Most planets and moons will have an atmosphere due to their own gravity, but within the solar system, we won't find a planet that allows us to breathe freely.
The atmospheres of these planets or moons are either very dense or very thin, and they have almost no gaseous oxygen.
The earth's atmosphere now has 21% oxygen and 78% nitrogen, and it is not particularly dense, whether it is animals or plants are more suitable for such atmospheric composition and atmospheric pressure.
So why is it that only the Earth's atmosphere has such a huge amount of oxygen? Or how does Earth's atmosphere evolve, and why is it so different from other planets?
Earth's former atmosphere is not as easy to leave evidence as rocks, and it will be more difficult to reconstruct the history of the Earth's atmosphere, although some evidence may provide references, including the interaction of gases and Earth rocks, as well as the existing gas conditions on other planets.
Capture gas period
It is now widely believed in science that the solar system originated in the solar nebula, which is a "bulk material" composed of gas and dust, the two most abundant of which are hydrogen and helium.
There is now a lot of evidence for the hypothesis that the solar nebula will eventually become the solar system, including more than 700 young stars observed in the 2.5 light-year region of the center of the Orion Nebula (M42), and more than 150 protoplanetary disks that are likely to give birth to new planetary systems.
Protoplanetary disk in M42, Source: ESA/Hubble
When Earth is born from a protoplanetary disk and becomes large enough, its gravitational pull can attract material from its surroundings, and much of the hydrogen and helium will be pocketed.
But even so, the atmosphere of the most primitive Earth was not composed of these two gases.
This is because the speed of motion of gas molecules is related to its mass and temperature, and the sun and earth were very hot at that time, so these gas molecules easily reached escape speeds.
The temperature of the Earth's upper atmosphere is now between 1000-2000 degrees Celsius, considering that at a temperature of 2000 K (about 1727 ° C), the average speed of compound molecules with a molecular weight greater than 10 is less than 11.2 km / s (the escape rate of the Earth), and the molecular weight of hydrogen and helium is 2 and 4 respectively, so they are basically difficult to retain.
The current mainstream view is that the volatile substances of the most primitive Earth are mainly hydrogen-containing compounds (helium is an inert gas that is not easy to retain in the form of compounds), such as methane, ammonia, water, etc., which are abundant on gas giant planets; in addition, some inert gases with molecular weights greater than 10, such as neon, etc., are still rare gases that exist on the earth.
Early Solar System, Source: NASA/JPL-Caltech
Earth's own exhaust
According to the formation theory of the "protoplanetary disk", the early solar system had many "planetary embryos", and the most primitive Earth was only one of them, which continued to grow and grow larger in "accretion" and collisions.
Some scientists believe that Mars is now the only surviving "planetary embryo" because it is too small relative to its orbit.
Lunar impact hypothesis, source: NASA/JPL-Calte
"Accretion" and collisions release enormous amounts of energy, which turned the early Earth into an "ocean of magma," and even entire planets went into a state of molten after being hit by a primordial planet known as Theia.
The molecules of the solar nebula's initial volatile material are not only trapped and lost in atmospheric form, but some also cover the surface of solid particles of rock material.
When these solids melt after the collision, the gas adsorbed on it volatilizes into the initial atmosphere, and after the formation of the Earth or the stabilization of the solar system, the first atmosphere of the Earth is made of these volatile substances.
Early Earth, Source: SwRI/Simone Marchi
So what kind of atmosphere would this be?
First, the gas released by the "magma ocean" is not the same as the gas emitted by the current volcanic eruption.
Our current volcanic eruptions spew out gases such as carbon dioxide and sulfur dioxide, which is actually because these gases adsorbed on rocks are re-released at the high temperatures inside the Earth and enter the atmosphere as the volcano erupts.
However, the volatile material adsorbed by the early Earth material was not these, but gases adsorbed from the solar nebula, so the gases emitted at that time were mainly hydrogen, helium, methane and ammonia.
Note: Iron oxide sample
Since helium is an inert gas and can easily escape, there is little need to be considered, and when we consider the remaining gases, we must determine the oxygenation of the Earth, because these substances will react with oxygen.
