 4,000°C – 6,000°CState: LiquidComposition: iron, nickel, sulphur and oxygen

This liquid layer of iron and nickel is 5,150km deep. The outer core flows around the centre of the Earth, and the movement of the metals creates our planet’s magnetic field.

Lower MantleTemperature: 3,000°CState: solidComposition: iron, oxygen, silicon, magnesium and aluminium The lower mantle is found between 670km and 2,890km below the surface, and is made from  solid rock. The rock is hot enough to melt, but is solid because of the pressure pushing down on it.

Upper MantleTemperature: 1,400°C – 3,000°CState: liquid / solidComposition: iron, oxygen, silicon, magnesium and aluminium This layer is up to 670km below the Earth’s surface. The lower part of the upper mantle is made from both solid and melted rock (liquid), while the rock in the upper region is stiffer, because it’s cooler.

CrustTemperature: Around 22°CState: Solid Composition: Oceanic crust made up of iron, oxygen, silicon, magnesium and aluminium.                        Continental crust made up of granite, sedimentary rocks and metamorphic rocks. The Earth's surface is covered by its thinnest layer, the crust. Land is made of continental crust, which is 8km to 70km thick and made mostly from a rock called granite. The layer beneath the ocean bed is made of oceanic crust, which is about 8km thick and made mainly from a rock called basalt.

How Do We Know?

If scientists have never studied any materials from a depth below 7 miles, then how is it that we know what is in the center of the Earth? How can we know what the core of the Earth is made of, if we have never seen it?

The answer is actually quite simple. While it is true that we can not study the Earth’s core using visible light, we can study it using other senses. The most important thing we use to sense the Earth’s core are seismic waves. Seismic waves are waves of energy caused either by earthquakes, or by massive manmade explosions. 

Scientists are able to measure these waves as they pass through the Earth. As these waves encounter different materials, they change in important ways, becoming longer, shorter, faster, or slower. Geologists study these changes in the waves, and are able to draw conclusions about what the core of the Earth must look like.

Geologists also can learn a lot about the core of our planet by looking at Earth’s magnetic field. The Magnetic field is created by massive circulations of hot liquid mantle beneath the Earth’s surface. These clues lead geologists to believe that the Earth is made of four distinct layers. These layers are the crust, the mantle, the outer core, and the inner core.

Atmosphere

The atmosphere is the air that is wrapped all around a planet. Not all planets have atmospheres. In order to have an atmosphere, the planet has to have enough gravity to hold on to light atoms like hydrogen and helium and keep them from floating away into space. That means that the planet has to have a lot of mass.

Because the force of gravity is stronger near the planet and gets weaker as you get further away, the atmosphere is thicker close to the ground and gradually gets thinner as you go further out into space. There is no sharp edge to the atmosphere. An altitude of 120 km (75 mi) is where atmospheric effects become noticeable during atmospheric reentry of spacecraft. The Kármán line, 62 miles above the Earth is often regarded as the boundary between atmosphere and outer space

How was the atmosphere formed?

When the Earth first formed, about four and a half billion years ago, Earth's atmosphere was almost entirely made of hydrogen and helium atoms, because they were the lightest atoms and floated to the top. But the Earth was still so hot, and the Sun heated the Earth so much, that most of the hydrogen and helium atoms ended up drifting off into space.

Soon after that, about 4.4 billion years ago, the Earth cooled down a lot. But there were still a lot of volcanoes that shot out steam, carbon dioxide, and ammonia. This created a new atmosphere made mostly of carbon dioxide and water, with some nitrogen.

Then about three billion years ago, early prokaryotic cells, one of the earliest forms of life on Earth, began to use photosynthesis to get food for themselves.

They made their food out of what was available - sunlight, carbon dioxide, and water. And they excreted (pooped out) what they didn't need - mainly oxygen. At first most of these oxygen atoms bonded with other atoms to form molecules, like this iron that has turned red by combining with oxygen.

After almost a billion years of millions of cells shooting out oxygen, everything that oxygen could join with had enough oxygen, and the leftovers began to pile up in the atmosphere. Quickly there got to be a lot of oxygen in the atmosphere, or in the air. By 2.2 billion years ago, the atmosphere was about 20 percent oxygen. We can see this early oxygen in old rocks, where about three billion years ago the iron in the rocks begins

to be red from combining with the oxygen in the air (rusting). All this oxygen poisoned many kinds of early cells and they died off. But other cells evolved to use oxygen for their energy, and these cells also benefited from the development of an ozone layer, a layer of one kind of oxygen high in the atmosphere that keeps most of the Sun's ultraviolet light from reaching the Earth and causing sunburns.

