Origin and Evolution of Earth’s Atmosphere and Oceans

Abstract

Both Venus and Mars most likely retain their proto-atmospheres with more than 95% CO₂ plus a few percent of N₂.  The Earth is situated between the two; there is no compelling reason that the Earth’s proto-atmosphere should be any different from those of Venus and Mars.  Thus, today’s Earth atmosphere is rather different from her proto-atmosphere.  After completion of accretion, the magma ocean on the Earth surface started to solidify.  Then a Mars-like impactor hit the Earth to form the Moon.  The Moon-making giant impact also released a large quantity of supercritical H₂O from the magma ocean entrapped inside the Earth.  The supercritical H₂O thus released would then quickly react with CO₂ in the proto-atmosphere to form a supercritical H₂O-CO₂ mixture.  When the Earth’s surface cooled down to 450-300 ^oC, the dense supercritical H₂O-CO₂ mixture precipitated to form the Earth’s indigenous oceans which were hot soda supercritical H₂O.  When the surface temperature further cooled down, the indigenous oceans expanded at the expense of CO₂ in the proto-atmosphere.  The atmospheric pressure also decreased simultaneously. The removal of CO₂ from the proto-atmosphere was accelerated and then completed when the indigenous oceans reacted with the most abundant surface mineral plagioclase to form carbonates and clay minerals, leaving Na⁺ in the oceans.  Once CO₂ in the proto-atmosphere was completely removed, N₂ naturally became the most abundant component in the Earth’s atmosphere as observed today.  Some supercritical H₂O at high altitude would likely dissociate into O₂ and H₂. The latter would then escape to the outer-space and O₂ remained in the atmosphere.  Alternatively, O₂ in the Earth’s atmosphere may be explained by the increase in photosynthetic organisms in the oceans, metabolizing carbon from CO₂ and releasing O₂ into the atmosphere.