atmosphere - National Geographic Society atmosphere Our atmosphere—the thin blue line. Photograph courtesy NASA Atmospheric Orbit Although the International Space Station orbits in the thermosphere, most satellites orbit the Earth outside its atmosphere. GPS satellites, for instance, are in orbit more than 20,000 kilometers (12,400 miles) above the Earth. Magnetosphere Earths magnetosphere is not considered part of the atmosphere. The magnetosphere, formed by the Earths magnetic fields, protects the atmosphere by preventing it from being blown away by powerful solar wind. Ingredients for Life Scientists have gathered enough information about other planets in our solar system to know that none can support life as we know it. Life is not possible without a stable atmosphere containing the right chemical ingredients for living organisms: hydrogen, oxygen, nitrogen, and carbon. These ingredients must be balanced—not too thick or too thin. Life also depends on the presence of water. Jupiter, Saturn, Uranus, and Neptune all have atmospheres made mostly of hydrogen and helium. These planets are called gas giants, because they are mostly made of gas and do not have a solid outer crust. Mercury and Mars have some of the right ingredients, but their atmospheres are far too thin to support life.

The atmosphere of Venus is too thick—the planet's surface temperature is more than 460 degrees Celsius (860 degrees Fahrenheit). Jupiter's moon Europa has a thin atmosphere rich with oxygen. It is likely covered by a huge ocean of liquid water. Some astrobiologists think that if life will develop elsewhere in the solar system, it will be near vents at the bottom of Europa's ocean. We live at the bottom of an invisible ocean called the atmosphere , a layer of gases surrounding our planet . Nitrogen and oxygen account for 99 percent of the gases in dry air , with argon , carbon dioxide , helium , neon , and other gases making up minute portions. Water vapor and dust are also part of Earth ’s atmosphere. Other planets and moons have very different atmospheres, and some have no atmospheres at all. The atmosphere is so spread out that we barely notice it, yet its weight is equal to a layer of water more than 10 meters (34 feet) deep covering the entire planet. The bottom 30 kilometers (19 miles) of the atmosphere contains about 98 percent of its mass . The atmosphere—air—is much thinner at high altitudes. There is no atmosphere in space.

Scientists say many of the gases in our atmosphere were ejected into the air by early volcanoes. At that time, there would have been little or no free oxygen surrounding the Earth. Free oxygen consists of oxygen molecules not attached to another element , like carbon (to form carbon dioxide) or hydrogen (to form water). Free oxygen may have been added to the atmosphere by primitive organisms, probably bacteria , during photosynthesis . Photosynthesis is the process a plant or other autotroph uses to make food and oxygen from carbon dioxide and water. Later, more complex forms of plant life added more oxygen to the atmosphere. The oxygen in today’s atmosphere probably took millions of years to accumulate . The atmosphere acts as a gigantic filter , keeping out most ultraviolet radiation while letting in the sun’s warming rays. Ultraviolet radiation is harmful to living things, and is what causes sunburns. Solar heat, on the other hand, is necessary for all life on Earth. Earth’s atmosphere has a layered structure. From the ground toward the sky, the layers are the troposphere , stratosphere , mesosphere , thermosphere , and exosphere . Another layer, called the ionosphere , extends from the mesosphere to the exosphere. Beyond the exosphere is outer space .

The boundaries between atmospheric layers are not clearly defined, and change depending on latitude and season . Troposphere The troposphere is the lowest atmospheric layer. On average, the troposphere extends from the ground to about 10 kilometers (6 miles) high, ranging from about 6 kilometers (4 miles) at the poles to more than 16 kilometers (10 miles) at the Equator . The top of the troposphere is higher in summer than in winter. Almost all weather develops in the troposphere because it contains almost all of the atmosphere’s water vapor. Clouds, from low-lying fog to thunderheads to high- altitude cirrus , form in the troposphere. Air masses, areas of high-pressure and low-pressure systems, are moved by winds in the troposphere. These weather systems lead to daily weather changes as well as seasonal weather patterns and climate systems, such as El Nino . Air in the troposphere thins as altitude increases. There are fewer molecules of oxygen at the top of Mount Everest , Nepal, for example, than there are on a beach in Hawaii. This is why mountaineers often use canisters of oxygen when climbing tall peaks. Thin air is also why helicopters have difficulty maneuvering at high altitudes.

