Technical Comments

"Killer" Impacts and Life's Origins

Science  12 Sep 1997:
Vol. 277, Issue 5332, pp. 1687-1688
DOI: 10.1126/science.277.5332.1687

The recent report by Christopher P. McKay and William J. Borucki (1) brought the consequences of impact shocks for the origin of life on Earth into focus anew. One could, however, imagine further positive consequences of large and small impacts for the prebiotic environment.

If one accepts the “RNA-world” scenario (2) for the origin of life on Earth, one has to assume a massive production of activated ribonucleotides or RNA-oligonucleotides. Whereas nucleotide bases and sugars could be produced from hydrogen cyanide (HCN) and formaldehyde—for which plausible prebiotic synthesis-mechanisms in the early Earth atmosphere exist (3)—the triphosphate part of the ribonucleotide would have to be mobilized from the lithosphere and brought into contact with the sugars and bases. Furthermore, a ready source of free enthalpy would be required to drive the cycles of polymerization (reproduction) and destruction (selection) of RNA chains until ribozymes (or even enzymes) arose that could couple the self-replicating RNA system to other, not so easily available sources of free energy, like, for example, solar radiation. Several researchers have proposed polyphosphate as this primary source of free enthalpy [see references in (4)].

In this respect, the paper by Yamagata et al. (5) about polyphosphate synthesis by heating of phosphate rocks with water vapor of over 1000°C and subsequent rapid cooling is most instructive. But instead of a synthesis route consisting of production of HCN and formaldehyde by lightning or ultraviolet radiation followed by rain out and subsequent reaction with polyphosphates produced in volcanoes (5), a second, maybe more efficient, way of ribonucleotide synthesis should be examined, which is also in line with current thought about the accretion of the Earth by planetesimals and its continuing bombardment even after the oceans and atmosphere had formed.

Fegley et al. (6) demonstrated the synthesis of HCN by meteoric impact on Earth's early atmosphere, while Sleep et al. (7) described the consequences of large impacts for the prebiotic enviroment. Large impacts could have repeatedly vaporized not only the entire ocean of the early Earth, but also enough rock to create 100 bar of rock vapor and suspended droplets with a temperature of 2000°C (7). Smaller impacts that vaporize only the photic zone of the oceans were also discussed by Sleep et al. (7). It would be interesting to examine whether, under these conditions, (poly-)phosphates would be produced and in what quantities. Probably no activated polyphosphate would survive the cooling time after a so-called “ocean blaster” had vaporized the whole ocean, but even after somewhat smaller impacts the resulting rock vapor probably produced polyphosphates like the heated phosphate rock/basalt mixure used by Yamagata et al. (5). As the extraction efficiency rises with increasing temperature (5), the higher temperatures should compensate for the fact that natural occurring rocks contain over one order of magnitude less phosphate than the model substances (4). The ocean would boil under the influence of the infrared radiation of the rock vapor, but the ocean depths would remain cool (7) and could act as a cold trap as in Yamagata's experiment.

The polyphosphate yield of this process would be determined not only by the production, but also by the (hydrolytic) destruction rate of polyphosphate before it could reach the deep ocean. A detailed discussion has to take into account the following considerations (8), among others: Phosphate would condense out of rock vapor after much of the silicate had condensed. So, rock rain would concentrate phosphate in the remaining air. The rock rain drops would fall through the surface layer and would be quenched in seconds in the ocean. Water rain would not fall until later, when the atmosphere became cooler than the critical point. This water rain would initially be buoyant on saline water and cool slowly.

In a reduced rock vapor with metallic iron, P is not volatile; it condenses in solid solution, and then as Fe3P. This would happen in a major impact, when metallic iron is present. Phosphorous becomes a lithosphile at lower temperatures as phosphate. If iron drops quench, then P might react at low temperature. On the other hand, iron phosphide in Fe metal drops may be a good starting material for interesting prebiotic reactions. In a more oxidizing impact of silicate without metallic iron, phosphate may become concentrated more in the final vapor. [The moon is somewhat depleted in P relative to the Earth, so the final vapor that was lost to space was somewhat enriched in P (9)].

Finally, a further point has to be taken into account: Keefe and Miller (4) pointed out that if the partial pressure of water vapor exceeds 6 bar, the entropy-driven condensation reaction of phosphate into polyphosphate and water would be driven in the reverse direction. This limits the size of the impactor useful for producing polyphosphates to about 90-km diameter (8). Much bigger blasts in earlier times (even bigger than ocean blaster) would have destroyed any complex molecules, including polyphosphates, [the water vapor atmosphere lasts 3000 years (7)], but could still be beneficial for the later origin of life by reworking the upper crust thoroughly and “leaching out” the phosphate fraction. Because phosphate probably remained airborne longer than the rock fraction (8), it would be more concentrated in the upper layers of the Earth's crust after re-condensation. There it could be mobilized again by not so massive and deep penetrating impacts. Such large impactors, however, would have an iron core. So the above-described process has to compete with iron droplets in a large impact. Most of the Earth's P today is in the core (8).

The admittedly optimistic scenario described above could be a counterargument to Miller's computation (which was intended as a reductio ad absurdum) that even if the phosphate of the upper 1 km of Earth's crust could be extracted, the resulting solution in the ocean would be only 0.03 M (4). Of course, the circumstances described above would be difficult to simulate in a laboratory, but it would be interesting if the outcome of such experiments supported the hypothesis that those “killer impacts,” through repeated “fractionation” and enrichment of polyphosphates in the future biosphere, got life started in the first place (10).

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