Scientists have determined this by using samples of the current mantle, and the key to this is to see how much oxygen is chemically bonded to iron.
Using existing samples, in 2020, scientists simulated the atmosphere of early Earth[1], which is likely to be an atmosphere containing 97% carbon dioxide and 3% nitrogen, which may be the first relatively stable atmosphere after the Earth has cooled.
But surprisingly, the atmospheric pressure in the first atmosphere was about 70 times that of today.
Note: Earth and Venus
At this point, I don't know what you have in mind?
In fact, this ratio of CO2 to N2 is very similar to the atmosphere on Venus today, and with such a high atmospheric pressure, the early Earth is likely to be very similar to the current Venus.
If you continue to ask why the Earth became what it is, and Venus retained its original appearance, the only answer you can find is that Venus is too close to the Sun and the Earth is a little farther away, so it changed in the days that followed.
The emergence of organisms and the modification of the atmosphere
In the first atmosphere we mentioned earlier, the two main substances are CO2 and N2, in fact, before the Earth cools, the proportion of water vapor will be very large, and may even be higher than CO2 and N2.
But since The Earth is far enough away from the Sun, when it cools down, water vapor begins to exist on Earth in the form of water and forms early oceans, which is the key.
Because the ocean absorbs carbon dioxide, life on Earth may have been born in the process.
Illustration: Schematic diagram of the birth of life on Earth
And Venus is too close to the sun, coupled with the greenhouse effect created by carbon dioxide, every corner of Venus, day and night, is above 400 degrees Celsius, and water can only exist in the form of steam forever.
When we talk about the loss and collection of atmosphere by the Earth, many aspects are not considered, such as the solar wind and photochemical reactions.
We know that without the presence of a magnetic field, the solar wind will mercilessly strip the planet's atmosphere, and in the case of the same planetary mass, the larger the molecular weight, the less likely it is to be stripped, and water (H2O) is the smallest molecular weight oxide, and Venus has almost no magnetic field, so now there are few water molecules left on it.
After the Earth cooled, the heavier elements, molten nickel and iron, entered the core and created a magnetic field under the rotation, firmly locking the atmosphere and water vapor.
When life began to form in the oceans, the Earth's atmosphere entered the era of life modification.
Life's transformation of the Earth's atmosphere mainly includes two aspects, one is the mainstream carbon cycle, and the other is the nitrogen cycle that may have occurred.
In 2016, scientists found bubbles containing the atmospheric composition of the past in the lava of Australia 2.7 billion years ago, and if these bubbles represent the atmospheric composition of the past, then it can be determined that the atmosphere at that time was only half as dense as it is now, and the main reduction was nitrogen.
At that time, the explanation given by scientists was that some bacteria could "fix nitrogen", just as plants now turn carbon dioxide into non-volatile sugars.
However, the study or findings have not been accepted by mainstream science, after all, the only evidence is only bubbles from 2.7 billion years ago, and more evidence is that the atmosphere of the early Earth was thicker than it is now.
Note: Cyanobacteria are still ubiquitous
The most important thing about the carbon cycle is photosynthesis, as well as the activity of the Earth itself (such as volcanic eruptions), and one of the most crucial "reforms" is the Great Oxidation Event.
About 2.5 billion years ago, cyanobacteria evolved photosynthesis, which absorbed carbon dioxide and expelled oxygen as exhaust gases.
The original maximum carbon dioxide in the earth's atmosphere is sucked up in the ocean, the plants are sucked up, and they are exhausted in the past, and the earth's atmosphere has become the current nitrogen, which is mainly based on nitrogen, which is only 3%, and supplemented by oxygen.
The change in the composition of the Earth's atmosphere is actually quite interesting, our current atmosphere is so suitable for life to survive, but in fact, it is created by life itself.
Maybe it is indeed possible to use microorganisms to change the environment of other planets, but I don't know what this cycle will be?
Resources:
[1]https://theconversation.com/ancient-earth-had-a-thick-toxic-atmosphere-like-venus-until-it-cooled-off-and-became-liveable-150934