In fact, a helicopter was not able to land on Mount Everest until 2005. As air in the troposphere thins, temperature decreases. This is why mountaintops are usually much colder than the valleys beneath. Scientists used to think temperature continued to drop as altitude increased beyond the troposphere. But data collected with weather balloons and rockets have showed this is not the case. In the lower stratosphere, temperature stays almost constant. As altitude increases in the stratosphere, temperature actually increases. Solar heat penetrates the troposphere easily. This layer also absorbs heat that is reflected back from the ground in a process called the greenhouse effect . The greenhouse effect is necessary for life on Earth. The atmosphere’s most abundant greenhouse gases are carbon dioxide, water vapor, and methane . Fast-moving, high-altitude winds called jet streams swirl around the planet near the upper boundary of the troposphere. Jet streams are extremely important to the airline industry. Aircraft save time and money by flying in jet streams instead of the lower troposphere, where air is thicker. Stratosphere The troposphere tends to change suddenly and violently, but the stratosphere is calm.

The stratosphere extends from the tropopause , the upper boundary of the troposphere, to about 50 kilometers (32 miles) above the Earth’s surface. Strong horizontal winds blow in the stratosphere, but there is little turbulence. This is ideal for planes that can fly in this part of the atmosphere. The stratosphere is very dry and clouds are rare. Those that do form are thin and wispy. They are called nacreous clouds. Sometimes they are called mother-of-pearl clouds because their colors look like those inside a mollusk shell. The stratosphere is crucial to life on Earth because it contains small amounts of ozone , a form of oxygen that prevents harmful UV rays from reaching Earth. The region within the stratosphere where this thin shell of ozone is found is called the ozone layer . The stratosphere’s ozone layer is uneven, and thinner near the poles. The amount of ozone in the Earth’s atmosphere is declining steadily. Scientists have linked use of chemicals such as chlorofluorocarbons (CFCs) to ozone depletion .

Mesosphere The mesosphere extends from the stratopause (the upper boundary of the stratosphere) to about 85 kilometers (53 miles) above the surface of the Earth. Here, temperatures again begin to fall. The mesosphere has the coldest temperatures in the atmosphere, dipping as low as -120 degrees Celsius (-184 degrees Fahrenheit, or 153 kelvin). The mesosphere also has the atmosphere’s highest clouds. In clear weather, you can sometimes see them as silvery wisps immediately after sunset. They are called noctilucent clouds, or night-shining clouds. The mesosphere is so cold that noctilucent clouds are actually frozen water vapor— ice clouds. Shooting stars—the fiery burnout of meteors, dust, and rocks from outer space—are visible in the mesosphere. Most shooting stars are the size of a grain of sand and burn up before entering the stratosphere or troposphere. However, some meteors are the size of pebbles or even boulders. Their outer layers burn as they race through the mesosphere, but they are massive enough to fall through the lower atmosphere and crash to Earth as meteorites.

The mesosphere is the least-understood part of Earth’s atmosphere. It is too high for aircraft or weather balloons to operate, but too low for spacecraft . Sounding rockets have provided meteorologists and astronomers their only significant data on this important part of the atmosphere. Sounding rockets are unmanned research instruments that collect data during sub-orbital flights. Perhaps because the mesosphere is so little understood, it is home to two meteorological mysteries: sprites and elves . Sprites are reddish, vertical electrical discharges that appear high above thunderheads, in the upper stratosphere and mesosphere. Elves are dim, halo-shaped discharges that appear even higher in the mesosphere. Ionosphere The ionosphere extends from the top half of the mesosphere all the way to the exosphere. This atmospheric layer conducts electricity . The ionosphere is named for ions created by energetic particles from sunlight and outer space. Ions are atoms in which the number of electrons does not equal the number of protons, giving the atom a positive (fewer electrons than protons) or negative (more electrons than protons) charge. Ions are created as powerful x-rays and UV rays knock electrons off atoms.

