The prebiotic atmosphere is the second atmosphere present on Earth before today's biotic, oxygen-rich third atmosphere, and after the first atmosphere (which was mainly water vapor and simple hydrides) of Earth's formation. The formation of the Earth, roughly 4.5 billion years ago, involved multiple collisions and coalescence of planetary embryos. This was followed by an over 100 million year period on Earth where a magma ocean was present, the atmosphere was mainly steam, and surface temperatures reached up to 8,000 K (14,000 °F). Earth's surface then cooled and the atmosphere stabilized, establishing the prebiotic atmosphere. The environmental conditions during this time period were quite different from today: the Sun was about 30% dimmer overall yet brighter at ultraviolet and x-ray wavelengths; there was a liquid ocean; it is unknown if there were continents but oceanic islands were likely; Earth's interior chemistry (and thus, volcanic activity) was different; there was a larger flux of impactors (e.g. comets and asteroids) hitting Earth's surface.
Studies have attempted to constrain the composition and nature of the prebiotic atmosphere by analyzing geochemical data and using theoretical models that include our knowledge of the early Earth environment. These studies indicate that the prebiotic atmosphere likely contained more CO2 than the modern Earth, had N2 within a factor of 2 of the modern levels, and had vanishingly low amounts of O2. The atmospheric chemistry is believed to have been "weakly reducing", where reduced gases like CH4, NH3, and H2 were present in small quantities. The composition of the prebiotic atmosphere was likely periodically altered by impactors, which may have temporarily caused the atmosphere to have been "strongly reduced".
