Technical Comments

Organic Shielding of Greenhouse Gases on Early Earth

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Science  06 Feb 1998:
Vol. 279, Issue 5352, pp. 779
DOI: 10.1126/science.279.5352.779a

Carl Sagan and Christopher F. Chyba (1) propose that particulates of organic polymers (tholins) were produced by ultraviolet (UV) light high in a primitive Earth atmosphere with CO2/CH4 < 1. Suspended in the stratosphere, the particulates would protect CH4 and NH3below the haze layer from UV photolysis, thus allowing for an enhanced early greenhouse. This proposal is designed to resolve the conflict between the low luminosity of the early sun and the geological evidence for liquid water on the primitive Earth. The atmosphere proposed by Sagan and Chyba (1) has many logical consequences, several of which we point out here.

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 and3CH2, but the survival of HCN against photolysis at H Ly α (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 CH4ratio 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 ~ 10−8 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 ~ 10−12, 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 cm−2 yr−1 for 100 Myr would require 2 × 1020 moles (40 moles cm−2) Fe0 and 1 × 1020 moles (20 moles cm−2) 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 cm−2, or about 1% of the total C in carbonates today.

Additionally, native metals would act as catalysts for CO2reduction 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.


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.


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