The ionosphere—a layer of free electrons and ions—reflects radio waves. Guglielmo Marconi , the “Father of Wireless,” helped prove this in 1901 when he sent a radio signal from Cornwall, England, to St. John’s, Newfoundland, Canada. Marconi’s experiment demonstrated that radio signals did not travel in a straight line, but bounced off an atmospheric layer—the ionosphere. The ionosphere is broken into distinct layers, called the D, E, F1, and F2 layers. Like all other parts of the atmosphere, these layers vary with season and latitude. Changes in the ionosphere actually happen on a daily basis. The low D layer, which absorbs high-frequency radio waves, and the E layer actually disappear at night, which means radio waves can reach higher into the ionosphere. That’s why AM radio stations can extend their range by hundreds of kilometers every night. The ionosphere also reflects particles from solar wind , the stream of highly charged particles ejected by the sun. These electrical displays create auroras (light displays) called the Northern and Southern Lights . Thermosphere The thermosphere is the thickest layer in the atmosphere.

Only the lightest gases—mostly oxygen, helium, and hydrogen—are found here. The thermosphere extends from the mesopause (the upper boundary of the mesosphere) to 690 kilometers (429 miles) above the surface of the Earth. Here, thinly scattered molecules of gas absorb x-rays and ultraviolet radiation. This absorption process propels the molecules in the thermosphere to great speeds and high temperatures. Temperatures in the thermosphere can rise to 1,500 degrees Celsius (2,732 degrees Fahrenheit, or 1,773 kelvin). Though the temperature is very high, there is not much heat. How is that possible? Heat is created when molecules get excited and transfer energy from one molecule to another. Heat happens in an area of high pressure (think of water boiling in a pot). Since there is very little pressure in the thermosphere, there is little heat transfer. The Hubble Space Telescope and the International Space Station (ISS) orbit the Earth in the thermosphere.

Even though the thermosphere is the second-highest layer of Earth’s atmosphere, satellites that operate here are in “ low-Earth orbit .” Exosphere The fluctuating area between the thermosphere and the exosphere is called the turbopause . The lowest level of the exosphere is called the exobase . At the upper boundary of the exosphere, the ionosphere merges with interplanetary space , or the space between planets. The exosphere expands and contracts as it comes into contact with solar storms. In solar storms particles are flung through space from explosive events on the sun, such as solar flares and coronal mass ejections (CMEs). Solar storms can squeeze the exosphere to just 1,000 kilometers (620 miles) above the Earth. When the sun is calm, the exosphere can extend 10,000 kilometers (6,214 miles). Hydrogen, the lightest element in the universe , dominates the thin atmosphere of the exosphere. Only trace amounts of helium, carbon dioxide, oxygen, and other gases are present. Many weather satellites orbit Earth in the exosphere.

The lower part of the exosphere includes low-Earth orbit, while medium-Earth orbit is higher in the atmosphere. The upper boundary of the exosphere is visible in satellite images of Earth. Called the geocorona , it is the fuzzy blue illumination that circles the Earth. Extraterrestrial Atmospheres All the planets in our solar system have atmospheres. Most of these atmospheres are radically different from Earth’s, although they contain many of the same elements. The solar system has two major types of planets: terrestrial planets (Mercury, Venus, Earth, and Mars) and gas giants (Jupiter, Saturn, Uranus, and Neptune). The atmospheres of the terrestrial planets are somewhat similar to Earth’s. Mercury’s atmosphere contains only a thin exosphere dominated by hydrogen, helium, and oxygen. Venus’ atmosphere is much thicker than Earth’s, preventing a clear view of the planet. Its atmosphere is dominated by carbon dioxide, and features swirling clouds of sulfuric acid . The atmosphere on Mars is also dominated by carbon dioxide, although unlike Venus, it is quite thin. Gas giants are composed of gases. Their atmospheres are almost entirely hydrogen and helium.

