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An atmosphere is a layer or a set of layers of gases surrounding or composing part of a planet or star that is held in place by the body's gravity. An atmosphere is more likely to be retained if the gravity it is subject to is high and atmosphere possesses a low temperature.
The atmosphere extends hundreds of kilometers into space but 99 percent of its air is in the 80 km closest to Earth. The gases that make up the atmosphere are:
and many other trace gases such as helium, water vapor, neon, krypton, and hydrogen. The important greenhouse gas Carbon Dioxide comprises a tiny 0.04% of the air. The amount of water vapor varies, but is usually around 1%. The Earth's atmosphere is made out of four (or five) primary layers.
- The Thermosphere (80 to 480 kilometers) is warmer than the underlying layers due to the strength of the Sun’s rays.
- The Mesosphere (45 to 180 kilometers) is where meteors burn up as they enter the atmosphere.
- The Stratosphere (10 to 45 kilometers) contains the ozone layer which absorbs most of the harmful UV light from the Sun. Aircrafts fly in its lower levels.
- The Troposphere, which extends from ground level to 10 kilometers in altitude. The Troposphere contains 80% of the mass of the air and is where clouds form. Temperatures decrease as you gain altitude down to below -50 degrees. This is where most of the greenhouse effect occurs.
- The Ionosphere overlaps the thermosphere and the mesosphere and is made up of charged ions produced by the strength of the sunlight. Swirling patterns of light called auroras can be seen in the ionosphere near the poles.
In other planets
Mercury's atmosphere is so small, it cannot blow up a party balloon to full size. It furthermore consists of mainly sodium gas clouds. Due to Mercury's thin atmosphere, Mercury is extremely hot during the day, but without any air to trap the heat, Mercury's temperature drops to well below freezing at night.