|
E. Linacre |
11/'97 |
The atmosphere is nowadays thought to have been created at the time the Earth was being formed, about 4.5 billion years ago (4.5 GaBP). Asteroids struck the growing planet and caused degassing, chiefly steam, but also hydrogen, nitrogen, carbon dioxide and carbon monoxide. It is now believed that much of this primordial atmosphere was removed quite early after the formation of the Earth by the impact of a body about the size of Mars, which incidentally created the Moon. More outgassing resulted from continued impacts, and at the same time the Earth started to cool. Water vapour condensed and the Earth became covered by oceans. Gravity was insufficient to hold the lightweight outgassed hydrogen (H2), so that escaped to outer space. Most of the carbon dioxide combined with calcium and other minerals to form carbonate rock, but there was enough left for a ‘greenhouse effect’, preventing the oceans from freezing. CO2 concentrations were perhaps 300 times what they are now. The consequence was that the Earth's atmosphere was warmer than today, and no polar ice caps existed prior to about 2.5 GaBP. The early Earth’s atmosphere was not very different from that of Mars or Venus, except that only on Earth water existed in a liquid state.
There was an abrupt increase of free oxygen in the atmosphere to 15% of present concentrations at 2 GaBP, as discussed in Chapter 1.