• atmosphere
    Our atmosphere—the thin blue line.

    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.

    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 balancednot 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 thickthe planets surface temperature is more than 460 degrees Celsius (860 degrees Fahrenheit).

    Jupiters 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 Europas 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.


    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.

    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.


    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.


    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.


    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.”


    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 Part of Speech Definition Encyclopedic Entry
    absorb Verb

    to soak up.

    accumulate Verb

    to gather or collect.

    air Noun

    layer of gases surrounding Earth.

    Encyclopedic Entry: air
    aircraft Noun

    vehicle able to travel and operate above the ground.

    airline Noun

    system or business that provides air transportation.

    air mass Noun

    a large volume of air that is mostly consistent, horizontally, in temperature and humidity.

    Encyclopedic Entry: air mass
    altitude Noun

    the distance above sea level.

    Encyclopedic Entry: altitude
    ammonia Noun

    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 Noun

    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 Noun

    area of the solar system between the orbits of Mars and Jupiter filled with asteroids.

    astrobiologist Noun

    person who studies the possibility of life in outer space.

    astronomer Noun

    person who studies space and the universe beyond Earth's atmosphere.

    atmosphere Noun

    layers of gases surrounding a planet or other celestial body.

    Encyclopedic Entry: atmosphere
    atom Noun

    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.

    Encyclopedic Entry: aurora
    autotroph Noun

    organism that can produce its own food and nutrients from chemicals in the atmosphere, usually through photosynthesis or chemosynthesis.

    Encyclopedic Entry: autotroph
    bacteria Plural Noun

    (singular: bacterium) single-celled organisms found in every ecosystem on Earth.

    beach Noun

    narrow strip of land that lies along a body of water.

    Encyclopedic Entry: beach
    belt Noun

    dark-colored band of clouds on Jupiter or Saturn.

    boulder Noun

    large rock.

    boundary Noun

    line separating geographical areas.

    Encyclopedic Entry: boundary
    canister Noun

    container, usually shaped like a long tube.

    carbon Noun

    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 Noun

    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 Noun

    thin, high-altitude cloud.

    climate Noun

    all weather conditions for a given location over a period of time.

    Encyclopedic Entry: climate
    cloud Noun

    visible mass of tiny water droplets or ice crystals in Earth's atmosphere.

    Encyclopedic Entry: cloud
    complex Adjective


    contract Verb

    to shrink or get smaller.

    coronal mass ejection Noun

    huge burst of solar wind and other charged particles.

    crater Noun

    bowl-shaped depression formed by a volcanic eruption or impact of a meteorite.

    Encyclopedic Entry: crater
    crucial Adjective

    very important.

    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 Plural Noun

    (singular: datum) information collected during a scientific study.

    dominate Verb

    to overpower or control.

    dry air Noun

    standard measurement of gases that make up air at sea level, excluding water vapor.

    dust Noun

    tiny, dry particles of material solid enough for wind to carry.

    Encyclopedic Entry: dust
    Earth Noun

    our planet, the third from the Sun. The Earth is the only place in the known universe that supports life.

    Encyclopedic Entry: Earth
    eject Verb

    to get rid of or throw out.

    electricity Noun

    set of physical phenomena associated with the presence and flow of electric charge.

    electron Noun

    negatively charged subatomic particle.

    element Noun

    chemical that cannot be separated into simpler substances.

    El Nino Noun

    irregular, recurring weather system that features a warm, eastern-flowing ocean current in the eastern Pacific Ocean.

    Encyclopedic Entry: El Niño
    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 Noun

    imaginary line around the Earth, another planet, or star running east-west, 0 degrees latitude.

    Encyclopedic Entry: equator
    erode Verb

    to wear away.

    exobase Noun

    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 Verb

    to grow.

    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 Verb

    to constantly change back and forth.

    fog Noun

    clouds at ground level.

    Encyclopedic Entry: fog
    food Noun

    material, usually of plant or animal origin, that living organisms use to obtain nutrients.

    Encyclopedic Entry: food
    free electron Noun

    electron that has been temporarily knocked off an atom.

    free oxygen Noun

    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 Noun

    fuzzy blue layer of hydrogen surrounding the Earth at the upper boundary of the exosphere.

    gigantic Adjective

    very large.

    Great Red Spot Noun

    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.

    Encyclopedic Entry: greenhouse effect
    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 Noun

    (1874-1937) Italian electrical engineer and inventor.

    helicopter Noun

    aircraft that flies using rotating blades on top of the body of the craft.

    helium Noun

    a light, colorless gas with the chemical symbol He.

    hot air balloon Noun

    bag filled with lighter-than-air gas able to float in the atmosphere.

    Hubble Space Telescope Noun

    (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 Noun

    chemical compound gas responsible for the foul odor of rotten eggs.

    ice Noun

    water in its solid form.

    Encyclopedic Entry: ice
    indicate Verb

    to display or show.

    International Space Station (ISS) Noun

    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 Noun

    outer layer of the Earth's atmosphere, 80-400 kilometers (50-250 miles) above the surface.

    jet stream Noun

    winds speeding through the upper atmosphere.

    Encyclopedic Entry: jet stream
    latitude Noun

    distance north or south of the Equator, measured in degrees.

    Encyclopedic Entry: latitude
    lightning Noun

    sudden electrical discharge from clouds.

