# What is it that distinguishes one atmospheric layer from another?

The atmosphere of a planetary body (assuming it has an atmosphere) is described as being made up of distinct layers.

For example, Earth, Saturn and Jupiter all have a stratosphere and a troposphere.

What is it that defines where one layer stops and another starts?

What is it that defines where one layer stops and another starts?

Temperature. More specifically, it's whether temperature rises or falls with increasing altitude.

In the troposphere, temperature generally decreases with increasing altitude, at an average rate of 6.4 °C/km (the environmental lapse rate). This decrease stops at the tropopause, the boundary between the troposphere and the stratosphere. The ozone layer is in the stratosphere, making temperatures in the stratosphere increase with increasing altitude. While this boundary is a bit fuzzy, it is still very real. It takes an incredibly strong thunderstorm (think hurricanes, storms powerful enough to spawn tornados, and very tall and strong storms in the Inter-Tropical Convergence Zone) to penetrate that boundary.

Temperatures in the stratosphere stop rising at the stratopause, the very fuzzy boundary between the stratosphere and the mesosphere. Like the troposphere, temperatures in the mesosphere fall (and fall very sharply) with increasing altitude. Thermodynamics makes the boundary between the stratosphere and mesosphere very different (and not nearly as clear) as the boundary between the troposphere and the mesosphere.

The boundary between the mesosphere and the thermosphere is similar to the boundary between the troposphere and stratosphere. Temperatures rise with increasing altitude in the thermosphere. This change from falling temperatures in the mesosphere to rising temperatures in the thermosphere makes for a rather stable boundary.

Something else happens near that boundary between the mesosphere and the thermosphere. Long-lived gases are fairly well-mixed in the troposphere, stratosphere, and mesosphere thanks to turbulence. Gases in the thermosphere and exosphere act more like a bunch of individual particles rather than a gas. Unlike the dense layers below, gases in the thermosphere and exosphere are differentiated. The makeup tends toward lighter and lighter particles (e.g., helium and hydrogen) with increasing altitude. Eventually, all one finds are hydrogen and helium. These are the gases that escape from the atmosphere.

The problematic notion here is the "distinct" layers, those effectively don't exist. Earth's atmosphere is a continuum, as any atmosphere is.

This stems from the hydrostatic pressure law $abla P = - g ho$, that gives together with the ideal gas law $P = ho k_B T / mu$ an exponentially decaying solution with height, for pressure as well as for density. Here P is the pressure, $ho$ is the local mass-density, g is the local gravitational acceleration, $k_B$ the Boltzmann constant and $T$ the temperature.

However there are some caveats to "layers" as a manner of characterizing the atmosphere:

• An inversion layer can be a distinct layer where the temperature decrease with height is inversed. Thus the temperature increases for a bit, then decreases again. Therefore there will be exactly two points where the temperature increase is 0. Between those two points the definition of a distinct layer makes sense.
• In a similar way the whole atmospheric temperature on a global scale has a funny height-dependency (see e.e. here). Again one can define layers between the points where something interesting in the T-profile happens. This is where the characterization into troposphere, stratosphere, etc. stems from.

Now we can choose variables other than temperature to characterize the atmosphere. Like ionization state, mixture state, dynamical movements, chemical makeup and then define layers that make sense only when talking about this particular variable.

Summarizing: Layers can be defined, even physically meaningful, but only in the context of a particular variable.

The stratosphere and the troposphere are defined by the variation in temperature with height.

Within a troposphere, the temperature drops as altitude increases, and drops fast enough for convection to occur. Whereas within a stratosphere, the temperature rises with increasing altitude. Since convection requires that the rate of temperature cooling with height (the "lapse rate") exceeds the rate at which a gas cools due to a reduction in pressure, in the stratosphere the atmosphere will be stable. Tropos means "turning" and stratos means layered. The stratosphere is warmed by the absorbtion of solar radiation, particularly UV radiation (in the ozone layer), whereas the troposphere is warmed mostly from the ground, or in the case of Jupiter, from Jupiter's internal heat.

The result is that in troposphere the atmosphere will be well mixed, and clouds will form, whereas in a stratosphere there will be less mixing and far fewer clouds.

In Jupiter the tropopause (the point of lowest temperature) occurs at a pressure of about 0.1 bars. (source)

## Science Made Simple: Earth’s Upper Atmosphere

Credit: NASA/Noctilucent Clouds, Jan Erik Paulsen Barrel Image, NASA / NSF.

The Earth’s atmosphere has four primary layers: the troposphere, stratosphere, mesosphere, and thermosphere. These layers protect our planet by absorbing harmful radiation.

Thermosphere 53–375 Miles – In the thermosphere, molecules of oxygen and nitrogen are bombarded by radiation and energetic particles from the Sun, causing the molecules to split into their component atoms and creating heat. The thermosphere increases in temperature with altitude because the atomic oxygen and nitrogen cannot radiate the heat from this absorption.

Mesosphere 31–53 Miles – Studying the mesosphere is essential to understanding long-term changes in the Earth’s atmosphere and how these changes affect climate. Since the mesosphere is responsive to small changes in atmospheric chemistry and composition, it could provide clues for scientists, such as how added greenhouse gases may contribute to a change in temperature or water composition in the atmosphere.

