Prebiotic Organic Syntheses

The origin of the atmosphere and of the oceans that has just been described has direct connections with the origins of life on Earth. In particular, the carbonaceous chondrites that were brought onto the Earth (Table 2.2) from the asteroid belt, mostly during the first 200 million years (Fig. 2.4), contained numerous amino acids. As an example, 74 different amino acids were extracted from the Murchison meteorite, a CM carbonaceous chondrite that fell in 1969 near Murchison, Australia (Cronin et al., 1988). Among these amino acids, eight of the protein amino acids (glycine, alanine, valine, leucine, isoleucine, proline, aspartic acid, and glutamic acid) have been identified along with 11 less common amino acids used by life. The total concentration of amino acids in the Murchison meteorite is about 60 parts per million (ppm). Assuming from the statistics (Sears and Dodd, 1988) that 3% of the meteorites are carbonaceous chondrites, this is still 6 x 1019 g (6 x 107 million tons!) of amino acids that would have reached the Earth. Assuming that everything would be destroyed by heat during the first 130 million years, 6 x 105 million tons would still reach the atmosphere later, and 6 x 103 million tons after the first 260 million years.

The Murchison meteorite also contains a complex mixture of aliphatic and aromatic hydrocarbons, of carboxylic acids and of nitrogen heterocycles. The latter are of particular interest because of the use of purines and pyrimidines as coding elements of the biological DNA and RNA. One pyrimidine (uracil) and four purines (xanthine, hypoxanthine, guanine, and adenine) used in biological DNA and RNA have been found at the ppm level in the Murchison meteorite (Cronin et al., 1988).

Comets have also brought to the Earth a much larger amount of organic compounds than the carbonaceous chondrites, and on the average much later. Their flux to the Earth has drastically subsided, but in a way it is not yet finished (Fig. 2.4). Their chemistry is still very poorly known, and the information available comes almost entirely from comet Halley. Its dust particles revealed a large amount of fine grains coated with mostly unsaturated organic material (Krueger and Kissel, 1987). The presence of purines and pyrimidines were inferred from the mass spectra, but amino acids were not detected; if present, they were at least a factor of 30 less abundant than the purines and pyrimidines.

The argument that the heat of the impacts with the Earth is going to destroy all this organic material, is brought to rest when one realizes that a large fraction of it is brought to the Earth by cometary dust. This dust, visible in the beautiful dust trails of comets, is braked by the upper atmosphere and gently brought to the ground. Prebiotic organic compounds came from space, and were brought again and again for aeons until they found the right "little pond" to get life started.

2.17 Summary

We have first established that the volcanic origin of our atmosphere and our oceans is an assumption that has never been demonstrated by any empirical data. It was based only on a blind extrapolation of the present recycling of volcanic gases back to a period that cannot be explored by geophysical means.

The only way to handle the problem is to consider the formation of the Earth in the more general paradigm of the origin of the solar system and the formation of the planets. This paradigm has received considerable support from recent observational evidence. It explains that Nature has found a way to make single stars by getting rid of the excess angular momentum through an accretion disk. Most of the mass falls first onto the accretion disk, before being fed into the central star. When the mass stops falling from afar, the turbulence stops in the disk and its dust sediments to the midplane of the disk, making big flat rings of solid particles that are the possible source of a planetary system. In the case of the Sun, these particles were mainly silicate and metallic iron dust for the terrestrial planets.

This sedimentation is convincingly documented by meteorites that came from the asteroid belt: the undifferentiated chondrites. Chondrites tell us that the gas dust separation occurred at a temperature close to 450 K at a distance of 2.6 AU from the Sun. Theory and observations concur to a value for the temperature gradient in the nebula, which implies that the dust particles that were going to agglomerate to form the Earth, were removed from the nebular gas in a range of temperatures going from 800 to 1,200 K (Fig. 2.6).

These temperatures imply that the silicate and metallic iron particles were outgassed before their agglomeration into larger objects. So water was in nebular steam, carbon in gaseous CO, and nitrogen in gaseous N2. The first 99% of the protoearth were therefore completely devoid of volatiles. Isotopic abundances of the noble gases on the Earth confirm that there is no trace

Agglomerate

Fig. 2.6. This plot of the reduced Fe/Si ratio versus the oxidized Fe/Si ratio for chondritic meteorites is usually referred to as a Urey-Craig diagram. The position of the comet Halley grains is indicated, as well as the place that would represent the Earth if it were homogenized. The arrows indicate the limits of our ignorance as far as a reduced phase of iron in the mantle is concerned. One understands that the outgassed and partially reduced fraction, collected near 1,000 K, would correspond rather well to the enstatite chondrites of group H. An admixture of a small volatile fraction coming from comets would easily bring the mixture onto the Earth's position. Such a model should not be overinterpreted, because we do not know the zone of origin of the H chondrites.

Fig. 2.6. This plot of the reduced Fe/Si ratio versus the oxidized Fe/Si ratio for chondritic meteorites is usually referred to as a Urey-Craig diagram. The position of the comet Halley grains is indicated, as well as the place that would represent the Earth if it were homogenized. The arrows indicate the limits of our ignorance as far as a reduced phase of iron in the mantle is concerned. One understands that the outgassed and partially reduced fraction, collected near 1,000 K, would correspond rather well to the enstatite chondrites of group H. An admixture of a small volatile fraction coming from comets would easily bring the mixture onto the Earth's position. Such a model should not be overinterpreted, because we do not know the zone of origin of the H chondrites.

left of an early atmosphere. Volatile metals are depleted as shown by upper mantle samples.

The giant planets formed at a location where the temperature was much lower; hence, their early building blocks were not stony, but icy, with a large fraction of volatile materials: the comets. The mass of the giant planets became large enough to deflect comets to the inner solar system; the final 1% of the Earth mass was a veneer of volatile material brought about by a late cometary bombardment due to the growth of the giant planets. In this veneer, gases were depleted by a factor of 2,000, and water by a factor of 100, by the numerous collisions with larger and larger objects predicted for the final stages of accretion. Some of the collisions with lunar- or Mars-sized bodies, as the one that has been proposed for the formation of the Moon, are likely to have completely vaporized and lost all the volatiles to space, which implies that the buildup of the atmosphere and the oceans started anew from the last of these collisions, whose epoch is difficult to specify since it was a stochastic event.

A small fraction of the volatile veneer, at most of the order of 0.l%, has been brought onto the Earth before the major accretion was complete, hence was buried in the mantle. The residual 0.9% has been brought mostly during the first half billion years of the Earth's existence, as testified by the ages of the lunar craters that were the relics of the same bombardment. In a general way, this bombardment has considerably rarefied but is not entirely finished, since comets must eventually hit the Earth from time to time. In particular, short-period comets still come from the Kuiper belt by the same orbital diffusion process that started some four and a half billion years ago, and long-period comets derive from those "new" comets coming from the Oort cloud, where they were also stored by an orbital diffusion that started in the same way.

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