The presence of methane in the atmospheres of Uranus and Neptune give the planets their bright blue color. In the lower atmospheres of Jupiter and Saturn, clouds of water, ammonia , and hydrogen sulfide form clear bands. Fast winds separate light-colored bands, called zones, from dark-colored bands, called belts. Other weather phenomena , such as cyclones and lightning , create patterns in the zones and belts. Jupiter’s Great Red Spot is a centuries-old cyclone that is the largest storm in the solar system. The moons of some planets have their own atmospheres. Saturn’s largest moon , Titan , has a thick atmosphere made mostly of nitrogen and methane. The way sunlight breaks up methane in Titan’s ionosphere helps give the moon an orange color. Most celestial bodies, including all the asteroids in the asteroid belt and our own moon, do not have atmospheres. The lack of an atmosphere on the Moon means it does not experience weather. With no wind or water to erode them, many craters on the Moon have been there for hundreds and even thousands of years. The way a celestial body ’s atmosphere is structured and what it’s made of allow astrobiologists to speculate what kind of life the planet or moon may be able to support.

Atmospheres, then, are important markers in space exploration. A planet or moon’s atmosphere must contain specific chemicals to support life as we know it. These chemicals include hydrogen, oxygen, nitrogen, and carbon . Although Venus, Mars, and Titan have similar atmospheric gases, there is nowhere in the solar system besides Earth with an atmosphere able to support life. Venus’ atmosphere is far too thick, Mars’ far too thin, and Titan’s far too cold. Term vehicle able to travel and operate above the ground. airline system or business that provides air transportation. air mass a gas (NH3) important to food production. AM radio Noun (amplitude modification) method of radio communication using amplitude modification, or varying the strength of the radio signal. argon chemical element (gas) with the symbol Ar. asteroid Noun irregularly shaped planetary body, ranging from 6 meters (20 feet) to 933 kilometers (580 miles) in diameter, orbiting the sun between Mars and Jupiter. asteroid belt area of the solar system between the orbits of Mars and Jupiter filled with asteroids. astrobiologist person who studies the possibility of life in outer space.

astronomer person who studies space and the universe beyond Earth's atmosphere. atmosphere the basic unit of an element, composed of three major parts: electrons, protons, and neutrons. aurora Noun brightly colored bands of light, visible around Earth's geomagnetic poles, caused by solar wind interacting with particles in Earth's magnetic field. autotroph Noun organism that can produce its own food and nutrients from chemicals in the atmosphere, usually through photosynthesis or chemosynthesis. (singular: bacterium) single-celled organisms found in every ecosystem on Earth. beach dark-colored band of clouds on Jupiter or Saturn. boulder container, usually shaped like a long tube. carbon chemical element with the symbol C, which forms the basis of all known life. carbon dioxide Noun greenhouse gas produced by animals during respiration and used by plants during photosynthesis. Carbon dioxide is also the byproduct of burning fossil fuels. celestial body natural object in space, such as a planet or star. Also called an astronomical object. chlorofluorocarbon (CFC) Noun chemical compound mostly used in refrigerants and flame-retardants. Some CFCs have destructive effects on the ozone layer. cirrus to shrink or get smaller.

coronal mass ejection huge burst of solar wind and other charged particles. crater cyclone Noun weather system that rotates around a center of low pressure and includes thunderstorms and rain. Usually, hurricanes refer to cyclones that form over the Atlantic Ocean. data (singular: datum) information collected during a scientific study. dominate standard measurement of gases that make up air at sea level, excluding water vapor. dust Earth Noun our planet, the third from the Sun. The Earth is the only place in the known universe that supports life. to get rid of or throw out. electricity set of physical phenomena associated with the presence and flow of electric charge. electron chemical that cannot be separated into simpler substances. El Nino Noun elves Plural Noun (Emissions of Light and Very low-frequency perturbations from Electromagnetic pulse Sources) halo-shaped electrical discharge in the upper atmosphere, usually appearing above sprites. Equator lowest level of the exosphere layer of Earth's atmosphere. exosphere Noun outermost layer of Earth's atmosphere, beginning at an altitude of about 550 kilometers (341 miles) above the Earth's surface.

expand filter Verb to remove particles from a substance by passing the substance through a screen or other material that catches larger particles and lets the rest of the substance pass through. fluctuate to constantly change back and forth. fog electron that has been temporarily knocked off an atom. free oxygen oxygen molecules that are not attached to other atoms or molecules. gas Noun state of matter with no fixed shape that will fill any container uniformly. Gas molecules are in constant, random motion. gas giant Noun one of the four enormous outermost planets in the solar system (Jupiter, Saturn, Neptune, Uranus), composed mostly of gases instead of rock. Also called a Jovian planet. geocorona fuzzy blue layer of hydrogen surrounding the Earth at the upper boundary of the exosphere. gigantic enormous storm in Jupiter's Southern Hemisphere, which has been observed for more than 100 years. greenhouse effect Noun phenomenon where gases allow sunlight to enter Earth's atmosphere but make it difficult for heat to escape. greenhouse gas Noun gas in the atmosphere, such as carbon dioxide, methane, water vapor, and ozone, that absorbs solar heat reflected by the surface of the Earth, warming the atmosphere.