Stratosphere 10–31 Miles – The ozone layer lies within the stratosphere and absorbs ultraviolet radiation from the Sun.

Troposphere 0–10 Miles – The troposphere is the layer of the Earth’s atmosphere where all human activity takes place.

Ionosphere – The ionosphere is a layer of plasma formed by the ionization of atomic oxygen and nitrogen by highly energetic ultraviolet and x-ray solar radiation. The Ionosphere extends from the middle of the mesosphere up to the magnetosphere. This layer cycles daily as the daytime exposure to solar radiation causes the ionization of the atoms that can extend down as far as the mesosphere. However, these upper atmospheric layers are still mostly neutral, with only one in a million particles becoming charged daily. At night, the ionosphere mostly collapses as the Sun’s radiation ceases to interact with the atoms in the thermosphere. There are still small amounts of charged atoms caused by cosmic radiation.

Communication – A unique property of the ionosphere is that it can refract shortwave radio waves, enabling communication over great distances by “bouncing” signals off this ionized atmospheric layer. Variability of the ionosphere can interrupt satellite communication, such as errors in GPS signals for commercial air navigation. During solar storms, this layer can even shut down communication between ground stations and satellites.

Rockets, Balloons, and Satellites – NASA scientists use balloons to collect in-situ measurements in the atmosphere. However, the mesosphere and thermosphere are too high for balloons to reach, so scientists use instruments on sounding rockets and satellites to gather more detailed measurements of the upper atmosphere.

Noctilucent Clouds in the Mesosphere – Evidence of change in the behavior of noctilucent clouds has been observed by the AIM mission. Recent data show dramatically lower ice content, leading scientists to speculate about changes in weather conditions and pole-to-pole atmospheric circulation.

Aeronomy of Ice in the Mesosphere (AIM) – NASA’s AIM satellite can remotely sense night-shining clouds in the mesosphere. These noctilucent clouds are made of ice crystals that form over the summer poles at an altitude too high and a temperature too cold for water-vapor clouds.

BARREL – The Balloon Array for Radiation-belt Relativistic Electron Losses (BARREL) is a balloon-based mission to augment the measurements of NASA’s RBSP spacecraft. BARREL seeks to measure the precipitation of relativistic electrons from the radiation belts during two multi-balloon campaigns operated in the Southern Hemisphere.

## Gases in Earth's Atmosphere

Nitrogen and oxygen are by far the most common dry air is composed of about 78% nitrogen (N2) and about 21% oxygen (O2). Argon, carbon dioxide (CO2), and many other gases are also present in much lower amounts each makes up less than 1% of the atmosphere's mixture of gases. The atmosphere also includes water vapor. The amount of water vapor present varies a lot, but on average is around 1%. There are also many small particles - solids and liquids - "floating" in the atmosphere. These particles, which scientists call "aerosols", include dust, spores and pollen, salt from sea spray, volcanic ash, smoke, and more.

## 2. Stratosphere

If we start from the top of the troposphere and go further into the sky, we reach the layer known as the stratosphere.

If we start from the top of the troposphere and go further into the sky, we reach the layer known as the stratosphere. This layer goes up around 50 km above the Earth’s ground. In this layer, the temperature rises as you go further up, and it has something to do with the ozone layer that is found inside the stratosphere.

The ozone layer serves a vital role in the protection of our planet, as the molecules of ozone prevent ultraviolet light from the Sun to hit our planet without stopping. The UV light is not technically stopped, but the conversion from UV light to heat happens (which is why holes in the ozone layer are so dangerous).

## Weather and Climate

Figure 2. Storm from Space: This satellite image shows Hurricane Irene in 2011, shortly before the storm hit land in New York City. The combination of Earth’s tilted axis of rotation, moderately rapid rotation, and oceans of liquid water can lead to violent weather on our planet. (credit: NASA/NOAA GOES Project)

All planets with atmospheres have weather, which is the name we give to the circulation of the atmosphere. The energy that powers the weather is derived primarily from the sunlight that heats the surface. Both the rotation of the planet and slower seasonal changes cause variations in the amount of sunlight striking different parts of Earth. The atmosphere and oceans redistribute the heat from warmer to cooler areas. Weather on any planet represents the response of its atmosphere to changing inputs of energy from the Sun (see Figure 2 for a dramatic example).

Climate is a term used to refer to the effects of the atmosphere that last through decades and centuries. Changes in climate (as opposed to the random variations in weather from one year to the next) are often difficult to detect over short time periods, but as they accumulate, their effect can be devastating. One saying is that “Climate is what you expect, and weather is what you get.” Modern farming is especially sensitive to temperature and rainfall for example, calculations indicate that a drop of only 2 °C throughout the growing season would cut the wheat production by half in Canada and the United States. At the other extreme, an increase of 2 °C in the average temperature of Earth would be enough to melt many glaciers, including much of the ice cover of Greenland, raising sea level by as much as 10 meters, flooding many coastal cities and ports, and putting small islands completely under water.

The best documented changes in Earth’s climate are the great ice ages, which have lowered the temperature of the Northern Hemisphere periodically over the past half million years or so (Figure 3). The last ice age, which ended about 14,000 years ago, lasted some 20,000 years. At its height, the ice was almost 2 kilometers thick over Boston and stretched as far south as New York City.

