If someone asks, what is the main gas in the earth's atmosphere? Many people may first think of oxygen as the answer, but the actual situation is that the most important gas in the earth's atmosphere is nitrogen. Measurement data shows that the nitrogen content in the earth's atmosphere is as high as 78, while the oxygen content is only about 21. In comparison, the nitrogen content in the earth's atmosphere is about 3.7 times that of oxygen.
The solar system was formed from a huge primitive nebula, called the "solar nebula". About 4.6 billion years ago, this nebula underwent gravitational collapse. As the size of the nebula continued to shrink, its core The material in the area becomes denser and denser, and the temperature and pressure continue to rise. When it reaches a certain level, the sun is born.
After that, the remaining material in the nebula continued to revolve around the sun, and gradually formed various celestial bodies in the solar system, and our earth is one of them.
As shown in the figure above, the proportion of elements in the universe is very uneven. Hydrogen and helium occupy about 98% of the mass. Among the other 2, oxygen, carbon, neon, iron, The eight elements of nitrogen, silicon, magnesium, and sulfur account for the vast majority, and the proportion of elements in the "Solar Nebula" is basically the same. This means that the nitrogen content of the "Solar Nebula" is actually relatively high. , which also laid the material foundation for the earth to have a large amount of nitrogen.
In the "Solar Nebula", hydrogen accounts for a very high proportion, and the second-ranked helium element is the least reactive element among the known elements, so the nitrogen in the "Solar Nebula" Elements are basically combined with hydrogen to form compounds, the main one being ammonia (NH3).
Since ammonia is relatively heavy (the molecular weight is about 17), the earth’s gravity is enough to bind it, so the earth’s original atmosphere contains a large amount of ammonia. In the following time, sunlight The short-wave radiation in will continuously decompose ammonia into nitrogen and hydrogen (the reaction equation is: "2NH3 = 3H2 N2").
Since the molecular weight of hydrogen is about 2, the gravity of the earth cannot effectively bind it, so the hydrogen will quickly escape from the top of the earth's atmosphere. The molecular weight of nitrogen is about 28, which is heavier than ammonia. , so they will remain in the earth's atmosphere. As this process continues, there will be a large amount of nitrogen in the earth's atmosphere. Because nitrogen is an inert gas, this nitrogen will remain for a long time.
Jupiter, Saturn, Uranus and Neptune in the solar system are all giant planets, and their gravity can bind hydrogen and helium. As mentioned above, the vast majority of elements in the "solar nebula" are hydrogen and helium, so the atmospheres of these four giant planets are dominated by hydrogen and helium. Relatively speaking, the proportion of ammonia accreted by them is very small.
Due to the difference in gravity (that is, heavy matter sinks and light matter floats up), ammonia will sink to the bottom of the atmosphere of these giant planets. In this case, ammonia cannot Decomposed by shortwave radiation in sunlight.
In addition to the Earth, there are three rocky planets in the solar system. Among them, Mercury is too small and very close to the sun, so it has no atmosphere, while Mars is not big and has a very thin atmosphere. So it's understandable that there isn't much nitrogen in its atmosphere, leaving only Venus.
The nitrogen content in the atmosphere of Venus is about 3.5. In comparison, the nitrogen content in the Earth's atmosphere is as high as 78. It seems that there is more nitrogen in the Earth's atmosphere than Venus, but this is not the case. .
You must know that the atmosphere of Venus is much denser than that of the Earth, so that its surface atmospheric pressure is about 92 times that of the Earth. However, more than 95% of the gas in the atmosphere of Venus is carbon dioxide, which makes its atmosphere The nitrogen content appears to be very low.
In fact, the total mass of nitrogen in the atmosphere of Venus is more than three times that of the Earth. In fact, this is not difficult to understand. After all, Venus was formed closer to the sun, so more nitrogen can be accreted. Source - Ammonia.
It is worth mentioning that there are two mechanisms for consuming carbon dioxide on the earth. One is that carbon dioxide in the earth’s atmosphere can react with liquid water to form carbonic acid, and carbonic acid can react with other elements (such as calcium , magnesium) react to form carbonates. Next, these carbonates will continue to precipitate and enter the interior of the earth along with the earth's geological movement over a long period of time.
Another mechanism is the photosynthesis of plants and algae on the earth. They can use the energy of sunlight to convert carbon dioxide into oxygen and organic matter (which is why the oxygen in the earth’s atmosphere comes from). Under the action of these two mechanisms, carbon dioxide on Earth will not accumulate in the atmosphere like Venus.