What is the Atmosphere?
The atmosphere is a mixture of gases that surrounds the planet. On Earth, the atmosphere helps make life possible. Besides providing us with something to breathe, it shields us from most of the harmful ultraviolet (UV) radiation coming from the Sun, warms the surface of our planet by about 33° C (59° F) via the greenhouse effect, and largely prevents extreme differences between daytime and nighttime temperatures. The other planets in our solar system also have an atmosphere, but none of them have the same ratio of gases and layered structure as Earth's atmosphere.
Gases in Earth's Atmosphere
Nitrogen and oxygen are by far the most common; dry air is composed of about 78% nitrogen (N2) and about 21% oxygen (O2). Argon, carbon dioxide (CO2), and many other gases are also present in much lower amounts; each makes up less than 1% of the atmosphere's mixture of gases. The atmosphere also includes water vapor. The amount of water vapor present varies a lot, but on average is around 1%. There are also many small particles - solids and liquids - "floating" in the atmosphere. These particles, which scientists call "aerosols", include dust, spores and pollen, salt from sea spray, volcanic ash, smoke, and more.
Layers of Earth's Atmosphere
The atmosphere grows thinner (less dense and lower in pressure) as one moves upward from Earth's surface. It gradually gives way to the vacuum of outer space. There is no precise "top" of the atmosphere. Air becomes so thin at altitudes between 100 and 120 km (62-75 miles) up that for many purposes that range of heights can be considered the boundary between the atmosphere and space. However, there are very thin but measurable traces of atmospheric gases hundreds of kilometers/miles above Earth's surface.
There are several different regions or layers in Earth's atmosphere. Each has characteristic temperatures, pressures, and phenomena. We live in the troposphere, the lowest layer, where most clouds are found and almost all weather occurs. Some jet aircraft fly in the next higher layer, the stratosphere, which contains the jet streams and the ozone layer. Temperatures reacj their lowest in the mesosphere, because the there are almost no air molecules there to absorb heat energy. The sky also changes from blue to black in the mesosphere, because there are so few molecules for light to refract off of there. And fartherst from the surface we have the thermosphere, which is the widest layer of the atmosphere and absorbs much of the harmful radiation that reaches Earth from th Sun. The exosphere represents the transition from Earth's atmosphere to space.
Earth is not the only world with an atmosphere. All of the planets - and even a few moons - in our solar system have atmospheres. Some have clouds, wind, rain and powerful storms. Recently scientists have also begun to get glimpses of atmospheres of planets in other solar systems.
Each of the planets in our solar system has an uniquely structured atmosphere. The atmosphere of Mercury is extremely thin and is not very different from the vacuum of space. All four giant planets in our solar system - Jupiter, Saturn, Uranus and Neptune - have very thick, deep atmospheres. The smaller, rocky planets - Earth, Venus and Mars - have much thinner atmospheres hovering above their solid surfaces. The atmospheres on moons in our solar sytem are typically quite thin. Saturn's moon Titan is an exception - air pressure at the surface of Titan is higher than on Earth! Of the five officially recognized dwarf planets, Pluto has a thin, seasonal atmosphere containing nitrogen, methane and carbon monoxide, and Ceres may have an extremely thin atmosphere of water vapor. But only Earth's atmosphere has the layered structure that allows enough light energy to enter and be trapped for warmth, but also shields us from too much harmful radiation. This important balance is necessary to maintain life on Earth.
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