Figure 3. Ice Age: This computer-generated image shows the frozen areas of the Northern Hemisphere during past ice ages from the vantage point of looking down on the North Pole. The area in black indicates the most recent glaciation (coverage by glaciers), and the area in gray shows the maximum level of glaciation ever reached. (credit: modification of work by Hannes Grobe/AWI)

These ice ages were primarily the result of changes in the tilt of Earth’s rotational axis, produced by the gravitational effects of the other planets. We are less certain about evidence that at least once (and perhaps twice) about a billion years ago, the entire ocean froze over, a situation called snowball Earth.

The development and evolution of life on Earth has also produced changes in the composition and temperature of our planet’s atmosphere, as we shall see in the next section.

### Key Concepts and Summary

The atmosphere has a surface pressure of 1 bar and is composed primarily of N2 and O2, plus such important trace gases as H2O, CO2, and O3. Its structure consists of the troposphere, stratosphere, mesosphere, and ionosphere. Changing the composition of the atmosphere also influences the temperature. Atmospheric circulation (weather) is driven by seasonally changing deposition of sunlight. Many longer term climate variations, such as the ice ages, are related to changes in the planet’s orbit and axial tilt.

The atmosphere can be divided into layers based on its temperature, as shown in the figure below. These layers are the troposphere, the stratosphere, the mesosphere and the thermosphere. A further region, beginning about 500 km above the Earth's surface, is called the exosphere.

### The Troposphere

This is the lowest part of the atmosphere - the part we live in. It contains most of our weather - clouds, rain, snow. In this part of the atmosphere the temperature gets colder as the distance above the earth increases, by about 6.5°C per kilometre. The actual change of temperature with height varies from day to day, depending on the weather.

The troposphere contains about 75% of all of the air in the atmosphere, and almost all of the water vapour (which forms clouds and rain). The decrease in temperature with height is a result of the decreasing pressure. If a parcel of air moves upwards it expands (because of the lower pressure). When air expands it cools. So air higher up is cooler than air lower down.

The lowest part of the troposphere is called the boundary layer. This is where the air motion is determined by the properties of the Earth's surface. Turbulence is generated as the wind blows over the Earth's surface, and by thermals rising from the land as it is heated by the sun. This turbulence redistributes heat and moisture within the boundary layer, as well as pollutants and other constituents of the atmosphere.

The top of the troposphere is called the tropopause. This is lowest at the poles, where it is about 7 - 10 km above the Earth's surface. It is highest (about 17 - 18 km) near the equator.

### The Stratosphere

This extends upwards from the tropopause to about 50 km. It contains much of the ozone in the atmosphere. The increase in temperature with height occurs because of absorption of ultraviolet (UV) radiation from the sun by this ozone. Temperatures in the stratosphere are highest over the summer pole, and lowest over the winter pole.

By absorbing dangerous UV radiation, the ozone in the stratosphere protects us from skin cancer and other health damage. However chemicals (called CFCs or freons, and halons) which were once used in refrigerators, spray cans and fire extinguishers have reduced the amount of ozone in the stratosphere, particularly at polar latitudes, leading to the so-called "Antarctic ozone hole".

Now humans have stopped making most of the harmful CFCs we expect the ozone hole will eventually recover over the 21 st century, but this is a slow process.

### The Mesosphere

The region above the stratosphere is called the mesosphere. Here the temperature again decreases with height, reaching a minimum of about -90°C at the "mesopause".

### The Thermosphere and Ionosphere

The thermosphere lies above the mesopause, and is a region in which temperatures again increase with height. This temperature increase is caused by the absorption of energetic ultraviolet and X-Ray radiation from the sun.

The region of the atmosphere above about 80 km is also caused the "ionosphere", since the energetic solar radiation knocks electrons off molecules and atoms, turning them into "ions" with a positive charge. The temperature of the thermosphere varies between night and day and between the seasons, as do the numbers of ions and electrons which are present. The ionosphere reflects and absorbs radio waves, allowing us to receive shortwave radio broadcasts in New Zealand from other parts of the world.

### The Exosphere

The region above about 500 km is called the exosphere. It contains mainly oxygen and hydrogen atoms, but there are so few of them that they rarely collide - they follow "ballistic" trajectories under the influence of gravity, and some of them escape right out into space.

### The Magnetosphere

The earth behaves like a huge magnet. It traps electrons (negative charge) and protons (positive), concentrating them in two bands about 3,000 and 16,000 km above the globe - the Van Allen "radiation" belts. This outer region surrounding the earth, where charged particles spiral along the magnetic field lines, is called the magnetosphere.

## Explainer: Our atmosphere — layer by layer

A view of Earth’s cloudscape and the rest of our atmosphere taken from a height of more than 30,000 feet. Not visible in such photos are the atmosphere’s many layers and how their features differ as they reach up and into outer space.

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December 1, 2020 at 6:30 am

Earth’s atmosphere is all around us. Most people take it for granted. But don’t. Among other things, it shields us from radiation and prevents our precious water from evaporating into space. It keeps the planet warm and provides us with oxygen to breathe. In fact, the atmosphere makes Earth the livable, lovable home sweet home that it is.