Since that time, the levels of these atoms in the air have changed from time to time, even though the atmosphere has continued to have a lot of oxygen in it. About 500 million years ago, the atmosphere was about 7 percent carbon dioxide. That was good for plants. About 300 million years ago, carbon dioxide was about where it is now, less than 1 percent. At about the same time, oxygen went up to about 35% of the atmosphere. Then carbon dioxide went up again, so that about 100 million years ago, the atmosphere was about 3% carbon dioxide. Big forests of giant ferns grew up because of all the carbon dioxide in the air, and the Earth got so warm that dinosaurs could live near the South Pole. Oxygen levels have also gone up and down over the last two billion years, though we don't understand the changes as well.

Today, Earth's atmosphere, or air, is about 78 percent nitrogen (mostly from the ammonia shot out by volcanoes), 21 percent oxygen (from photosynthesizing cells, mainly one-celled algae in the ocean), and less than one percent each of argon, carbon dioxide, and water. But because people are burning so much oil and coal that are made of carbon, we are releasing a lot more carbon dioxide into the air, and the percentage of carbon dioxide is going up. Right now the percentage of carbon dioxide is higher than it has been any time in the last 650,000 years (since the earliest people were beginning to leave Africa and travel around the Earth), and it is still going up quickly. This will cause the Earth to get warmer. Nobody knows exactly what this will mean for plants and animals on Earth, or for us.

The atmosphere protects life on Earth by absorbing ultraviolet solar radiation, warming the surface by trapping heat (greenhouse effect), and reducing temperature extremes between day and night. Mercury, a planet with practically no atmosphere has extreme temperature differences throughout the plant. Sometimes the difference is up to 700

degrees! Earth’s atmosphere makes it so the half of the planet that isn’t facing the sun doesn’t freeze solid while the other half burns up.

The atmosphere has distinct layers, each with specific characteristics such as temperature or composition. The layers are of the atmosphere are from bottom up: Troposphere, Stratosphere (which includes the ozone layer), Mesosphere, Thermosphere, Exosphere and Magnetosphere.

How was the Earth formed?

Most of the molecules that smashed together to make the Earth were iron - the smashing together released a lot of heat, so the whole center of the Earth is made of hot melted iron. Iron is heavier than most other metals, so it sank to the center of the Earth, while lighter molecules like silicon and carbon rose to the top. At the outermost part of the Earth were the lightest molecules, hydrogen, helium, and nitrogen, making our atmosphere. Most of the hydrogen and helium was so light that it floated back out into space, so more than three-quarters of our air is made of nitrogen. Perhaps about the same time, because Jupiter's gravity was messing with their orbits, a lot of comets made of ice smashed into the Earth. When they reached Earth, the ice melted into water, and the water boiled to make a steamy atmosphere. Ultraviolet light from the sun (the rays that give you sunburns now) hit the water molecules and broke them apart into hydrogen and oxygen. A layer of oxygen (the ozone layer) formed and blocked most of the ultraviolet rays. Along with the comets, in the ice, came amino acids that had formed in space out of carbon and hydrogen atoms.

By about three and a half billion years ago, long strings of amino acids began cooperating to make the earliest living cells. Soon these cells began cooperating with other cells, uniting to form more complicated cells that could use photosynthesis to make energy out of sunlight and carbon dioxide. These cells broke apart the carbon dioxide and used the energy to eat and the carbon to repair and reproduce themselves, but they didn't want the oxygen. So the atmosphere gradually got more and more oxygen molecules in it. By around 2,500,000,000 years ago, there was so much oxygen that some cells began to use oxygen for energy instead of carbon dioxide, and by about 1,500,000,000 years ago, plants and animals with more than one cell began to develop.

How do we know? We weren’t around to see a lot of this happening, so here is some evidence. 1) Seismic waves: Earthquake data regarding the velocity of seismic waves inside the core fit nicely with experimental laboratory data regarding the velocity of seismic waves in iron-nickel alloys under extremely high temperatures and pressures. Seismic waves also tell us that the core exists in the first place (because of the very strong change in seismic wave velocity that occurs at the core/mantle boundary).

