We might commonly think of Earth as having an oxygen-dominated atmosphere, but in reality the molecule makes up only a fifth of our air. Most of what surrounds us is nitrogen, at 78 percent. Astrobiologists are beginning to see nitrogen — and not just oxygen— as a key indicator of a planet’s habitability. Nitrogen is essential for life on Earth and could signal an atmosphere thick enough to stabilize liquid water on a planet’s surface, fundamental to creating habitable conditions.
Nitrogen, in fact, was even more abundant in Earth’when volcanoes and other internal processes began replacing our planet’s original envelope of hydrogen and helium. The result of those geological processes, as well as the contributions added by early life, was the evolution of a “secondary” atmosphere made up of nitrogen, oxygen (mainly from photosynthetic life like plants), and trace constituents such as water and argon.
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