The atmosphere extends from Earth’s surface to more than 10,000 kilometers (6,200 miles) above the planet. Those 10,000 kilometers are divided into five distinct layers. From the bottom layer to the top, the air in each has the same composition. But the higher up you go, the further apart those air molecules are.

Ready to reach for the sky? Here’s an overview, layer by layer:

#### Troposphere: Earth’s surface to between 8 and 14 kilometers (5 and 9 miles)

Go ahead, stick your head right into the troposphere (TROH-poh-sfear). This lowest layer of the atmosphere starts at the ground and extends 14 kilometers (9 miles) up at the equator. That’s where it’s thickest. It’s thinnest above the poles, just 8 kilometers (5 miles) or so. The troposphere holds nearly all of Earth’s water vapor. It’s where most clouds ride the winds and where weather occurs. Water vapor and air constantly circulate in turbulent convection currents. Not surprisingly, the troposphere also is by far the densest layer. It contains as much as 80 percent of the mass of the whole atmosphere. The further up you go in this layer, the colder it gets. Want snow in summer? Head to where the upper troposphere bathes the highest peaks. The boundary between the troposphere and the next layer up is known as the tropopause.

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#### Stratosphere: 14 to 64 km (9 to about 31 miles)

Unlike the troposphere, temperatures in this layer increase with elevation. The stratosphere is very dry, so clouds rarely form here. It also contains most of the atmosphere’s ozone, triplet molecules made from three oxygen atoms. At this elevation, ozone protects life on Earth from the sun’s harmful ultraviolet radiation. It’s a very stable layer, with little circulation. For that reason, commercial airlines tend to fly in the lower stratosphere to keep flights smooth. This lack of vertical movement also explains why stuff that gets into in the stratosphere tends to stay there for a long time. That “stuff” might include aerosol particles shot skyward by volcanic eruptions, and even smoke from wildfires. This layer also has accumulated pollutants, such as chlorofluorocarbons (Klor-oh-FLOR-oh-kar-buns). Better known as CFCs, these chemicals can destroy the protective ozone layer, thinning it greatly. By the top of the stratosphere, called the stratopause, air is only a thousandth as dense as at Earth’s surface.

In this image taken from the International Space Station, the lowest layer of the atmosphere — the troposphere — appears orange. Above in blue is the bottom of the stratosphere. NASA

#### Mesosphere: 64 to 85 km (31 to 53 miles)

Scientists don’t know quite as much about this layer. It’s just harder to study. Airplanes and research balloons don’t operate this high and satellites orbit higher up. We do know that the mesosphere (MAY-so-sfere) is where most meteors harmlessly burn up as they hurtle towards Earth. Near the top of this layer, temperatures drop to the lowest in Earth’s atmosphere — about -90° Celsius (-130° Fahrenheit). The line marking the top of the mesosphere is called, you guessed it, the mesopause. If you ever travel that far, congratulations! You are officially a space traveler — aka astronaut — according to the U.S. Air Force.

The mesopause is also known as the Karman line. It’s named for the Hungarian-born physicist Theodore von Kármán. He was looking to determine the lower edge of what might constitute outer space. He set it at about 80 kilometers (50 miles) up. Some agencies of the U.S. government have accepted that as defining where space begins. Other agencies argue this imaginary line is a bit higher: at 100 kilometers (62 miles).

The ionosphere is a zone of charged particles that extends from the upper stratosphere or lower mesosphere all the way to the exosphere. The ionosphere is able to reflect radio waves this allows radio communications.

Time-lapse image of Earth showing the atmosphere, from the International Space Station NASA

#### Thermosphere: 85 to 600 km (53 to 372 miles)

The next layer up is the thermosphere. It soaks up x-rays and ultraviolet energy from the sun, protecting those of us on the ground from these harmful rays. The ups and downs of that solar energy also make the thermosphere vary wildly in temperature. It can go from really cold to as hot as about 1,980 ºC (3,600 ºF) near the top. The sun’s varying energy output also causes the thickness of this layer to expand as it heats and to contract as it cools. With all the charged particles, the thermosphere is also home to those beautiful celestial light shows known as auroras. This layer’s top boundary is called the thermopause.

#### Exosphere: 600 to 10,000 km (372 to 6,200 miles)

The uppermost layer of Earth’s atmosphere is called the exosphere. Its lower boundary is known as the exobase. The exosphere has no firmly defined top. Instead, it just fades further out into space. Air molecules in this part of our atmosphere are so far apart that they rarely even collide with each other. Earth’s gravity still has a little pull here, but just enough to keep most of the sparse air molecules from drifting away. Still, some of those air molecules — tiny bits of our atmosphere — do float away, lost to Earth forever.

