Prebiotic atmosphere
The environmental conditions during this time period were quite different from today: the Sun was ~30% dimmer overall yet brighter at ultraviolet and x-ray wavelengths,[5][6] there was a liquid ocean, it is unknown if there were continents but oceanic islands were likely,[7][8] Earth's interior chemistry (and thus, volcanic activity) was different,[9] and there was a larger flux of impactors (e.g. comets and asteroids) hitting Earth's surface.
[10] 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.
[11] Constraining the composition of the prebiotic atmosphere is key to understanding the origin of life, as it may facilitate or inhibit certain chemical reactions on Earth's surface believed to be important for the formation of the first living organism.
[4] Earth's surface in the aftermath of the Moon-forming impact was characterized by high temperatures (~2,500 K), an atmosphere made of rock vapor and steam, and a magma ocean.
[3] As the Earth cooled by radiating away the excess energy from the impact, the magma ocean solidified and volatiles were partitioned between the mantle and atmosphere until a stable state was reached.
The oldest direct evidence for life on Earth is around 3.5 billion years old, such as fossil stromatolites from North Pole, Western Australia.
[14] Putative evidence of life on Earth from older times (e.g. 3.8 and 4.1 billion years ago[15][16]) lacks additional context necessary to claim it is truly of biotic origin, so it is still debated.
[8] This adds uncertainty to the interaction between Earth's prebiotic surface and atmosphere, as the presence of exposed land determines the rate of weathering processes and provides local environments that may be necessary for life to form.
Additionally, the oxidation state of Earth's mantle was likely different at early times, which changes the fluxes of chemical species delivered to the atmosphere from volcanic outgassing.
[23][24][25] Carbon dioxide (CO2) is an important component of the prebiotic atmosphere because, as a greenhouse gas, it strongly affects the surface temperature; also, it dissolves in water and can change the ocean pH.
[9] Evidence suggests that the carbonate-silicate cycle regulates Earth's atmospheric carbon dioxide abundance on timescales of about 1 million years.
[27] It has been proposed that the processes of the carbonate-silicate cycle would result in high CO2 levels in the prebiotic atmosphere to offset the lower energy input from the faint young Sun.
[31] One extensive modeling study suggests that CO2 was roughly 20 times higher in the prebiotic atmosphere than the preindustrial modern value (280 ppm), which would result in a global average surface temperature around 259 K (6.5 °F) and an ocean pH around 7.9.
[31] This is in agreement with other studies, which generally conclude that the prebiotic atmospheric CO2 abundance was higher than the modern one,[9][29][28][32] although the global surface temperature may still be significantly colder due to the faint young Sun.
[9] Despite this, atmospheric N2 was at least moderately important to the prebiotic environment because it impacts the climate via Rayleigh scattering and it may have been more photochemically active under the enhanced x-ray and ultraviolet radiation from the young Sun.
[35] However, this study fits into a larger body of work that generally constrains the prebiotic N2 abundance to be between half and double the present level.
Incoming solar photons or lightning can break up CO2 and H2O molecules, freeing oxygen atoms and other radicals (i.e. highly reactive gases in the atmosphere).
[11] The prebiotic atmosphere can supply chemical ingredients and facilitate environmental conditions that contribute to the synthesis of organic compounds involved in the origin of life.
[51] Proposed important ingredients for the origin of life include (but are not limited to) methane (CH4), ammonia (NH3), phosphate, hydrogen cyanide (HCN), various organics, and various photochemical byproducts.
