Photodecomposition of CH₄ and NH₃ has been the main criticism of a strongly reducing early Earth atmosphere (2). The atmosphere proposed by Sagan and Chyba is similar to that proposed by Oparin (3) and by Urey (4) and simulated in prebiotic electric discharge experiments producing amino acids (5). Yellow-brown tholins are also produced efficiently in spark discharge experiments, but these are mostly water soluble in contrast to compounds produced high in the atmosphere in the relative absence of water.

It has been proposed (6) that an early atmosphere of ~10 bar of CO₂ was significant in aerobraking incoming comets, thus reducing the comet impact velocities sufficiently that cometary organics might have survived the impact. The requirement of CO₂ < CH₄ for efficient tholin production precludes the existance of an extensive CO₂ atmosphere, suggesting that aerobraking was not as important as previously maintained (6). In contrast, the efficiency of delivery of organics by interplanetary dust particles (IDPs) might be higher as a result of the reducing atmosphere because there would be less oxidation of organics liberated during ablative heating of IDPs. Because the CH₄ in the atmosphere proposed by Sagan and Chyba (1) has an endogenous source (that is, mid-ocean ridge vents), there is no role for exogenous sources of CH₄ and other organic compounds (6, 8).

The UV protection needed for NH₃ in the lower atmosphere diminishes the role of UV light as an energy source for prebiotic synthesis in the lower atmosphere. Thus, prebiotic syntheses employing hot hydrogen atoms (9) or CH₄ photolysis fragments (10) would be restricted to regions of the stratosphere above the haze layer. The formation of HCN could still occur by reaction of N with CH₃ and ³CH₂, but the survival of HCN against photolysis at H Ly  (121.6 nm) will only occur below the haze layer; shielding by CO₂ is unimportant for CO₂ ~ present-day levels (11). Thus, the amount of HCN delivered to the oceans is dependent on how quickly HCN is transported below the haze layer. The formation of H₂CO in the troposphere (12) would most likely be lower as a result of a reduction in H₂O and CO₂ photolysis below the haze. More generally, below the haze layer, lightning and corona discharge would be the dominant energy sources for prebiotic syntheses, even though UV (<250 nm) is at least a factor of 100 times greater in energy flux than are electric discharges.

Central to the feasibility of the model by Sagan and Chyba (1) is an adequate source of CH₄. They assume that most of the C outgassed from the oceanic vents was CH₄. At present, the CO₂ to CH₄ ratio is ~100 to 1 for the vents (14), corresponding to an apparent equilibrium of the reaction CO₂ + 4H₂ = CH₄ + 2H₂O at 500°C and 500 bars (7), assuming the quartz-fayalite-magnatite (QFM) buffer. The present oxygen fugacity in the mantle (15) is buffered approximately at QFM, which corresponds to fO₂ ~ 10⁸ at ~ 1200°C. As has been discussed by Kasting (13), a lower oxgen fugacity in the mantle, near the IW (iron-wustite) buffer with fO₂ ~ 10¹², would produce CO₂/CH₄ ~1 for present-day mantle conditions. Definitive evidence for a more reducing early mantle is lacking, but it is not unreasonable to expect that core formation was an imperfect process, and that some fraction of native metal (that is, iron) was left in the mantle. Oxidation of Fe⁰ to FeO (wustite) by reaction with H₂O, followed by reduction of CO₂ to CH₄, requires four moles of Fe⁰ for each mole of CH₄ produced (net reaction: 4Fe⁰ + CO₂ + 2H₂O = 4FeO + CH₄). As an example, a CH₄ flux of 100 nmole cm² yr¹ for 100 Myr would require 2 × 10²⁰ moles (40 moles cm²) Fe⁰ and 1 × 10²⁰ moles (20 moles cm²) H₂O, or ~0.0005% of the terrestrial inventory of iron and ~0.1% of the present ocean reservoir of water. Whether the early mantle contained such amounts of Fe⁰ and H₂O after core formation is unknown, but would seem to be plausible. The total C implied by the above CH₄ flux for 100 Myr corresponds to ~100 gC cm², or about 1% of the total C in carbonates today.

Additionally, native metals would act as catalysts for CO₂ reduction at lower temperatures, thus allowing CH₄ to reach equilibrium at the ~350°C hydrothermal vent temperatures. Without a catalyst, CH₄ formation is kinetically inhibited at temperatures below ~500°C at typical hydrothermal vent pressures. For a QFM buffer and at 500 bars, CO₂/CH₄ ~ 1 at 400°C; for a PPM (pyrite-pyrhotite-magnetite) buffer, CO₂/CH₄ ~ 1 at 275°C.

