The earth was formed by a relatively short lived, but intense, gravitational accretion of rather large planetesimals, orbiting the newly formed sun some 4560 Ma ago. The kinetic energy of the impacting bodies was sufficient to raise the temperature to a few thousand degrees Celsius. This would have melted the entire Earth, resulting in loss of the primary atmosphere.

Earth cooled rapidly by radiation when the initial bombardment abated, and the very high temperature (Hadean) phase is not thought to have lasted longer than a few hundred Ma. The crust solidified, but massive outgassing continued, resulting in an atmosphere mainly comprising carbon dioxide and steam (Table 1.2) as probably occurred on Venus and Mars.^1,2 In the case of Earth, the water vapour condensed to surface water, and there is good evidence that oceans existed about 3800 Ma ago and perhaps even earlier.³ Once Earth’s crust was cool, and surface water was in existence, it was possible for comets and meteorites to leave a secondary veneer of their contents, including water and a wide range of organic compounds.⁴

Table 1.2 Average composition of gas evolved from Hawaiian volcanoes

(Data are from reference 5, reproduced from reference 2 by permission of the Geologists’ Association.)

Important physico-chemical changes occurred in the early secondary atmosphere. Helium and hydrogen tended to be lost from the Earth’s gravitational field. Ammonia dissociated to nitrogen and hydrogen, the former retained and the latter lost from the atmosphere. Some carbon dioxide might have been reduced by hydrogen to form traces of methane, but very large quantities slowly reacted with surface silicates to become trapped as carbonates, while forming silica (weathering). Traces of water vapour underwent photodissociation to hydrogen and oxygen. However, oxygen from this source was present in only minimal quantities, and the early atmosphere is no longer thought to have been as strongly reducing as was formerly believed.⁶

The initial very high partial pressure of carbon dioxide, and probably some methane, would have provided a powerful greenhouse effect to offset the early minimal weak solar radiation, which was some 30% less than today (Figure 1.1). However, the Sun commenced its main sequence of thermo-nuclear fusion of hydrogen to helium about 3000 Ma ago. Since then solar radiation has been increasing steadily as the Sun proceeds remorselessly towards becoming a red giant, which will ultimately envelop the inner planets. It is fortunate that increasing solar radiation has been approximately offset by a diminishing greenhouse effect, due mainly to decreasing levels of carbon dioxide (see below). As a result, Earth’s temperature has remained relatively stable, permitting the existence of surface water for the last 3800 Ma.

Fig. 1.1 Solar luminosity plotted against the age of the Sun, with the open circles giving a qualitative impression of the diameter of the Sun. Superimposed is an indication of the life of the Earth and Moon, which is now about half way through the main sequence of the Sun deriving its energy from hydrogen fusion to helium. The times can only be very approximate.

(After reference 7 with kind permission of Springer Link and Business Media.)

Small bodies, such as Mercury and most of the planets’ satellites, have a gravitational field which is too weak for the retention of any significant atmosphere (Figure 1.2). The gas-giants (Jupiter, Saturn, Uranus and Neptune) have a gravitational field which is sufficiently strong to retain all gases, including helium and hydrogen, thereby ensuring the retention of a reducing atmosphere. The gravitational field of the Earth is intermediate, resulting in a differential retention of the heavier gases (oxygen, carbon dioxide and nitrogen), while permitting the escape of hydrogen and helium. This is essential for the development of an oxidising atmosphere and life as we know it. Water vapour (molecular weight only 18) would be lost from the atmosphere were it not for the cold trap at the tropopause.

Fig. 1.2 The planets and some of their larger satellites, plotted according to distance from the Sun (abscissa), and mass (ordinate), both scales being logarithmic and relative to Earth. Mean surface temperatures are shown. Potential for life as we know it exists only within the parallelogram surrounding the Earth.

Surface temperature of a planetary body is crucial for the existence of liquid water, which is essential for life and therefore the composition of our atmosphere. To a first approximation, temperature is dependent on the distance of a planet from the Sun, and the intensity of solar radiation (Figure 1.2). The major secondary factor is the greenhouse effect of any atmosphere which the planet may possess. Mercury and Venus have surface temperatures far above the boiling point of water. All planets (and their satellites) which are further away from the Sun than Earth have a surface temperature too cold for liquid water to exist today. However, there is now evidence that Mars had liquid surface water in the past,⁸ now present only as ice.⁹

Earth is the only planet in the solar system which has both a mass permitting retention of an oxidising atmosphere, and a distance from the Sun at which the temperature permits liquid water to exist on its surface. It is difficult to see how there could be life as we know it anywhere in the solar system outside the small parallelogram in Figure 1.2. However, an environment similar to that of the earth may well exist on some planets of the 10²² other sun-like stars in the universe.

Amino acids and a wide range of organic compounds are found in a type of meteorite known as carbonaceous chondrites.⁴ Therefore, whether or not such compounds were actually synthesised on the early Earth, as Stanley Miller had proposed,⁶ it is highly likely that a wide range of organic compounds were available on the pre-biotic Earth when liquid oceans were formed.

It is less easy to explain the next stage in the evolution of life. An essential feature of all life is the synthesis of proteins using a ribonucleic acid (RNA) template, usually transcribed from the genetic code carried on deoxyribonucleic acid (DNA). There would appear to have been a classical ‘chicken and egg’ situation. Useful proteins could not be formed without the appropriate sequences in RNA or DNA: RNA and DNA could not be polymerised without appropriate enzymes which are normally proteins. Nevertheless, life did appear, perhaps in the first instance with the genetic code carried only on RNA, or even the much simpler peptide nucleic acid (PNA).¹⁰

An essential requirement for life is the availability of bio-usable forms of energy. The forms of available energy and their location at the dawn of life remain a mystery. However, one cannot ignore the possibility of hydrothermal vents, such as the black smokers along the mid-ocean ridges at great depths, which still support very simple life forms on the basis of chemoautotrophy. They are totally independent of sunlight, and exploit the profound chemical disequilibrium between the emerging hot, reducing and acid water, containing hydrogen sulphide, methane, ammonia, phosphorus and a range of metals, and the surrounding sea water.¹¹

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