2) Meteorites, Part I: Because we have much data suggesting that our solar system and the meteorites, asteroids, and planets within it all formed at the same time (about 4.55 billion years ago), we can get a good idea of the average composition of the entire earth by careful analysis of the sun (from a distance)

and of meteorites (up close). Data from many meteorites suggest that the earth should be significantly richer in iron than the composition of our crust and mantle allow. Therefore, there should be a very iron-rich component deep inside the earth, and a core made of iron and nickel is just right for balancing the iron composition of the earth.

3) By 2.2 billion years ago, the atmosphere was about 20 percent oxygen. We can see this early oxygen in old rocks, where about three billion years ago the iron in the rocks begins to be red from combining with the oxygen in the air (rusting).

4) Since that time, the levels of these atoms in the air have changed from time to time, even though the atmosphere has continued to have a lot of oxygen in it. The amount of carbon dioxide and oxygen levels have gone up and down over the last two billion years. We can tell this from the fossils we find. When the amount of carbon dioxide was higher we find huge fossil plants that couldn’t live today.

Why is the earth's core so hot? And how do scientists measure its temperature?

Quentin Williams, associate professor of earth sciences at the University of California at Santa Cruz offers this explanation:

There are three main sources of heat in the deep earth: (1) heat from when the planet formed and accreted, which has not yet been lost; (2) frictional heating, caused by denser core material sinking to the center of the planet; and (3) heat from the decay of radioactive elements.

It takes a rather long time for heat to move out of the earth. This occurs through both "convective" transport of heat within the earth's liquid outer core and solid mantle and slower "conductive" transport of heat through nonconvecting boundary layers, such as the earth's plates at the surface. As a result, much of the planet's primordial heat, from when the earth first accreted and developed its core, has been retained.

The amount of heat that can arise through simple accretionary processes, bringing small bodies together to form the proto-earth, is large: on the order of 10,000 kelvins (about 18,000 degrees Farhenheit). The crucial issue is how much of that energy was deposited into the growing earth and how much was reradiated into space. Indeed, the currently accepted idea for how the moon was formed involves the impact or accretion of a Mars-size object with or by the proto-earth. When two objects of this size collide, large amounts of heat are generated, of which quite a lot is retained. This single episode could have largely melted the outermost several thousand kilometers of the planet.

Additionally, descent of the dense iron-rich material that makes up the core of the planet to the center would produce heating on the order of 2,000 kelvins (about 3,000 degrees F). The magnitude of the third main source of heat--radioactive heating--is uncertain. The precise abundances of radioactive elements (primarily potassium, uranium and thorium) are is poorly known in the deep earth.

In sum, there was no shortage of heat in the early earth, and the planet's inability to cool off quickly results in the continued high temperatures of the Earth's interior. In effect, not only do the earth's plates act as a blanket on the interior, but not even convective heat transport in the solid mantle provides a particularly efficient mechanism for heat loss. The planet does lose some heat through the processes that drive plate tectonics, especially at mid-ocean ridges. For comparison, smaller bodies such as Mars and the Moon show little evidence for recent tectonic activity or volcanism.

How do we know?:We derive our primary estimate of the temperature of the deep earth from the melting behavior of iron at ultrahigh pressures. We know that the earth's core depths from 2,886

kilometers to the center at 6,371 kilometers (1,794 to 3,960 miles), is predominantly iron, with some contaminants. How? The speed of sound through the core (as measured from the velocity at which seismic waves travel across it) and the density of the core are quite similar to those seen in of iron at high pressures and temperatures, as measured in the laboratory. Iron is the only element that closely matches the seismic properties of the earth's core and is also sufficiently abundant present in sufficient abundance in the universe to make up the approximately 35 percent of the mass of the planet present in the core.

The earth's core is divided into two separate regions: the liquid outer core and the solid inner core, with the transition between the two lying at a depth of 5,156 kilometers (3,204 miles). Therefore, If we can measure the melting temperature of iron at the extreme pressure of the boundary between the inner and outer cores, then this lab temperature should reasonably closely approximate the real temperature at this liquid-solid interface. Scientists in mineral physics laboratories use lasers and high-pressure devices called diamond-anvil cells to re-create these hellish pressures and temperatures as closely as possible.’

Those experiments provide a stiff challenge, but our estimates for the melting temperature of iron at these conditions range from about 4,500 to 7,500 kelvins (about 7,600 to 13,000 degrees F). As the outer core is fluid and presumably convecting (and with an additional correction for the presence of impurities in the outer core), we can extrapolate this range of temperatures to a temperature at the base of Earth's mantle (the top of the outer core) of roughly 3,500 to 5,500 kelvins (5,800 to 9,400 degrees F) at the base of the earth's mantle.