The model proposed by Sagan and Chyba (1) brings us back to the earlier models of a reducing primitive Earth atmosphere, with its many attractive implications for the origin of life.

Stanley L. Miller
James R. Lyons
Department of Chemistry and Biochemistry,
University of California, San Diego,
La Jolla, CA 92093-0506, USA
E-mail: jrlyons{at}ucsd.edu

REFERENCES

1. C. Sagan and C. Chyba, Science 276, p. 1217 (1997) .
2. J. F. Kasting, ibid. 259, 920 (1993) ; J. C. G. Walker, Evolution of the Atmosphere (Macmillan, New York, 1977); H. D. Holland, The Chemical Evolution of the Atmosphere and Oceans (Princeton Univ. Press, Princeton, NJ, 1984).
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Response: The late Carl Sagan and I noted in the introduction to our article (1) that a high-altitude organic aerosol on early Earth would make the persistence of a reducing atmosphere more likely, and that such an atmosphere favors organic synthesis, making the origin of life easier to envision. As we stated, atmospheres rich in CO₂ have been considered more likely candidates for early terrestrial atmospheres for two reasons. One is the rapid UV photodissociation of methane and ammonia. Our article suggests that a reducing "Miller-Urey" atmosphere, when treated self-consistently, avoids this difficulty because such an atmosphere would become self-shielding against UV photolysis. The second long-standing objection to an early reducing atmosphere is that most models for the oxidation state of the mantle suggest that an intermediate oxidation state atmosphere was most likely on early Earth. This argument is unaffected by our article. However, as Miller and Lyons point out, and as we stated, the issue remains an active area of research.

Extraterrestrial organics would have been delivered to Earth regardless of the nature of the early atmosphere. In a full-blown Miller-Urey atmosphere, exogenous sources are unlikely to have been quantitatively important, whereas they may have been the dominant source for prebiotic organics on an early Earth with a CO₂-rich atmosphere (2). I therefore do not agree with the statement by Miller and Lyons that in light of our article, "there is no role for exogenous sources of CH₄ and other organic compounds." Both exogenous and endogenous sources contributed to the prebiotic organic inventory of early Earth: They were not in competition. Which sources dominated depended on the nature of the early atmosphere, and there may have been specific molecules for which one source or the other was critical. Some recent work (3) hints that exogenous sources may be more important than we earlier estimated (2). Much remains to be understood, including the possible role of exogenous organics in the origin of biological homochirality (4).

It no longer appears correct that aerobraking of small coments or asteroids in an early dense atmosphere (5) would have allowed these impactors to have collided with sufficiently low velocities for organics to survive. Work on catastrophic explosions of small asteroids and comets in the atmosphere (6, 7) suggests that, apart from rare iron impactors, objects smaller than ~100 meters in diameter airburst in the terrestrial atmosphere prior to impact with the ground. This atmospheric "filtering" of impactors would have been even more severe in a putative denser early atmosphere. While studies show that some organics might survive airbursts (2), this work calls earlier estimates (5) of the role of ~100 m objects into question (7). Estimates of the importance of the likely dominant exogenous source, delivery by interplanetary dust particles (2, 8), remain unaffected by these arguments.

Whatever the sources of prebiotic organics--exogenous and endogenous--the formation of organic monomers is one of the few parts of the origins-of-life puzzle that is reasonably well in hand. In this sense, it may no longer be among the most important issues in the field (9). We can both extract these organics from meteorites and synthesize them in the laboratory. Nearly all subsequent steps on the road to the last common ancestor are much less well understood.

Christopher F. Chyba
Department of Planetary Sciences,
University of Arizona,
Tucson, AZ 85721-0092, USA
E-mail: chyba{at}lpl.arizona.edu.

REFERENCES

1. C. Sagan and C. Chyba, Science 276, p. 1217 (1997) .
2. C. Chyba and C. Sagan, Nature 355, 125 (1992) ; in Comets and the Origin and Evolution of Life, P. J. Thomas, C. F. Chyba, C. P. McKay, Eds. (Springer-Verlag, New York, 1997), pp. 147-173.
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4. J. R. Cronin and S. Pizzarello, ibid. 275, 951 (1997) ; M. Engel and S. Macko, Nature 389 (1997); C. F. Chyba, ibid., p. 234.
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8. E. Anders, Nature 342, 255 (1989) .
9. C. F. Chyba and G. D. McDonald, Annu. Rev. Earth Planet. Sci. 24, 215 (1995) [CrossRef].