What is the Earth’s atmosphere?
The Earth’s atmosphere is a thin layer of gases that surrounds the Earth. It seals the planet and protects us from the vacuum of space. It protects us from electromagnetic radiation given off by the Sun and small objects flying through space such as meteoroids.
Atmospheres in the solar system
Here on Earth, we tend to take our atmosphere for granted, and not without reason. Our atmosphere has a lovely mix of 78% nitrogen and 21% oxygen with trace amounts of water vapor, carbon dioxide and other gaseous molecules. In short, our atmosphere is plentiful and life-sustaining. But what about the other planets of the Solar System? How do they stack up in terms of atmospheric composition? We know for a fact that they are not breathable by humans and cannot support life. But just what is the difference between these balls of rock and gas and our own?
Our next-door neighbor, Venus, has a thick carbon-dioxide atmosphere with no oxygen, but with sulfuric acid clouds. Mars also has an atmosphere that is primarily carbon-dioxide, but on Mars, the atmosphere is extremely thin, and prone to high winds. The giant jovian planets- Jupiter, Saturn, Uranus, and Neptune have extremely thick atmospheres of hydrogen, helium, methane, and ammonia. In fact, the jovian planets are often called gaseous planets because they are largely composed of their very thick gaseous atmospheres.
The presence of hydrogen and hydrogen-rich molecules in the atmospheres of the outer planets is easy to rationalize. Most of the universe is hydrogen. The only reason hydrogen and helium are rare in the inner solar system is that these light gases were blown away in the early stages of the solar system via radiation pressure and the solar wind. Planets like Jupiter kept their hydrogen, and, since the hydrogen atom likes to combine with other atoms, you get things like methane, and ammonia.
The thickness of a planet's atmosphere depends on the planet's gravity and the temperature of the atmosphere. A planet with weaker gravity (because it is smaller) does not have as strong a hold on the molecules that make up its atmosphere as a planet with stronger gravity, so the gas molecules will be more likely to escape the planet's gravity.
If the atmosphere is cool enough, then the gas molecules will not be moving fast enough to escape the planet's gravity. But how strong is ``strong enough'' and how cool is ``cool enough'' to hold onto an atmosphere? To answer that you need to consider the following about a planet:
