The atmosphere

Formation of the earth and the prebiotic atmosphere

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 from volcanoes continued, resulting in an atmosphere mainly comprising carbon dioxide and steam ( Table 34.2 ), as probably also occurred on Venus and Mars. ^, 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. 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.

Important physicochemical 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.

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 34.1 ). However, the Sun commenced its main sequence of thermonuclear 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, caused mainly by decreasing levels of carbon dioxide (see later). As a result, Earth’s temperature has remained relatively stable, permitting the existence of surface water for the last 3800 Ma.

Significance of the mass of earth and its distance from sun

Small bodies, such as Mercury and most planetary satellites, have a gravitational field which is too weak for the retention of any significant atmosphere ( Fig. 34.2 ). The gas giants (Jupiter, Saturn, Uranus and Neptune) have gravitational fields which are 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 oxidizing atmosphere and life as we know it. Water vapour (with a molecular weight of only 18) would be lost from the atmosphere were it not for the ‘cold trap’ at the tropopause, where the temperature falls sufficiently low to turn water vapour into ice.

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 ( Fig. 34.2 ). 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, now present only as ice.

Earth is the only planet in the solar system which has both a mass permitting retention of an oxidizing 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 shown in Figure 34.2 . However, an environment similar to that of Earth may well exist on some planets of the 10 other sun-like stars in the universe.

Origin of life and the development of photosynthesis

Amino acids and a wide range of organic compounds are found in a type of meteorite known as carbonaceous chondrites. Therefore, whether or not such compounds were actually synthesized on the early Earth, as Stanley Miller had proposed, 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 of RNA or DNA: RNA and DNA could not be polymerized 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.

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 midocean ridges at great depths, which still support very simple life forms whose biochemistry is based on 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. It is likely that there have been hydrothermal vents on Earth for as long as surface water has coexisted with volcanic activity. Chemoautothrophs might, therefore, have appeared as early as 3800 Ma ago.

Hydrothermal vents provide an extremely constrained and hazardous environment for life, dependent on the continued existence of the energy supply. A much more attractive alternative was to utilize the limitless availability of energy in the form of solar visible light. The most familiar of such reactions is the oxygenic photosynthesis of glucose, summarized as follows:

The biochemical adaptation from thermal detection in hydrothermal vents to photosynthesis does not seem to have been insuperable, and it was thought that photosynthesizing cyanobacteria (blue-green algae) may have existed 2700 Ma ago. However, it has recently been suggested that this crucial development may have occurred later, closer to 2400 Ma ago when oxygen first appeared in the atmosphere. At a later date, cyanobacteria underwent symbiotic incorporation into the cells of certain eukaryotes to become chloroplasts, which then conferred the biochemical benefits of photosynthesis on their hosts, which include all plants.