How high up does the atmosphere reach?

The end of the atmosphere is not like the horizon, where you can definitely say, "This is where the Earth ends and the atmosphere begins." Rather, as one travels higher and higher, the atmosphere gets thinner and thinner. One can say, for practical purposes, that the upper atmosphere begins to be indistinguishable from outer space at about 435 miles (700 kilometers) altitude, but that is really just a random place to draw the borderline. The density of the atmosphere is getting very thin indeed at an elevation of 370 miles (about 600 kilometers). At this height, there are about six miles (10 kilometers) between each molecule (this gap is known as the "mean free path." The air pressure here is, effectively, zero.

How did Earth's atmosphere form?

Some of Earth's atmosphere was probably gas captured from the solar nebula four and a half billion years ago, when our planet was forming. It is thought that most of Earth's atmosphere was trapped beneath Earth's surface, escaping through volcanic eruptions and other crustal cracks and fissures. Water vapor was the most plentiful gas to spew out, and it condensed to form the oceans, lakes, and other surface water. Carbon dioxide was probably the next most plentiful gas, and much of it dissolved in the water or combined chemically with rocks on the surface. Nitrogen came out in smaller amounts, but did not undergo significant condensation or chemical reactions. This is why scientists think it is the most abundant gas in our atmosphere.

The high concentration of oxygen in our atmosphere is very unusual for planets, because oxygen is highly reactive and combines easily with other elements. In order to maintain oxygen in gaseous form, it must constantly be replenished. On

Where does the word "gas" come from?

The person who is credited with coming up with the word "gas" is Flemish physician Jan Baptista van Helmont (1577-1644). His experiments with gas and volume taught him that gases always take up all the space within a container (they do not leave a vacuum). Thus, he surmised that substances in a gaseous state exist in a chaotic form. The Flemish pronunciation for "chaos" sounds like "gas," and thus the word was born.

Earth, this is accomplished by plants and algae that conduct photosynthesis, removing carbon dioxide from the atmosphere and adding oxygen into it. It was during the Carboniferous Period about 300 million years ago that plant growth dramatically changed the atmosphere, increasing the amount of oxygen to 35 percent. Today, the oxygen content is not quite as rich (21 percent).

How is Earth's atmosphere important to life on Earth?

Very few life forms on Earth can survive for any length of time at all without Earth's atmosphere. We breathe the atmosphere; and it blocks harmful radiation from space. The pressure it provides keeps surface water liquid, and the greenhouse effect it produces keeps us warm.

How thick is Earth's atmosphere?

Earth's atmosphere extends hundreds of miles beyond its surface, but it is much denser at the surface than at high altitudes. About half of the gas in Earth's atmosphere is within a few kilometers of the surface, and 95 percent of the gas is found within 12 miles (19 kilometers) of the surface.

Are we losing our atmosphere?

Yes, but don't worry; the number of molecules and ions escaping our atmosphere is very tiny and will not deplete our atmosphere significantly for billions of years. Scientists monitoring the magnetosphere learned that periodic changes in the magnetosphere help to accelerate particles, especially ions, with enough speed to escape Earth's gravity. If the Earth's gravity were weaker, however, this interaction might have caused our planet to lose its atmosphere at a significant rate. Indeed, some astronomers speculate that this may be what happened to Mars's atmosphere.

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