As it rises out toward space, Earth’s atmosphere changes in density and much more. The depth of each layer can vary by the day and the latitude and are depicted here artistically (not drawn to scale). VectorMine/iStock/Getty Images

#### Fun facts

• Shock waves from earthquakes, volcanic eruptions and explosions on Earth’s surface can ripple through the atmosphere.
• The International Space Station orbits Earth at an average altitude of about 400 kilometers (250 miles). That’s within the thermosphere. Satellites also operate in this region and higher, into the exosphere.
• The thermosphere is cluttered with human-made debris, such as old satellites and bits of rockets. Each year, collisions between these items create even more debris. Orbiting at incredible rates of speed, even a pea-sized particle can cause serious damage to working satellites. The International Space Station has had several near misses with space debris and now and then changes its position in orbit to avoid collisions. such as carbon dioxide, methane, water vapor and nitrous oxide occur naturally in the atmosphere. But human activity has boosted their levels. They absorb heat from Earth and radiate it back to the surface again, boosting warming.

### Power Words

aerosol: (adj. aerosolized) A tiny solid or liquid particle suspended in air or as a gas. Aerosols can be natural, such as fog or gas from volcanic eruptions, or artificial, such as smoke from burning fossil fuels.

atmosphere: The envelope of gases surrounding Earth or another planet.

atom: The basic unit of a chemical element. Atoms are made up of a dense nucleus that contains positively charged protons and uncharged neutrons. The nucleus is orbited by a cloud of negatively charged electrons.

aurora: A light display in the sky caused when incoming energetic particles from the sun collide with gas molecules in a planet’s upper atmosphere. The best known of these is Earth’s aurora borealis, or northern lights. On some outer gas planets, like Jupiter and Saturn, the combination of a fast rate of rotation and strong magnetic field leads to high electrical currents in the upper atmosphere, above the planets’ poles. This, too, can cause auroral “light” shows in their upper atmosphere.

average: (in science) A term for the arithmetic mean, which is the sum of a group of numbers that is then divided by the size of the group.

carbon dioxide: (or CO2) A colorless, odorless gas produced by all animals when the oxygen they inhale reacts with the carbon-rich foods that they’ve eaten. Carbon dioxide also is released when organic matter burns (including fossil fuels like oil or gas). Carbon dioxide acts as a greenhouse gas, trapping heat in Earth’s atmosphere. Plants convert carbon dioxide into oxygen during photosynthesis, the process they use to make their own food.

celestial: (in astronomy) Of or relating to the sky, or outer space.

chemical: A substance formed from two or more atoms that unite (bond) in a fixed proportion and structure. For example, water is a chemical made when two hydrogen atoms bond to one oxygen atom. Its chemical formula is H2O.

cloud: A plume of molecules or particles, such as water droplets, that move under the action of an outside force, such as wind, radiation or water currents. (in atmospheric science) A mass of airborne water droplets and ice crystals that travel as a plume, usually high in Earth’s atmosphere. Its movement is driven by winds.

commercial: (in research and economics) An adjective for something that is ready for sale or already being sold. Commercial goods are those caught or produced for others, and not solely for personal consumption.

convection: The rising and falling of material in a fluid or gas due to uneven temperatures. This process occurs in the outer layers of some stars.

debris: Scattered fragments, typically of trash or of something that has been destroyed. Space debris, for instance, includes the wreckage of defunct satellites and spacecraft.

earthquake: A sudden and sometimes violent shaking of the ground, sometimes causing great destruction, as a result of movements within Earth’s crust or of volcanic action.

elevation: The height or altitude at which something exists.

equator: An imaginary line around Earth that divides Earth into the Northern and Southern Hemispheres.

eruption: (in geoscience) The sudden bursting or spraying of hot material from deep inside a planet or moon and out through its surface. Volcanic eruptions on Earth usually send hot lava, hot gases or ash into the air and across surrounding land. In colder parts of the solar system, eruptions often involve liquid water spraying out through cracks in an icy crust. This happens on Enceladus, a moon of Saturn that is covered in ice.

force: Some outside influence that can change the motion of a body, hold bodies close to one another, or produce motion or stress in a stationary body.

gravity: The force that attracts anything with mass, or bulk, toward any other thing with mass. The more mass that something has, the greater its gravity.

greenhouse gas: A gas that contributes to the greenhouse effect by absorbing heat. Carbon dioxide is one example of a greenhouse gas.

International Space Station: An artificial satellite that orbits Earth. Run by the United States and Russia, this station provides a research laboratory from which scientists can conduct experiments in biology, physics and astronomy — and make observations of Earth.

ionosphere: A layer of Earth’s atmosphere lying around 75 and 1,000 kilometers (47 and 620 miles) above Earth’s surface. It absorbs the sun’s harmful extreme-ultraviolet rays. That energy strips electrons from atoms and molecules, creating a zone full of free-floating ions. The share of ions present, here, affects radio and other signals passing through it.

Karman line: Also known as the mesopause, it’s named for the Hungarian-born physicist Theodore von Kármán. At about 80 kilometers (50 miles) up, it’s an imaginary line that Karman selected to mark where outer space begins.

latitude: The distance from the equator measured in degrees (up to 90). Low latitudes are closer to the equator high latitudes are closer to the poles.

mass: A number that shows how much an object resists speeding up and slowing down — basically a measure of how much matter that object is made from.

meteor: (adj. meteoritic) A lump of rock or metal from space that hits the atmosphere of Earth. In space it is known as a meteoroid. When you see it in the sky it is a meteor. And when it hits the ground it is called a meteorite.