The bottom line here is simply that a large part of the interior of the planet (the outer core) is composed of somewhat impure molten iron alloy. The melting temperature of iron under deep-earth conditions is high, thus providing prima facie evidence that the deep earth is quite hot.

Gregory Lyzenga is an associate professor of physics at Harvey Mudd College. He provided some additional details on estimating the temperature of the earth's core:How do we know the temperature? The answer is that we really don't--at least not with great certainty or precision. The center of the earth lies 6,400 kilometers (4,000 miles) beneath our feet, but the deepest that it has ever been possible to drill to make direct measurements of temperature (or other physical quantities) is just about 10 kilometers (six miles).

Ironically, the core of the earth is by far less accessible more inaccessible to direct probing than would be the surface of Pluto. Not only do we not have the technology to "go to the core," but it is not at all clear how it will ever be possible to do so.As a result, scientists must infer the temperature in the earth's deep interior indirectly. Observing the speed at which of passage of seismic waves pass through the earth allows geophysicists to determine the density and stiffness of rocks at depths inaccessible to direct examination. If it is possible to match up those properties with the

properties of known substances at elevated temperatures and pressures, it is possible (in principle) to infer what the environmental conditions must be deep in the earth.The problem with this is that the conditions are so extreme at the earth's center that it is very difficult to perform any kind of laboratory experiment that faithfully simulates conditions in the earth's core. Nevertheless, geophysicists are constantly trying these experiments and improving on them, so that their results can be extrapolated to the earth's center, where the pressure is more than three million times atmospheric pressure.

The bottom line of these efforts is that there is a rather wide range of current estimates of the earth's core temperature. The "popular" estimates range from about 4,000 kelvins up to over 7,000 kelvins (about 7,000 to 12,000 degrees F).

If we knew the melting temperature of iron very precisely at high pressure, we could pin down the temperature of the Earth's core more precisely, because it is largely made up of molten iron. But until our experiments at high temperature and pressure become more precise, uncertainty in this fundamental property of our planet will persist.

Deepest Hole ever

The world’s deepest hole tunnels miles into the Earth. However, we know more about certain distant galaxies than we do about what lies miles beneath our very own feet. For

that reason, Soviet scientists in the 1970s decided to probe deeper than humanity has ever done before. For the next 24 years, they drilled on and off into the Earth’s crust. 

The result was the Kola Superdeep Borehole and a drill-depth of more than 7.5 miles (12 kilometers). To put that in perspective, Kola descends further than the deepest point of the ocean, which lies at nearly 6.8 miles (11 kilometers). The borehole is located on the Kola Peninsula of Russia. 

So did we learn anything from these decades of labor? Thankfully, yes! Scientists found microscopic fossils of single-celled organisms at 4.3 miles (7 kilometers) down. And at nearly the same depth,  they discovered water.  They also found that the temperature at the bottom of the hole reached a blistering 356°F (180°C). Too hot to continue, drilling officially halted in 1994. 

However, what’s even more impressive is that scientists estimate that the distance to the center of the Earth is nearly 4,000 miles (6,400 kilometers). Turns out, 7.5 miles barely scratches the surface. 

Why is the earth's core so hot? And how do scientists measure its temperature?There are three main sources of heat in the deep earth: (1) heat from when the planet formed, which has not yet been lost; (2) frictional heating, caused by denser core material sinking to the center of the planet; and (3) heat from the decay of radioactive elements.

It takes a long time for heat to move out of the earth. As a result, much of the planet's original heat, from when the earth first formed, has been retained.

The amount of heat that can arise as planets form is large: on the order of 10,000 kelvins (about 18,000 degrees Farhenheit). Additionally, sinking of the dense iron-rich material that makes up the core of the planet to the center

would produce heating on the order of 2,000 kelvins (about 3,000 degrees F). The amount of the third main source of heat--radioactive heating--is uncertain. The precise amounts of radioactive elements (primarily potassium, uranium and thorium) are is poorly known in the deep earth.

How do we know?We get our main estimate of the temperature of the deep earth from the melting behavior of iron at really high pressures. If we can measure the melting temperature of iron under extreme pressure in the lab, then we can infer what the temperature of the core must be. Scientists in mineral physics laboratories use lasers and high-pressure devices called diamond-anvil cells to re-create these intense pressures and temperatures as closely as possible.