    Encyclopedic Entry: lightning
    low-Earth orbit Noun

    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 Noun

    a skillful movement.

    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 Noun

    between 2,000 kilometers (1,243 miles) and 36,000 kilometers (22,370 miles) above the Earths surface.

    mesopause Noun

    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

    rocky debris from space that enters Earth's atmosphere. Also called a shooting star or falling star.

    Encyclopedic Entry: meteor
    meteorite Noun

    type of rock that has crashed into Earth from outside the atmosphere.

    Encyclopedic Entry: meteorite
    meteorologist Noun

    person who studies patterns and changes in Earth's atmosphere.

    methane Noun

    chemical compound that is the basic ingredient of natural gas.

    minute Adjective

    very small amount.

    molecule Noun

    smallest physical unit of a substance, consisting of two or more atoms linked together.

    mollusk Noun

    type of invertebrate animal.

    moon Noun

    natural satellite of a planet.

    Encyclopedic Entry: moon
    mountaineer Noun

    someone who climbs mountains.

    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 Adjective

    pearly, or resembling the inside of a shell.

    neon Noun

    chemical element (gas) with the symbol Ne.

    nitrogen Noun

    chemical element with the symbol N, whose gas form is 78% of the Earth's atmosphere.

    noctilucent Adjective

    glowing, high-altitude clouds visible in the twilight sky.

    ocean Noun

    large body of salt water that covers most of the Earth.

    Encyclopedic Entry: ocean
    orbit Noun

    path of one object around a more massive object.

    outer space Noun

    space beyond Earth's atmosphere.

    oxygen Noun

    chemical element with the symbol O, whose gas form is 21% of the Earth's atmosphere.

    ozone Noun

    form of oxygen that absorbs ultraviolet radiation.

    ozone depletion Noun

    process of the Earth's atmosphere losing ozone.

    ozone layer Noun

    layer in the atmosphere containing the gas ozone, which absorbs most of the sun's ultraviolet radiation.

    Encyclopedic Entry: ozone layer
    particle Noun

    small piece of material.

    pebble Noun

    very small, rounded rock.

    penetrate Verb

    to push through.

    phenomena Plural 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 Noun

    person who steers a ship or aircraft.

    planet Noun

    large, spherical celestial body that regularly rotates around a star.

    Encyclopedic Entry: planet
    plant Noun

    organism that produces its own food through photosynthesis and whose cells have walls.

    pole Noun

    extreme north or south point of the Earth's axis.

    primitive Adjective

    simple or crude.

    propel Verb

    to push forward.

    proton Noun

    positively charged subatomic particle.

    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.

    Encyclopedic Entry: region
    rocket Noun

    device that moves through the atmosphere by release of expanding gas.

    sand Noun

    small, loose grains of disintegrated rocks.

    satellite imagery Noun

    photographs of a planet taken by or from a satellite.

    season Noun

    period of the year distinguished by special climatic conditions.

    Encyclopedic Entry: season
    shooting star Noun

    rocky debris from space that enters Earth's atmosphere. Also called a meteor.

    significant Adjective

    important or impressive.

    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 Noun

    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 Noun

    vehicle designed for travel outside Earth's atmosphere.

    specific Adjective

    exact or precise.

    speculate Verb

    to consider or guess.

    sprite Noun

    electrical discharge in the upper atmosphere, usually above thunderhead clouds.

    storm Noun

    severe weather indicating a disturbed state of the atmosphere resulting from uplifted air.

    stratopause Noun

    fluctuating area of the upper atmosphere between the stratosphere and the mesosphere.

    stratosphere Noun

    highest level of Earth's atmosphere, extending from 10 kilometers (6 miles) to 50 kilometers (31 miles) above the surface of the Earth.

    sub-orbital Adjective

    flight that has not reached the altitude or velocity to achieve orbit.

    sulfuric acid Noun

    toxic chemical made of hydrogen, sulfur, and oxygen.

    temperature Noun

    degree of hotness or coldness measured by a thermometer with a numerical scale.

    Encyclopedic Entry: temperature
    terrestrial planet Noun

    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 Noun

    low-level cloud that produces rain, thunder, and lightning. Also called cumulonimbus.

    Titan Noun

    largest moon of the planet Saturn.

    tropopause Noun

    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 Noun

    fluctuating area of the upper atmosphere between the thermosphere and the exosphere.

    turbulence Noun

    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 Noun

    all known matter, energy, and space.

    unmanned Adjective

    lacking the physical presence of a person.

    valley Noun

    depression in the Earth between hills.

    vapor Noun

    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.

    Encyclopedic Entry: volcano
    weather Noun

    state of the atmosphere, including temperature, atmospheric pressure, wind, humidity, precipitation, and cloudiness.

    Encyclopedic Entry: weather
    weather balloon Noun

    hydrogen-filled balloon equipped with tools to measure temperature, humidity, pressure, and other aspects of the atmosphere.

    weather pattern Noun

    repeating or predictable changes in the Earth's atmosphere, such as winds, precipitation, and temperatures.

    weather satellite Noun

    instrument that orbits the Earth to track weather and patterns in the atmosphere.

    weather system Noun

    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 Noun

    radiation in the electromagnetic spectrum with a very short wavelength and very high energy.

    zone Noun

    light-colored band of clouds on Jupiter or Saturn.