Guglielmo Marconi (1874-1937) Italian electrical engineer and inventor. helicopter aircraft that flies using rotating blades on top of the body of the craft. helium a light, colorless gas with the chemical symbol He. hot air balloon bag filled with lighter-than-air gas able to float in the atmosphere. Hubble Space Telescope (1990-present) large, versatile NASA telescope orbiting the Earth. hydrogen Noun chemical element with the symbol H, whose most common isotope consists of a single electron and a single proton. hydrogen sulfide chemical compound gas responsible for the foul odor of rotten eggs. ice satellite in low-Earth orbit that houses several astronauts for months at a time. interplanetary space Noun space within the solar system but outside the atmospheres of any planets or moons. Also called the interplanetary medium. ion Noun electrically charged atom or group of atoms, formed by the atom having gained or lost an electron. ionosphere outer layer of the Earth's atmosphere, 80-400 kilometers (50-250 miles) above the surface.

jet stream between 160 kilometers (100 miles) and 2,000 kilometers (1,240 miles) above Earth's surface. low-pressure system Noun weather pattern characterized by low air pressure, usually as a result of warming. Low-pressure systems are often associated with storms. maneuver mass Noun unit of measurement (abbreviated m) determined by an object's resistance to change in the speed or direction of motion. medium-Earth orbit between 2,000 kilometers (1,243 miles) and 36,000 kilometers (22,370 miles) above the Earths surface. mesopause fluctuating area of the upper atmosphere between the mesosphere and the thermosphere. mesosphere Noun region in Earth's atmosphere between the stratosphere and the thermosphere, about 50-80 kilometers (31-50 miles) above the Earth's surface. meteor Noun person who studies patterns and changes in Earth's atmosphere. methane chemical compound that is the basic ingredient of natural gas. minute smallest physical unit of a substance, consisting of two or more atoms linked together.

mollusk Mount Everest Noun highest spot on Earth, 8,850 meters (29,035 feet). Mount Everest is part of the Himalaya range, in Nepal and China. nacreous pearly, or resembling the inside of a shell. neon chemical element (gas) with the symbol Ne. nitrogen chemical element with the symbol N, whose gas form is 78% of the Earth's atmosphere. noctilucent glowing, high-altitude clouds visible in the twilight sky. ocean path of one object around a more massive object. outer space chemical element with the symbol O, whose gas form is 21% of the Earth's atmosphere. ozone form of oxygen that absorbs ultraviolet radiation. ozone depletion process of the Earth's atmosphere losing ozone. ozone layer Noun (singular: phenomenon) any observable occurrence or feature. photosynthesis Noun process by which plants turn water, sunlight, and carbon dioxide into water, oxygen, and simple sugars. pilot person who steers a ship or aircraft. planet organism that produces its own food through photosynthesis and whose cells have walls. pole extreme north or south point of the Earth's axis.

primitive radio wave Noun electromagnetic wave with a wavelength between 1 millimeter and 30,000 meters, or a frequency between 10 kilohertz and 300,000 megahertz. region Noun any area on Earth with one or more common characteristics. Regions are the basic units of geography. device that moves through the atmosphere by release of expanding gas. sand small, loose grains of disintegrated rocks. satellite imagery photographs of a planet taken by or from a satellite. season rocky debris from space that enters Earth's atmosphere. Also called a meteor. significant solar flare Noun explosion in the sun's atmosphere, which releases a burst of energy and charged particles into the solar system. solar storm Noun sudden change in the Earth's magnetosphere, caused by the solar wind interacting with the Earth's magnetic field. Also called a geomagnetic storm. solar system the sun and the planets, asteroids, comets, and other bodies that orbit around it. solar wind Noun flow of charged particles, mainly protons and electrons, from the sun to the edge of the solar system.

