Exploring Earth’s Atmosphere Imagine you are on a spaceship that has come to explore Earth. You are the scientist in charge of studying Earth’s atmosphere. What would you see first?

The Exosphere As your ship approaches within 190,000km of Earth, you begin to detect higher levels of hydrogen and helium than in “empty” space. Your ultra-violet sensors show you a glowing geocorona extending 100,000 km from the planet. You have entered the Exosphere.

Exosphere (250km-100Mm) The exosphere is a region of very low density gas beginning 250-500km above the earth and extending halfway to the moon. It is mostly hydrogen and helium atoms, so spread out they almost never bump into each other. The hydrogen gas reflects high energy UV light (120-125nm) creating an invisible glow around the earth.

The Thermosphere As you move into orbit 250km above the Earth, you begin to detect atoms of other gases, including oxygen. The density of the atmosphere is still very low, but it starts to slow your ship down. The captain has to use the ion drive to maintain altitude. He also has to be careful to avoid the many satellites orbiting in the thermosphere. As you pass near the poles, you see beautiful, multi-colored lights created by solar radiation striking the Earth’s magnetic field.

Thermosphere (80-250km) The thermosphere is composed mostly nitrogen (N2) and oxygen (O2)along with some hydrogen (H) and helium (He). Atoms in the thermosphere are hit with large amounts of solar radiation. This makes the atoms very hot, but the air density is so low, you wouldn’t feel the heat. Because the air is so thin, many satellites orbit in the thermosphere. This radiation also splits some nitrogen and oxygen molecules into individual atoms. The radiation also electrically charges the atoms creating a region called the ionosphere. Ions there interact with the solar wind to produce auroras.

The Mesosphere You’d like the captain to bring you down into the Mesosphere, between 50 and 80km up. The captain won’t do that because the increased density of the air would create more friction and burn up the ship. You have to settle for dropping a probe and enjoying the unusual weather phenomena.

Mesosphere (50-80km) Below the thermosphere is the mesosphere. Meso- means middle. The mesosphere has strange weather—glowing clouds and red lightning (sprites). The mesosphere is also the layer that makes meteors and returning spacecraft glow from the friction of passing through.

The Stratosphere As your probe enters the Earth’s stratosphere it detects a increase in O3 or ozone. Ozone is formed when ultraviolet radiation splits an oxygen molecule into two atoms and those atoms bond with other oxygen molecules. If an ozone molecule is hit by UV radiation, it will split back into an O2 and O atom. As the probe descends through this layer ozone concentrations increase and UV radiation decreases.

Stratosphere (10-50km) Below the mesosphere is the stratosphere. This is the layer where passenger jets fly. The oxygen is very thin, but dense enough block ultraviolet radiation. The ultraviolet radiation helps to form ozone (O3), which also blocks UV light. Without the ozone that builds up in the stratosphere, plants and animals would die from radiation.

The Troposphere At 10km from the ground the air is thick enough to open your probe’s parachute. The sensors show temperature increasing as it nears the ground and increasing amounts of water vapor in the air. Heat and water vapor mean this layer is full of weather and life.

Troposphere (0-10km) The troposphere extends from the ground up to 10km. The troposphere is thickest at the equator. (up to 16km) It is the most dense layer. (It contains 75% of the atmosphere’s mass but is the smallest layer.) Most weather occurs in the troposphere.

Layers Our atmosphere is divided into layers. The troposphere is the lowest. Next is the stratosphere. The mesosphere is in the middle. Above it is the thermosphere. The outer most layer is the exosphere.

Pauses Where does one layer stop and then next start? As you get higher in the troposphere, temperature goes down, but it starts to go back up in the stratosphere. The height where temperature starts to go back up is called the tropopause and marks the boundary between troposphere and stratosphere. The stratopause is where the temperature starts to go back down and the stratosphere turns into the mesosphere. The mesopause is the coldest part of the atmosphere, above it temperatures start to rise in the thermosphere.