methane: A hydrocarbon with the chemical formula CH4 (meaning there are four hydrogen atoms bound to one carbon atom). It’s a natural constituent of what’s known as natural gas. It’s also emitted by decomposing plant material in wetlands and is belched out by cows and other ruminant livestock. From a climate perspective, methane is 20 times more potent than carbon dioxide is in trapping heat in Earth’s atmosphere, making it a very important greenhouse gas.

molecule: An electrically neutral group of atoms that represents the smallest possible amount of a chemical compound. Molecules can be made of single types of atoms or of different types. For example, the oxygen in the air is made of two oxygen atoms (O2), but water is made of two hydrogen atoms and one oxygen atom (H2O).

orbit: The curved path of a celestial object or spacecraft around a galaxy, star, planet or moon. One complete circuit around a celestial body.

oxide: A compound made by combining one or more elements with oxygen. Rust is an oxide so is water.

ozone: A colorless gas made of molecules that contain three oxygen atoms. It can form high in the atmosphere or at ground level. When it forms at Earth’s surface, ozone is a pollutant that irritates eyes and lungs. It is also a major ingredient of smog.

ozone layer: A layer in Earth’s stratosphere. It contains a lot of ozone (a molecule made from three oxygen atoms), which helps block much of the sun’s biologically damaging ultraviolet radiation.

particle: A minute amount of something.

physicist: A scientist who studies the nature and properties of matter and energy.

planet: A large celestial object that orbits a star but unlike a star does not generate any visible light.

poles: (in Earth science and astronomy) The cold regions of the planet that exist farthest from the equator the upper and lower ends of the virtual axis around which a celestial object rotates.

pollutant: A substance that taints something — such as the air, water, our bodies or products. Some pollutants are chemicals, such as pesticides. Others may be radiation, including excess heat or light. Even weeds and other invasive species can be considered a type of biological pollution.

radiate: (in physics) To emit energy in the form of waves.

radio waves: Waves in a part of the electromagnetic spectrum. They are a type that people now use for long-distance communication. Longer than the waves of visible light, radio waves are used to transmit radio and television signals. They also are used in radar.

satellite: A moon orbiting a planet or a vehicle or other manufactured object that orbits some celestial body in space.

shock waves: Tiny regions in a gas or fluid where properties of the host material change dramatically owing to the passage of some object (which could be a plane in air or merely bubbles in water). Across a shock wave, a region’s pressure, temperature and density spike briefly, and almost instantaneously.

solar energy: The energy in sunlight that can be captured as heat or converted into heat or electrical energy. Some people refer to wind power as a form of solar energy. The reason: Winds are driven by the variations in temperatures and the density of the air, both of which are affected by the solar heating of the air, ground and surface waters.

sun: The star at the center of Earth’s solar system. It is about 27,000 light-years from the center of the Milky Way galaxy. Also a term for any sunlike star.

tropopause: A boundary between the two lower layers of Earth's atmosphere, the troposphere and the stratosphere. That boundary layer varies with latitude, running from a height of about 6 kilometers (4 miles) over the poles to 18 kilometers (11 miles) over the equator.

turbulent: (n. turbulence) An adjective for the unpredictable fluctuation of a fluid (including air) in which its velocity varies irregularly instead of maintaining a steady or calm flow.

ultraviolet: A portion of the light spectrum that is close to violet but invisible to the human eye.

vertical: A term for the direction of a line or plane that runs up and down, as the vertical post for a streetlight does. It’s the opposite of horizontal, which would run parallel to the ground.

water vapor: Water in its gaseous state, capable of being suspended in the air.

wave: A disturbance or variation that travels through space and matter in a regular, oscillating fashion.

weather: Conditions in the atmosphere at a localized place and a particular time. It is usually described in terms of particular features, such as air pressure, humidity, moisture, any precipitation (rain, snow or ice), temperature and wind speed. Weather constitutes the actual conditions that occur at any time and place. It’s different from climate, which is a description of the conditions that tend to occur in some general region during a particular month or season.

X-ray: A type of radiation analogous to gamma rays, but having somewhat lower energy.

### Citations

Journal: E. Astafyeva. Ionospheric detection of natural hazards. Reviews of Geophysics. Vol. 57, December 4, 2019, p. 1265. doi: 10.1029/2019RG000668.

Website: Center for Science Education. University Corporation for Atmospheric Research. Layers of Earth’s atmosphere.

Website: National Environmental Satellite, Data, and Information Service. Peeling Back the Layers of the Atmosphere. National Oceanic and Atmospheric Administration. February 22, 2016.

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## Space Littering Can Impact Earth’s Atmosphere

There is growing appreciation that outer space has become atrash bin, with the Earth encircled by dead or dying spacecraft, along withmenacing bits of orbital clutter - some of which burns up in the planet?satmosphere.

The big news of late was a smashup of a commercial Iridiumsatellite with a defunct Russian spacecraft earlier this year. Then there wasthat 2007 anti-satellite test by China, purposely destroying one of its agingweather satellites. These events produced largedebris fields in space ? adding to the swamp of cosmic compost.

But I sense a line of research that needs exploring: Theoverall impact of human-made orbital debris, solid and liquid propellantdischarges, and other space age substance abuse that winds up in a high-speeddive through Earth?s atmosphere.

There?s a convenient toss away line that is in vogue: thatsuch space refuse simply ?burns up? ? a kind of out of sight, out of minddeclaration.