sounding rocket Noun instrument that is launched but does not go into orbit, taking measurements, gathering data, and performing scientific experiments before falling back to Earth. southern lights Noun the bright bands of color around the South Pole caused by the solar wind and the Earth's magnetic field. Also known as the aurora australis. spacecraft vehicle designed for travel outside Earth's atmosphere. specific electrical discharge in the upper atmosphere, usually above thunderhead clouds. storm severe weather indicating a disturbed state of the atmosphere resulting from uplifted air. stratopause fluctuating area of the upper atmosphere between the stratosphere and the mesosphere. stratosphere Noun level of Earth's atmosphere, extending from 10 kilometers (6 miles) to 50 kilometers (31 miles) above the surface of the Earth. sub-orbital flight that has not reached the altitude or velocity to achieve orbit. sulfuric acid toxic chemical made of hydrogen, sulfur, and oxygen. temperature one of the four planets closest to the sun: Mercury, Venus, Earth, or Mars.

thermosphere Noun layer of the Earth's atmosphere located between 80 kilometers (50 miles) and 550 kilometers (341 miles) above the Earth's surface. thunderhead low-level cloud that produces rain, thunder, and lightning. Also called cumulonimbus. Titan largest moon of the planet Saturn. tropopause boundary between the troposphere and the stratosphere layers in the Earth's atmosphere. troposphere Noun lowest layer of the Earth's atmosphere, extending from the surface to about 16 kilometers (10 miles) above. turbopause fluctuating area of the upper atmosphere between the thermosphere and the exosphere. turbulence irregular, violent motion in the atmosphere. ultraviolet radiation Noun powerful light waves that are too short for humans to see, but can penetrate Earth's atmosphere. Ultraviolet is often shortened to UV. universe all known matter, energy, and space. unmanned lacking the physical presence of a person. valley depression in the Earth between hills. vapor visible liquid suspended in the air, such as fog.

volcano Noun an opening in the Earth's crust, through which lava, ash, and gases erupt, and also the cone built by eruptions. weather balloon Noun hydrogen-filled balloon equipped with tools to measure temperature, humidity, pressure, and other aspects of the atmosphere. weather pattern repeating or predictable changes in the Earth's atmosphere, such as winds, precipitation, and temperatures. weather satellite instrument that orbits the Earth to track weather and patterns in the atmosphere. weather system movement of warm or cold air. wind Noun movement of air (from a high pressure zone to a low pressure zone) caused by the uneven heating of the Earth by the sun. X-ray radiation in the electromagnetic spectrum with a very short wavelength and very high energy. zone Earth's Atmosphere | UCAR Center for Science Education Earth's Atmosphere The layered structure of Earth's atmosphere is visible in this sunset view from the International Space Station. Credit: Image Science & Analysis Laboratory, NASA Johnson Space Center Earth's atmosphere is a mixture of gases that surrounds our home planet. Besides providing us with something to breathe, the atmosphere helps make life on Earth possible in several ways.

It shields us from most of the harmful ultraviolet (UV) radiation coming from the Sun, warms the surface of our planet by about 33° C (59° F) via the greenhouse effect, and largely prevents extreme differences between daytime and nighttime temperatures. The atmosphere is a mixture of many different gases. 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. The atmosphere grows thinner (less dense and lower in pressure) as one moves upward from Earth's surface. It gradually gives way to the vacuum of outer space.

There is no precise "top" of the atmosphere. Air becomes so thin at altitudes between 100 and 120 km (62-75 miles) up that for many purposes that range of heights can be considered the boundary between the atmosphere and space. However, there are very thin but measurable traces of atmospheric gases hundreds of kilometers/miles above Earth's surface. There are several different regions or layers in the atmosphere. Each has characteristic temperatures, pressures, and phenomena. We live in the troposphere , the lowest layer, where most clouds are found and almost all weather occurs. Some jet aircraft fly in the next higher layer, the stratosphere , which contains the jet streams and the ozone layer. Higher still are the mesosphere , thermosphere and exosphere . © 2011 UCAR Earth's Atmosphere: Composition, Climate & Weather Earth's Atmosphere: Composition, Climate & Weather By Tim Sharp, Reference Editor | September 19, 2012 06:52pm ET MORE Earth is the only planet in the solar system with an atmosphere that can sustain life.

