To answer the question of what the main source of the atmosphere of our planet is, we need to take a look at the evolution of the atmosphere. More specifically, we need to focus on its evolution through geologic time. The process of the creation of our atmosphere is extremely complex.
It arose from the earlier conditions, but we luckily have plenty of evidence that can point us to the exact answer. Through ancient rocks and sediments, we can see which chemical reactions were occurring in the Earth's crust throughout history, and also determine the biochemical processes that gave birth to our atmosphere in its current state.
The Atmosphere Changed
In the original atmosphere of our planet, the main chemical components were methane, ammonia, water vapor, and neon. However, it did not contain any free oxygen. It took possibly millions of years before oxygen began accumulating in the atmosphere. It was originally produced by unicellular organisms, and it took a long time before it became a part of the atmosphere. So organisms that produced oxygen were definitely one source of the Earth's atmosphere we know today.
One key thing that needs to be mentioned about the Earth's atmosphere is that it is considered a part of the planet's crust by scientists. The crust includes not only solid materials but also the hydrosphere (oceans and surface waters) and the atmosphere. All of these portions of the crust interact with each other frequently, no matter if they are liquid, solid, or gaseous. It is incredibly hard to separate them because of the frequency of their interaction, and that interaction throughout history made our atmosphere into what it is today.
The Compounds Making Up The Atmosphere
The most important compounds that took part in the creation of our atmosphere are molecular nitrogen and oxygen, noble gasses such as helium, neon, krypton and xenon, and other variable components such as carbon dioxide and water vapor. Some other components that were less crucial were molecular hydrogen, methane, ammonia, nitrous oxide, hydrogen sulfide, carbon monoxide, and hydrogen chloride. However, multiple other components were involved as the source of our atmosphere, but these are the most important ones.
All of these components are made up of several similar compounds. They are almost always made up of hydrogen, carbon, nitrogen, and oxygen. We add the noble gasses to the list, and that is it, it actually ends up being more simple than it seems at first. When gas gets delivered to the atmosphere through a process, that process starts to be considered the original source of that gas.
If a process removes gas from the atmosphere, it is called a sink. Many sources and sinks throughout our planet's history made the atmosphere into what it is today. One process will consume a component, while a different one will produce it, and it ends up being a long-lasting cycle. When the strengths of the sources and sinks are in balance with each other, the composition of the atmosphere will remain the same.
One important source of our atmosphere today is gas releases throughout many processes on the planet. Volcanoes outgasses make one example. Throughout the past, these outgasses shaped our planet, as well as our atmosphere. They happen during volcanic eruptions, which were much different in the past than the ones that appear on our planet today.
Another important source of the atmosphere that occurred through outgassing were releases at submarine hydrothermal vents. While many sources shaped the atmosphere of our planet into what it is today, in this article, we tried to name the most important ones and explain in simplified terms the chemical processes and compounds that made it.