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.

atmosphere
Our atmosphere—the thin blue line.

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.

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.

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.

absorb
Verb

to soak up.

accumulate
Verb

to gather or collect.

Noun

layer of gases surrounding Earth.

aircraft
Noun

vehicle able to travel and operate above the ground.

airline
Noun

system or business that provides air transportation.

Noun

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

Noun

the distance above sea level.

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.

Noun

layers of gases surrounding a planet or other celestial body.

atom
Noun

the basic unit of an element, composed of three major parts: electrons, protons, and neutrons.

Noun

brightly colored bands of light, visible around Earth's geomagnetic poles, caused by solar wind interacting with particles in Earth's magnetic field.

Noun

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

Plural Noun

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

Noun

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

belt
Noun

dark-colored band of clouds on Jupiter or Saturn.

boulder
Noun

large rock.

Noun

line separating geographical areas.

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.

Noun

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

complex
Adjective

complicated.

contract
Verb

to shrink or get smaller.

coronal mass ejection
Noun

huge burst of solar wind and other charged particles.

Noun

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

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.

Noun

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

Noun

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

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.

Noun

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

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.

Noun

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

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.

Noun

clouds at ground level.

Noun

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

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.

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
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.

Noun

water in its solid form.

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.

Noun

winds speeding through the upper atmosphere.

Noun

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

Noun

sudden electrical discharge from clouds.

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.

Noun

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

Noun

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

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

large phylum of invertebrate animal, all possessing a mantle with a significant cavity used for breathing and excretion, a radula (except for bivalves), and the structure of the nervous system. 

Noun

natural satellite of a planet.

mountaineer
Noun

someone who climbs mountains.

Mount Everest
Noun

highest spot on Earth, approximately 8,850 meters (29,035 feet). Mount Everest is part of the Himalaya and straddles the border of 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.

Noun

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

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.

Noun

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

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.

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.

Noun

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

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.

Noun

any area on Earth with one or more common characteristics. Regions are the basic units of geography.

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.

Noun

period of the year distinguished by special climatic conditions.

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

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

suborbital
Adjective

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

sulfuric acid
Noun

toxic chemical made of hydrogen, sulfur, and oxygen.

Noun

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

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.

Noun

an opening in the Earth's crust, through which lava, ash, and gases erupt, and also the cone built by eruptions.

Noun

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

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.

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.