The blanket of gases not only contains the air that we breathe but also protects us from the blasts of heat and radiation emanating from the sun. It warms the planet by day and cools it at night. Astronauts aboard the International Space Station took this image showing Earth's atmosphere and moon on July 31, 2011. Credit: ISS Crew Earth Observations Experiment and Image Science & Analysis Laboratory/Johnson Space Center. Earth's atmosphere is about 300 miles (480 kilometers) thick, but most of it is within 10 miles (16 km) the surface. Air pressure decreases with altitude. At sea level, air pressure is about 14.7 pounds per square inch (1 kilogram per square centimeter). At 10,000 feet (3 km), the air pressure is 10 pounds per square inch (0.7 kg per square cm). There is also less oxygen to breathe. Composition of air The gases in Earth's atmosphere include: Nitrogen – 78 percent Water vapor and other gases exist in small amounts as well.

Atmosphere layers Earth's atmosphere is divided into five main layers, the exosphere, the thermosphere, the mesosphere, the stratosphere and the troposphere. The atmosphere thins out in each higher layer until the gases dissipate in space. There is no distinct boundary between the atmosphere and space, but an imaginary line about 68 miles (110 kilometers) from the surface, called the Karman line, is usually where scientists say atmosphere meets outer space. The troposphere is the layer closest to Earth's surface. It is 4 to 12 miles (7 to 20 km) thick and contains half of Earth's atmosphere. Air is warmer near the ground and gets colder higher up. Nearly all of the water vapor and dust in the atmosphere are in this layer and that is why clouds are found here. The stratosphere is the second layer. It starts above the troposphere and ends about 31 miles (50 km) above ground. Ozone is abundant here and it heats the atmosphere while also absorbing harmful radiation from the sun. The air here is very dry, and it is about a thousand times thinner here than it is at sea level.

Because of that, this is where jet aircraft and weather balloons fly. The mesosphere starts at 31 miles (50 km) and extends to 53 miles (85 km) high. The top of the mesosphere, called the mesopause, is the coldest part of Earth's atmosphere with temperatures averaging about minus 130 degrees F (minus 90 C). This layer is hard to study. Jets and balloons don't go high enough, and satellites and space shuttles orbit too high. Scientists do know that meteors burn up in this layer. The thermosphere extends from about 56 miles (90 km) to between 310 and 620 miles (500 and 1,000 km). Temperatures can get up to 2,700 degrees F (1,500 C) at this altitude. The thermosphere is considered part of Earth's atmosphere, but air density is so low that most of this layer is what is normally thought of as outer space. In fact, this is where the space shuttles flew and where the International Space Station orbits Earth. This is also the layer where the auroras occur.

Charged particles from space collide with atoms and molecules in the thermosphere, exciting them into higher states of energy. The atoms shed this excess energy by emitting photons of light, which we see as the colorful Aurora Borealis and Aurora Australis . The exosphere, the highest layer, is extremely thin and is where the atmosphere merges into outer space. It is composed of very widely dispersed particles of hydrogen and helium. Climate and weather Earth is able to support a wide variety of living beings because of its diverse regional climates, which range from extreme cold at the poles to tropical heat at the Equator. Regional climate is often described as the average weather in a place over more than 30 years. A region's climate is often described, for example, as sunny, windy, dry, or humid. These can also describe the weather in a certain place, but while the weather can change in just a few hours, climate changes over a longer span of time. Earth's global climate is an average of regional climates. The global climate has cooled and warmed throughout history. Today, we are seeing unusually rapid warming. The scientific consensus is that greenhouse gases, which are increasing because of human activities, are trapping heat in the atmosphere.

