Transport of Complex Organic Molecules to the Primordial Earth

9.2.3.1 The Meteoritic Bombardment

We have seen (Sect. 4.3.2.2) that the formation of planets by accretion favoured the presence of refractory, heavier elements, in relative proximity to the Sun, whereas elements such as C, N, and O were trapped in the form of ices at greater heliocentric distances, beyond the asteroid belt. This explains why carbon is more abundant at r > 2 AU (Fig. 9.8). Yet the presence of liquid water requires r < 2 AU. How can we reconcile these two factors that appear equally indispensable for the appearance of life? Meteoritic bombardment may offer a solution to the problem, as has been suggested by the astrobiologist Oro. The history of the Solar System's formation shows that an intense meteoritic bombardment occurred during the first 1000 million years, with a peak activity, the Late Heavy Bombardment (LHB) about 3800 million years ago. We see the traces of this bombardment most markedly on the surfaces of the Moon and Mercury, as well as on those of Phobos and Deimos, the two satellites of Mars, which are undoubtedly captured asteroids.

The fall of the Murchison meteorite (Fig. 9.9) in Australia in 1969, provided decisive support to the theory that prebiotic molecules arrived on the early Earth from outside. This primitive meteorite, a carbonaceous chondrite, proved to be rich in some twenty amino acids, in particular those that are found in proteins. The list of amino acids found in the Murchison meteorite, as well as their relative abundances, also shows a remarkable similarity to those formed in Miller and Urey's experiments (Table 9.3). This fundamental discovery reveals two important facts: (1) A complex prebiotic chemistry was at work from the beginning of the Solar System's history;

Fig. 9.8 The abundance ratio of carbon to the combined total of heavy elements as a function of heliocentric distance (After Gilmour and Sephton, 2003)
Fig. 9.9 The Murchison meteorite
Table 9.3 The abundance of amino acids synthesized in Miller and Urey's experiment, compared with those found in the Murchison meteorite [After Gilmour and Sephton, 2003]

Name of

Abundance in the

Abundance in the

Found in terrestrial

amino-acid

Miller-Urey experiment

Murchison meteorite

proteins (Y/N)

glycine

xxxx

xxxx

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