Gregory Lyzenga is an associate professor of physics at Harvey Mudd College, and he said, “How do we know the temperature? The answer is that we really don't--at least not with great certainty or precision. The center of the earth lies 6,400 kilometers (4,000 miles) beneath our feet, but the deepest that it has ever been possible to drill to make direct measurements of temperature (or other physical quantities) is just about 10 kilometers (six miles). Not only do we not have the technology to "go to the core," but it is not at all clear how it will ever be possible to do so.”

As a result, scientists have to gather indirect evidence. Geophysicists use seismic waves to determine the density and stiffness of rock deep in the Earth. Then they do experiments to try to imitate those conditions, and see what the temperature is like. The problem is that the conditions are so intense that it is really hard to do accurate experiments.

So we don’t know exactly how hot the Earth is. The "popular" estimates range from about 4,000 kelvins up to over 7,000 kelvins (about 7,000 to 12,000 degrees F). If we knew the melting temperature of iron very precisely at high pressure, we could pin down the temperature of the Earth's core more precisely, because it is largely made up of

molten iron. But until our experiments at high temperature and pressure become more precise, uncertainty in this fundamental property of our planet will persist.

Atmosphere

The atmosphere is the air that is wrapped all around a planet. Not all planets have atmospheres. In order to have an atmosphere, the planet has to have enough gravity to hold on to light atoms like hydrogen and helium and keep them from floating away into space. That means that the planet has to have a lot of mass.

Because the force of gravity is stronger near the planet and gets weaker as you get further away, the atmosphere is thicker close to the ground and gradually gets thinner as you go further out into space.

How was the atmosphere formed?

When the Earth first formed, about four and a half billion years ago, Earth's atmosphere was almost entirely made of hydrogen and helium atoms, because they were the lightest atoms and floated to the top. But the Earth was still so hot, and the Sun heated the Earth so much, that most of the hydrogen and helium atoms ended up drifting off into space.

Soon after that, about 4.4 billion years ago, the Earth cooled down a lot. But there were still a lot of volcanoes that shot out steam, carbon dioxide, and ammonia. This created a new atmosphere made mostly of carbon dioxide and water, with some nitrogen.

Then about three billion years ago, early cells began to use photosynthesis to get food for themselves, and this process makes oxygen. At first most of

these oxygen atoms bonded with other atoms to form molecules, like this iron that has turned red by combining with oxygen.

After almost a billion years of millions of cells shooting out oxygen, everything that oxygen could join with had enough oxygen, and the leftovers began to pile up in the atmosphere. Quickly there got to be a lot of oxygen in the atmosphere, or in the air. By 2.2 billion years ago, the atmosphere was about 20 percent oxygen. We can see this early oxygen in old rocks, where about three billion years ago the iron in the rocks begins to be red from combining with the oxygen in the air (rusting). All this oxygen poisoned many kinds of early cells and they died off. But other cells evolved to use oxygen for their energy, and

these cells also benefited from the development of an ozone layer, a layer of one kind of oxygen high in the atmosphere that keeps most of the Sun's ultraviolet light from reaching the Earth and causing sunburns.

Since that time, the levels of these atoms in the air have changed from time to time, even though the atmosphere has continued to have a lot of oxygen in it. The amount of carbon dioxide and oxygen levels have gone up and down over the last two billion years. We can tell this from the fossils we find.

Today, Earth's atmosphere, or air, is about 78 percent nitrogen (mostly from the ammonia shot out by volcanoes), 21 percent oxygen (from photosynthesizing cells, mainly one-celled algae in the ocean), and less than one percent each of argon, carbon dioxide, and water. But because people are burning so much oil and coal that are made of carbon, we are releasing a lot more carbon dioxide into the air, and the percentage of carbon dioxide is going up. Right now the percentage of carbon dioxide is higher than it has been any time in the last 650,000 years (since the earliest people were beginning to leave Africa and travel around the Earth), and it is still going up quickly. This will cause the Earth to get warmer. Nobody knows exactly what this will mean for plants and animals on Earth, or for us.

The atmosphere protects life on Earth by absorbing ultraviolet solar radiation, warming the surface by trapping heat (greenhouse effect), and reducing temperature extremes between day and night. Mercury, a planet with practically no atmosphere has extreme temperature differences throughout the plant. Sometimes the difference is up to 700

degrees! Earth’s atmosphere makes it so the half of the planet that isn’t facing the sun doesn’t freeze solid while the other half burns up.

The atmosphere has distinct layers, each with specific characteristics such as temperature or composition. The layers are of the atmosphere are from bottom up: Troposphere, Stratosphere (which includes the ozone layer), Mesosphere, Thermosphere, Exosphere and Magnetosphere.