— Tim Sharp, Reference Editor NOAA/NASA SciJinks :: How did Earth's atmosphere form? How did Earth's atmosphere form? Breathe! No one knows of any other planet where you can do this simple thing. Other planets and moons in our solar system have atmospheres, but none of them could support life as we know it. They are either too dense (as on Venus) or not dense enough (as on Mars), and none of them have much oxygen, the precious gas that we Earth animals need every minute. So how did our atmosphere get to be so special? Some scientists describe three stages in the evolution of Earth’s atmosphere as it is today. Just formed Earth: Like Earth, the hydrogen (H2) and helium (He) were very warm. These molecules of gas moved so fast they escaped Earth's gravity and eventually all drifted off into space. Earth’s original atmosphere was probably just hydrogen and helium, because these were the main gases in the dusty, gassy disk around the Sun from which the planets formed. The Earth and its atmosphere were very hot. Molecules of hydrogen and helium move really fast, especially when warm. Actually, they moved so fast they eventually all escaped Earth's gravity and drifted off into space.

Young Earth: Volcanoes released gases H2O (water) as steam, carbon dixoide (CO2), and ammonia (NH3). Carbon dioxide dissolved in seawater. Simple bacteria thrived on sunlight and CO2. By-product is oxygen (O2). Earth’s “second atmosphere” came from Earth itself. There were lots of volcanoes, many more than today, because Earth’s crust was still forming. The volcanoes released steam (H2O, with two hydrogen atoms and one oxygen atom), carbon dioxide (CO2, with one carbon atoms and two oxygen atoms), ammonia (NH3, with one nitrogen atom and three hydrogen atoms). Current Earth: Plants and animals thrive in balance. Plants take in carbon dioxide (CO2) and give off oxygen (O2). Animals take in oxygen (O2) and give off CO2. Burning stuff also gives off CO2. Much of the CO2 dissolved into the oceans. Eventually, a simple form of bacteria developed that could live on energy from the Sun and carbon dioxide in the water, producing oxygen as a waste product. Thus, oxygen began to build up in the atmosphere, while the carbon dioxide levels continued to drop.

Meanwhile, the ammonia molecules in the atmosphere were broken apart by sunlight, leaving nitrogen and hydrogen. The hydrogen, being the lightest element, rose to the top of the atmosphere and much of it eventually drifted off into space. Now we have Earth’s “third atmosphere,” the one we all know and love—an atmosphere containing enough oxygen for animals, including ourselves, to evolve. So plants and some bacteria use carbon dioxide and give off oxygen, and animals use oxygen and give off carbon-dioxide—how convenient! The atmosphere upon which life depends was created by life itself. The atmosphere life depends on was created by life itself? Woah! What's in Earth's atmosphere now? Note that this pie does not account for any water in the air. As you can see, air is mostly nitrogen (78%), with oxygen a distant runner up (21%). Argon and some other gases make up another small amount (about 1%). And carbon dioxide is only a very tiny slice (.0385% or only about 385 parts per million parts of air). But these proportions were quite likely different in Earth’s much younger days.

Composition of the Atmosphere | Climate Education Modules for K-12 Composition of the Atmosphere Composition of the Atmosphere body The atmosphere is concentrated at the earth’s surface and rapidly thins as you move upward, blending with space at roughly 100 miles above sea level. The atmosphere is actually very thin compared to the size of the earth, equivalent in thickness to a piece of paper laid over a beach ball. However, it is responsible for keeping our earth habitable and for producing weather. Figure A The atmosphere is composed of a mix of several different gases in differing amounts. The permanent gases whose percentages do not change from day to day are nitrogen, oxygen and argon. Nitrogen accounts for 78% of the atmosphere, oxygen 21% and argon 0.9%. Gases like carbon dioxide, nitrous oxides, methane, and ozone are trace gases that account for about a tenth of one percent of the atmosphere. Water vapor is unique in that its concentration varies from 0-4% of the atmosphere depending on where you are and what time of the day it is.

In the cold, dry artic regions water vapor usually accounts for less than 1% of the atmosphere, while in humid, tropical regions water vapor can account for almost 4% of the atmosphere. Water vapor content is very important in predicting weather. Greenhouse gases whose percentages vary daily, seasonally, and annually have physical and chemical properties which make them interact with solar radiation and infrared light (heat) given off from the earth to affect the energy balance of the globe. This is why scientists are watching the observed increase in greenhouse gases like carbon dioxide and methane carefully, because even though they are small in amount, they can strongly affect the global energy balance and temperature over time.

Question: Which gas forms about 78% of the Earth's atmosphere?
Answer: Nitrogen