Getting ready: why does Earth need an atmosphere?

• The unit opens with questions: Which planets in the solar system have an atmosphere?
• How thick is it?
• What are its components?
• What would happen if Earth had no atmosphere?

• Which planets have atmospheres? All the planets have some gas around them except for Mercury, which has only a vanishingly thin envelope. Venus has a very thick atmosphere of carbon dioxide (surface temperature about 465 °C because of a runaway greenhouse effect); Mars has a very thin atmosphere (mostly CO₂, pressure less than 1 % of Earth's); Jupiter, Saturn, Uranus and Neptune are surrounded by enormous atmospheres of hydrogen and helium.
• Earth's atmosphere is a thin shell compared with the size of the planet: about 99 % of its mass lies within the lowest 30 km, which is less than 0.5 % of Earth's radius (6371 km). Its outer edge fades gradually into space at about 1000 km.
• Without an atmosphere Earth would resemble the Moon or Mercury: no air to breathe; no liquid water (it would boil away or freeze because of the low pressure); extreme temperature differences (+120 °C in sunlight, −150 °C at night); no protection from harmful ultraviolet rays and meteors; no weather, wind or rain; no sound; and no life as we know it.

• In the previous unit you studied the hydrosphere, one of the four spheres of the environment.
• Now we move to another sphere — the atmosphere.

The atmosphere

• is a layer of gases that surrounds planet Earth.
• It is mainly composed of nitrogen, oxygen and carbon dioxide, along with argon and trace amounts of other gases.