What chemistry is involved given the high heating duringreentry of space leftovers made of tungsten, beryllium, aluminum and lots ofcomposite materials? The impact of these materials on Earth?s atmosphere - topto bottom ? would seem worthy of investigation.

As for total mass of uncontrolled objects that re-enter eachyear ? it?s in the range of 70 ? 80 metric tons. And that?s the trackable, bigstuff ? never mind smaller bits of orbital jetsam like bubbles ofstill-radioactive coolant that has been leaked from old nuclear-powered Sovietsatellites.

One study team that looked into the impact of de-orbitingspace debris on stratospheric ozone issued their findings back in 1994. Thework was done by an aerospace industry firm for the Environmental ManagementDivision of the Space and Missile Systems Center. They reported that objectsre-entering the atmosphere can affect ozone in several ways, but not on asignificant level globally.

Indeed, as an object plowsthrough the Earth?s stratosphere, a shock wave is created that producesnitric oxide, a known cause of ozone depletion. Spacecraft and rocket motorsare composed of metal alloys and composite materials that melt away duringre-entry. The researchers found that these materials, as they undergo intenseheating, also form chemicals that react directly or indirectly to consumeozone.

Overall, the study found that the physical and chemicalphenomena associated with deorbiting debris do not have ?a significant impact?on global stratospheric ozone.

Pass the collection plates

Then there?s the work of Michael Zolensky of AstromaterialsResearch and Exploration Science at NASA?s Johnson Space Center in Houston,Texas.

Some 20 years ago, Zolensky led a team that found a ten-foldincrease in the abundance of large solid particles in the stratosphere between1976 and 1984. Using high-altitude aircraft, the NASA sampling program wasdirected at snagging particles of dust from comets and asteroids as they filterdown through the atmosphere.

However, when the collection plates were later analyzed,exhaust residue from solid rocket motor firings, protective paints that shedfrom the outer hulls of spacecraft in orbit, and particles of mostly aluminumfrom re-entering space hardware were identified.

?I don?t think anyone ever followed up on this,? Zolensky toldme. More study is needed on the density of particles, types of particles, howlong they are suspended in the atmosphere, and whether or not the amount ofdeorbiting detritus has increased over time.

Another scientist flagging this issue is Martin Ross of TheAerospace Corporation in El Segundo, California. He points out that this typeof research is one where you need to have the science guys talking to theengineering community. ?And that usually doesn?t happen.?

Ross emphasized that orbital debris impacts onEarth?s atmosphere, at the moment, is not something to be too concernedabout. However, now is the time to get smart about what is taking place, hesaid.

But complicating that investigation, Ross noted, is thatairplane and balloons only operate at altitudes lower than where the re-entryprocess takes place. That upper stratosphere-lower mesosphere region has oftenbeen tagged as the ?Ignorosphere,? Ross said.

Even at balloon altitude there has been some recent,unexpected, insight. Scientists at the Indian Space Research Organizationannounced last March that ultraviolet-resistant bacteria had been found inEarth?s upper stratosphere, purportedly not found elsewhere on Earth.

?Everywhere we look on the Earth, we seem to find somethingthat we could call life,? Ross told SPACE.com. ?So I guess it wouldn?tbe too surprising that you?d find some layer of a particular microbe, orsomething, at various levels in the atmosphere.?

Ross, along with Darin Toohey of the University of Colorado,Boulder?s Atmospheric and Oceanic Sciences Department recently reported thatrocket launches may need regulation to prevent ozone depletion.

That study ? published in Astropolitics this pastMarch, an international journal of space politics and policy -- includesanalysis from Embry-Riddle Aeronautical University in Daytona, Florida andprovides a market analysis for estimating future ozone layer depletion based onthe expected growth of the space industry and known impacts of rocket launches.

In that assessment, the global market for rocket launchesmay require more stringent regulation in order to prevent significant damage toEarth?s stratospheric via ozone-destroying rocket emissions in the decades tocome.

The new study was designed to bring attention to the issue in hopes of sparkingadditional research, Ross said. Furthermore, getting a handle on the makeup ofhuman-made components and debris that speeds through the upper atmosphere ?from an accounting point of view -- would be a fairly simple thing to do, headded.

?All we really have right now are a small handful ofobservations of the emissions of a few rockets as they ascend to space. Eventhen, we lack critical observations in the plumes of many other types ofrockets to be confident in predictions of the impacts of the space launch fleetas a whole,? Toohey told SPACE.com.

?Add in the unknown impacts of vapors formed during reentry,and you can guess that we have some work to do to provide solid evidence neededby the space launch industry to design new vehicles that minimize thoseimpacts,? Toohey added.

Toohey said the good news is that, if the atmosphericsciences and space launch communities can come together to address this issue,?we have the expertise and tools to solve this before it ever becomes a seriousproblem.?

Space ? a Superfund clean-up site

While getting a research handle on the Ignorosphere appearscalled for, the bigger mess to deal with is how best to de-clutter lowEarth orbit.

