Answer: Our atmosphere is about 78% nitrogen and 21% oxygen, with traces of other things like water and carbon dioxide. It's an odd mix compared to the atmospheres of other planets. Jupiter and Saturn are dominated by hydrogen and helium, The thick atmosphere of Venus is about 96% carbon dioxide, and only 3% nitrogen, which is about the same ratio as the thin atmosphere of Mars. So why is our atmosphere so dominated by nitrogen?
It wasn't always this way. Like most planets Earth's earliest atmosphere was dominated by hydrogen and helium. These two elements are by far the most abundant in the Universe. About 92% of the atoms created by the big bang were hydrogen, and most of the rest were helium. All the other elements on the periodic table are formed through astrophysical processes such as nuclear fusion in the heart of a star. To this day they make up only a small trace of cosmic elements.
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