Changes in the Earth's composition

Escape of the primitive atmosphere. The Earth and other planets accreted by coalescence of planetesimals with a wide variety of compositions, including the volatile-rich carbon-aceous chondrites. Initial accretion was under cool conditions, but by the time 0.1 of the Earth's mass had accumulated gravity would have been so strong that incoming planetesimals arrived at a velocity fast enough to induce vaporization on impact. Volatile elements were released to form a primitive atmosphere, whereas non-volatile elements condensed to form an outer layer of molten rock.

It is therefore likely that the early Earth wascomposed of a cool, volatile-rich, solid interior, surrounded by an ocean of molten rock, a magma ocean more than 1000 km thick. Sinking of heavy molten iron through the magma ocean, the process of separating dense metallic iron towards the Earth's centre to form a core, released an additional tremendous quantity of gravitational energy as heat. Heating from the combined sources of impacting meteorites and core separation caused outgassing of volatile elements trapped in the Earth's interior to form an early atmosphere, chiefly of carbon monoxide (CO), hydrogen sulphide (H₂S), nitrogen (N₂), hydrogen (H₂) and water together with a proportion of the more volatile trace elements such as chromium, manganese and sodium.

This primitive atmosphere probably totalled about 10 per cent of the Earth's mass, but it was steadily blown off during the first billion years as outgassing proceeded by a combination of intense solar wind from the young Sun, rapid rotation of the Earth and turbulent mixing with the solar nebula.

The Earth's present atmosphere of nitrogen, oxygen, argon and carbon dioxide is essentially secondary, derived by outgassing of the Earth's hot interior while the mantle solidified. The dominant gaseous species of C, N and S were reduced in the primitive atmosphere, as dictated by the presence of reduced metallic iron throughout the Earth's volume. Once the dense metallic iron had been swept by gravity into the core, oxidized gaseous species of C, N and S were produced in reactions mediated by the more oxidized mantle. Further strong modification of the early atmosphere arose from biological photosynthesis and fermentation.

That the Earth's present atmosphere is secondary can be deduced from the low content of the noble gases neon (Ne), krypton (Kr) and xenon (Xe) relative to their abundance in the Sun and material from which the Earth accreted. For instance, the solar nitrogen/neon ratio is about 0.8, whereas the extant terrestrial atmospheric ratio is about 40000. Noble gases are relatively immune to losses, so that the current excess of nitrogen is due to secondary generation of this gas.

The Earth's atmosphere is now relatively stable, with minor losses induced by specialized processes. Only hydrogen and to a lesser extent helium can escape from the upper atmosphere into space, a process known as thermal evaporation. Hydrodynamical loss involves the escape of gases when local supersonic velocities are attained. Photo-dissociation, the break-up of gas molecules by incident solar radiation, may enhance the loss of hydrogen by converting ammonia (NH₃) into its constituent nitrogen and hydrogen molecules. Combined losses are not at present appreciable.