Origin of Earth

• In 1650 Archbishop Ussher used the Bible to calculate that the Earth was created in 4004BC. Later on in the mid-nineteenth century Charles Darwin believed that the Earth must be extremely old because he recognized that natural selection and evolution required vast amounts of time.

• It wasn't until the discovery of radioactivity that scientists began to put a timescale on the history of the Earth. Rocks often contain heavy radioactive elements which decay over long periods of time, the decay is unaffected by the physical and chemical conditions and different elements decay at different rates (These rates are slow and half-lifes of several hundred million years are not uncommon).

• Throughout this century the race has been on to discover the oldest rocks in the world. The oldest volcanic rock found so far has been dated at 3.75 billion years old, but this is not the whole story. Meteorites created at the same time as the Earth hit us all the time, radioactive dating shows that they are about 4.55 billion years old.

Birth of Moon

The moon came from a collision between the earth and another planet about 4.5 billion years ago, about 50 million years after earth first formed. When the earth was in the process of forming, you still had a great amount of debris still in the same orbit as the earth. The planet that ran into the earth was the largest piece of this debris, about the size of Mars.

Hot

• Primordial heat, collisions and compression during accretion, decay of short-lived radioactive elements.
• Consequences - Constant volcanism, surface temperature too high for liquid water or life as we know it, molten surface or thin, unstable basaltic crust.
• Atmosphere - early atmosphere probably completely different in composition (H2, He)

Cooling
• Primordial heat dissipated to space
• Condensation of water (rain), accumulation of surface water.
• Accumulation of new atmosphere due to volcanic out gassing
• Conditions appropriate for evolution of life

Atmosphere at the Time of Origin

The atmosphere of the Earth (and also of Venus and Mars) is generally believed to have its origin in relatively volatile compounds that were incorporated into the solids from which these planets accreted. Such compounds could include nitrides (a source of N2), water (which can be taken up by silica, for example), carbides, and hydrogen compounds of nitrogen and carbon.

Many of these compounds (and also some noble gases) can form clathrate complexes with water and some minerals which are fairly stable at low temperatures. The high temperatures developed during the later stages of accretion as well as subsequent heating produced by decay of radioactive elements presumably released these gases the surface. Even at the present time, large amounts of CO2 , water vapor, N2, HCl, SO2 and H2S are emitted from volcanos.

Origin of atmospheric oxygen

Free oxygen is never more than a trace component of most planetary atmospheres. Thermodynamically, oxygen is much happier when combined with other elements as oxides; the pressure of O2 in equilibrium with basaltic magmas is only about 10–7 atm. Photochemical decomposition of gaseous oxides in the upper atmosphere is the major source of O2 on most planets.The partial pressure of O2 in the prebiotic atmosphere is estimated to be no more than 103atm, and may have been several orders of magnitude less. The major source of atmospheric oxygen on the earth is photosynthesis carried out by green plants and certain bacteria:

H2O + CO2 ={CH2O} + O2

Carbon dioxide

Carbon dioxide has probably always been present in the atmosphere in the relatively small absolute amounts now observed (around 54 1015 mol = 54 Pmol). The reaction of CO2 with silicate-containing rocks to form precambrian limestones suggest a possible moderating influence on its atmospheric concentration throughout geological time. CaSiO3 + CO2 =CaCO3 + SiO2

The ozone layer was formed which started to filter out harmful ultraviolet rays. This allowed the evolution of new living organisms in the shallow seas.

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