thumb|The Pale Orange Dot: an artist's interpretation of what the Earth may have looked like when haze built up in its atmosphere. A paleoatmosphere (or palaeoatmosphere) is an atmosphere, particularly that of Earth, at some unspecified time in the geological past.
thumb|The Pale Orange Dot: an artist's interpretation of what the Earth may have looked like when haze built up in its atmosphere. A paleoatmosphere (or palaeoatmosphere) is an atmosphere, particularly that of Earth, at some unspecified time in the geological past.
When regarding geological history of Earth, the paleoatmosphere can be chronologically divided into the following phases: the Hadean first atmosphere or primary atmosphere (also known as the primordial atmosphere or proto-atmosphere), whose composition resembled that of the solar nebula; the Archean second atmosphere or secondary atmosphere (also known as the prebiotic atmosphere), which is a reducing atmosphere that became nitrogen-abundant due to volcanic outgassing and meteoric injections during the Late Heavy Bombardment, and; the Proterozoic and Phanerozoic third atmosphere or tertiary atmosphere (also known as the biotic atmosphere), which started to contain free elemental oxygen (dioxygen) due to ongoing biotic photosynthesis finally having released enough byproduct oxygen to overwhelm the reducing capability of the preceding second atmosphere. The appearance of free oxygen during the Neoarchean–Paleoproterozoic boundary, i.e. the Great Oxygenation Event (GOE), permanently changed the redox property of the atmosphere, and this new oxidative atmosphere can be subsequently further subdivided into several more periods of oxygenation and anoxic events associated with geological and climate catastrophes, mass extinctions and the evolution, diversification, radiations and successions of photoautotrophs (cyanobacteria, algae and plants) before it eventually reached the current Holocene state.
Discovered by embedding cosine similarity (sentence-transformers MiniLM, 384-dim).