The thermosphere rises several hundred miles above the Earth's surface, from 56 miles (90 km) up to between 311 and 621 miles (500–1,000 km). Temperature is very much affected by the sun here it can be 360 degrees Fahrenheit hotter (500 C) during the day than at night. Temperature increases with height and can rise to as high as 3,600 degrees Fahrenheit (2000 C). Nonetheless, the air would feel cold because the hot molecules are so far apart. This layer is known as the upper atmosphere, and it is where the auroras occur (northern and southern lights).

Extending from the top of the thermosphere to 6,200 miles (10,000 km) above Earth is the exosphere, where weather satellites are. This layer has very few atmospheric molecules, which can escape into space. Some scientists disagree that the exosphere is a part of the atmosphere and instead classify it actually as a part of outer space. There is no clear upper boundary, as in other layers.

## Shrinking atmospheric layer linked to low levels of solar radiation

Large changes in the sun's energy output may drive unexpectedly dramatic fluctuations in Earth's outer atmosphere.

Results of a new study link a recent, temporary shrinking of a high atmospheric layer with a sharp drop in the sun's ultraviolet radiation levels.

The research, led by scientists at the National Center for Atmospheric Research (NCAR) in Boulder, Colo., and the University of Colorado at Boulder (CU), indicates that the sun's magnetic cycle, which produces differing numbers of sunspots over an approximately 11-year cycle, may vary more than previously thought.

The results, published in the American Geophysical Union journal Geophysical Research Letters, are funded by NASA and by the National Science Foundation (NSF), NCAR's sponsor.

"This research makes a compelling case for the need to study the coupled sun-Earth system," says Farzad Kamalabadi, program director in NSF's Division of Atmospheric and Geospace Sciences, "and to illustrate the importance of solar influences on our terrestrial environment with both fundamental scientific implications and societal consequences."

The findings may have implications for orbiting satellites, as well as for the International Space Station.

"Our work demonstrates that the solar cycle not only varies on the typical 11-year time scale, but also can vary from one solar minimum to another," says lead author Stanley Solomon, a scientist at NCAR's High Altitude Observatory. "All solar minima are not equal."

The fact that the layer in the upper atmosphere known as the thermosphere is shrunken and dense means that satellites can more easily maintain their orbits.

But it also indicates that space debris and other objects that pose hazards may persist longer in the thermosphere.

"With lower thermospheric density, our satellites will have a longer life in orbit," says CU professor Thomas Woods, a co-author.

"This is good news for those satellites that are actually operating, but it is also bad because of the thousands of non-operating objects remaining in space that could potentially have collisions with our working satellites."

The sun's energy output declined to unusually low levels from 2007 to 2009, a particularly prolonged solar minimum during which there were virtually no sunspots or solar storms.

During that same period of low solar activity, Earth's thermosphere shrank more than at any time in the 43-year era of space exploration.

The thermosphere, which ranges in altitude from about 55 to more than 300 miles (90 to 500 kilometers), is a rarified layer of gas at the edge of space where the sun's radiation first makes contact with Earth's atmosphere.

It typically cools and becomes less dense during low solar activity.

But the magnitude of the density change during the recent solar minimum appeared to be about 30 percent greater than would have been expected by low solar activity.

The study team used computer modeling to analyze two possible factors implicated in the mystery of the shrinking thermosphere.

They simulated both the impacts of solar output and the role of carbon dioxide, a potent greenhouse gas that, according to past estimates, is reducing the density of the outer atmosphere by about 2 percent to 5 percent per decade.

Their work built on several recent studies.

Earlier this year, a team of scientists from the Naval Research Laboratory and George Mason University, measuring changes in satellite drag, estimated that the density of the thermosphere declined in 2007-09 to about 30 percent less than during the previous solar minimum in 1996.

Other studies by scientists at the University of Southern California and CU, using measurements from sub-orbital rocket flights and space-based instruments, have estimated that levels of extreme-ultraviolet radiation-a class of photons with extremely short wavelengths-dropped about 15 percent during the same period.

However, scientists remained uncertain whether the decline in extreme-ultraviolet radiation would be sufficient to have such a dramatic impact on the thermosphere, even when combined with the effects of carbon dioxide.

To answer this question, Solomon and his colleagues turned to an NCAR computer tool, known as the Thermosphere-Ionosphere-Electrodynamics General Circulation Model.

They used the model to simulate how the sun's output during 1996 and 2008 would affect the temperature and density of the thermosphere.

They also created two simulations of thermospheric conditions in 2008-one with a level that approximated actual carbon dioxide emissions and one with a fixed, lower level.

The results showed the thermosphere cooling in 2008 by 41 kelvins, or K (about 74 degrees Fahrenheit) compared to 1996, with just 2 K attributable to the carbon dioxide increase.

The results also showed the thermosphere's density decreasing by 31 percent, with just 3 percent attributable to carbon dioxide, and closely approximated the 30 percent reduction in density indicated by measurements of satellite drag.

"It is now clear that the record low temperature and density were primarily caused by unusually low levels of solar radiation at the extreme-ultraviolet level," Solomon says.

Woods says the research indicates that the sun could be going through a period of relatively low activity, similar to periods in the early 19th and 20th centuries.

This could mean that solar output may remain at a low level for the near future.

"If it is indeed similar to certain patterns in the past, then we expect to have low solar cycles for the next 10 to 30 years," Woods says.

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