The Environment of the Primitive Earth The Hydrosphere and Atmosphere

9.4.2.1 A Primordial Reducing Atmosphere?

The question of the composition of the primordial atmosphere remains far from being settled. Sources of hydrogen and of reduced compounds must have been significant on the early Earth. There would have been emission of volcanic H2; oxidation of iron by water, liberating H2; and the formation of CH4, NH3, and more complex organic molecules in the presence of H2 in hydrothermal systems. CH4 and NH3 are extremely fragile molecules once in the atmosphere, where they are oxidized and photodissociated by UV radiation. As for hydrogen, it escapes from the top of the atmosphere. It is the balance between the emission of H2, CH4, and NH3, and the photochemical destruction of CH4 and NH3 and the loss of hydrogen from the top of the atmosphere that determines whether the atmosphere was reducing or not, and the effectiveness of organic synthesis of the sort that Urey and Miller carried out.

The estimate of the escape of H has been recently drastically revised downwards (Tian et al., 2005), which has revived the discussion about the composition of the primordial atmosphere, because those authors found that a rate of emission of H2 comparable with current volcanic emissions, in an atmosphere of N2 (0.8 bar) and CO2 (0.2 bar) leads to an atmosphere containing more than 0.1 bar of H2, instead of values below 1 mbar, found in work that was regarded as standard (Kasting, 1993). With even more significant emissions, for example at 3.8 x 109 years BP, Tian et al. found even greater abundances, and atmospheres where H2 is the dominant component. The difference essentially arises from the fact that Kasting (1993) assumed that the loss of H to space was limited only by its diffusion through the atmosphere. So, if the planet's exosphere is too cold, it is the temperature that will limit any loss to space to far smaller amounts (unless non-thermal escape mechanisms are dominant) and maintain the abundance of atmospheric hydrogen at low values. This is a question that is still unanswered.

The time parameter is also very important. For a long time it has been believed that life could not have appeared and persisted until about 3.8 x 109 years ago, because the bombardment would have sterilized the Earth at earlier epochs. At a period 3.8 x 109 years ago the degassing of the Earth should be similar to present-day degassing, both in composition (because there are lavas of that age that have a 'modern' composition), and in intensity (to within a factor of 10), because the internal heat flux rapidly decreased during the first 500 million years. However, if we envisage a much earlier origin of life, around 4.3 x 109 years BP, then we can also imagine that the emission of gas from the primordial Earth was far more intense and rich in H2, CH4, and NH3, producing an atmosphere of the Urey-Miller type. It transpires that the story of the bombardment of the early Earth has been recently rewritten (Gomes et al., 2005). The peak bombardment at 3.9-3.8 x 109 years BP could have been far less intense than was thought, and be preceded by a much calmer phase between the end of the Earth's accretion phase and the peak, i.e., between about 4.4 and 3.9-3.8 x 109 years ago. The peak bombardment was a traumatic episode for the Earth, but if some sufficiently complex microbial life was already present, and was capable of adapting, it could well have survived. In that case, there is nothing to prevent us from considering the theory of very precocious life-forms arising in a reducing atmosphere. However, if we consider a primordial atmosphere, we then need to take account of the effects of the intense X-ray and EUV radiation that was emitted by the young Sun. This radiation heated the upper atmosphere and photodissociated the molecules, accelerating the loss of H to space. Once again, a coherent model remains to be developed.

In the absence of any strong constraints on the time of the atmosphere's origin, and on its nature, it is important to consider the possibility that in-situ organic synthesis was ineffective, and that an external source of organic material did play a role.

9.4.2.2 The Origin of the Earth's Water

Meteorites contain a greater or lesser fraction of water as a function of the orbital distance of their parent bodies. Those that formed at distances greater than about 2.5 AU contain more than 10 percent of water in the form of hydrated minerals. Still farther out, beyond the ice line, the typical proportion of water reaches 50 percent, and this time essentially in the form of ice. In contrast, the asteroids that are found to be within 2.5 AU are almost completely depleted in water. The fact that the asteroid populations have not merged suggest that their distribution is a real sign of the properties of the protosolar nebula at distances where we observe them nowadays, and that there has been no major migration of these populations.

So, if the Earth itself formed from material that essentially formed within 2 AU, which is what the models of planetary formation suggested until recently, it should initially have been without water. Water must have been brought by a bombardment by bodies coming from more distant regions, either asteroids or micrometeorites rich in water, or comets. We know that cometary ices could have made only a limited contribution (less than 10 percent) because the water that they contain is richer in deuterium than the terrestrial oceans. The isotopic composition of carbonaceous chondrites (rocks formed at distances > 2.5 AU) and of micrometeorites (microscopic, rocky debris from comets or asteroids), is compatible with that of water on Earth.

At first it was thought that the water could have been brought by asteroids at the time of the late bombardment at 3.9 x 109 years BP. We now know, however, thanks to the analysis of terrestrial zircons dating from more than 4.3 x 109 years ago, that the Earth already possessed oceans at that time (Martin et al., 2006). So we have to explain an extremely early influx. A continuous addition by micrometeorites has been suggested (Maurette et al., 2000) but, in fact, the problem of the origin of the water is in the process of being resolved quite simply, thanks to new numerical models of planetary formation, which have been made possible by the power of modern computers. These N-body models are, in fact, now able to work with larger and larger values of N, and thus begin with far more realistic distributions of planetesimals. The most recent models show that the Earth did not form solely from material in a narrow ring around its orbit at 1 AU, but that planetesimals that originated much farther out were involved in the formation of the Earth. So an ad-hoc later influx of water is no longer necessary, and we now find ourselves with the opposite situation, because the models form 'Earths' that are too rich in water, sometimes by as much as a factor of 100. But this result is actually logical, because the models do not include the phenomena of the loss of volatile components through impacts between planetesimals, nor the degassing associated with their differentiation. In fact, the planetary embryos were sufficiently hot to degas an atmosphere, but were not sufficiently massive to retain it effectively. The composition of planetary atmospheres, notably in the rare gases and their isotopes, retains a trace of the violent events accompanying atmospheric escape (see Fig. 7.22, Chap. 7). For a long time it was thought that this escape must have taken place from the primordial atmospheres of the planets, once these had formed. But it now seems more logical to believe that these violent escape processes took place on the planetesimals and planetary embryos. So it is perfectly normal - and even essential - for the quantity of water and volatile components that were initially present in all the planetesimals that were involved in the formation of the Earth, to greatly exceed present-day reservoirs on the planets.

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