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Catling speculates that the exobase of early Earth was hot andthat the ancient nonthermal escape rate was more than 1000 timesthe present rate. However, low oxygen and high carbon dioxideon early Earth yields a cold exobase, and nonthermal escaperates are limited and cannot balance the volcanic outgassingof hydrogen.
1 Laboratory for Atmospheric and Space Physics, University of Colorado, Boulder, CO 80309, USA. 2 Program in Atmospheric and Oceanic Science, University of Colorado, Boulder, CO 80309, USA.
* To whom correspondence should be addressed: E-mail: tian{at}colorado.edu
Although the supply of organic compounds by hydrothermal ventsand meteorites to the prebiotic Earth remains an interestingsubject of study, our modeling results (1) suggest that researchinto the origin of life should be refocused on chemistry inthe atmosphere, in the global oceans, and at the interface betweenthe atmosphere and ocean.
The abundance of hydrogen in the atmosphere is currently limitedby oxidation, not escape. We found that the hydrogen escaperate would have been 1000 times as high on prebiotic Earth asit is today (1 to 3 x 108 cm–2 s–1) (2) and wouldhave been the major sink for hydrogen. Therefore, our modelsupposes that hydrogen escape was energy-limited rather thandiffusion-limited.
The current high exobase temperature on Earth is caused by absorptionof sunlight by oxygen and the lack of an effective radiator.Catling (3) speculates that a high exobase temperature on earlyEarth could be caused by other gases. However, other gases areas likely to cool the atmosphere as they are to warm it. EarlyEarth analogs include Titan, Venus, and Mars, all of which havecold exobases (4). The model cited by Catling (5) is not appropriatefor early Earth (6) because it contains too much O2 and notenough CO2. Catling argues that the CO2 content in the earlyArchean atmosphere was <1% and speculates that even 30 timesthe current CO2 might not cause substantial cooling in the thermosphere.These arguments are not convincing. Geological evidence supportsvery high CO2 concentrations throughout the Archean (7). Italso has been shown that a doubling of CO2 content in the presentatmosphere, to 0.072%, can cause the thermosphere to cool by50 K (8). Thus the anoxic, CO2-rich atmosphere of early Earthshould have had a cold exobase even with only 1% CO2, and Jeansescape should have been slow. In response to some of Catling'sother criticisms, our model used a constant heating efficiencyof 15%, the same value as that used in a previous Venus hydrodynamicescape model (9). Detailed radiative transfer calculations willbe needed to determine how this assumption will affect the escaperate. The escape rate of atomic hydrogen depends on its abundance,which should be much smaller than that of H2 (10).
Catling's arguments on nonthermal escape are speculative andto some extent represent a misunderstanding of the escape processes.Helium in the contemporary atmosphere is in a balance betweennonthermal escape and degassing (11), a behavior similar tothat of hydrogen in early Earth's atmosphere. This does notlead to the conclusion that hydrogen escape from early Earth'satmosphere was dominated by nonthermal processes. Nonthermalescape of hydrogen is dominant under solar minimum conditionson present Earth (12). However, for nonthermal escape to balancethe outgassing of hydrogen on early Earth, the rate must be1000 times as fast as it is now. Analysis of the nonthermalescape processes for a water-rich early Venus atmosphere showsthat the upper limit of hydrogen nonthermal escape is 1 x 1010cm–2 s–1 (13), because the dominant escape mechanism,charge exchange, depends on the abundance of protons, whichis limited by photoionization. Such photoionization on Earthshould always be a factor of two or less than that on Venus.Regarding Catling's statement about hydrogen escape promotedby Earth's magnetic field, the permanent magnetic field canmake global nonthermal escape only smaller, not higher (14).No one has proposed a mechanism that would result in a highertotal nonthermal escape rate from a planet with a magnetic fieldas opposed to one without a magnetic field. Therefore, the ancientVenus nonthermal escape rate is an upper limit for the nonthermalescape on early Earth. The upper limit of nonthermal, Venus-likeescape of hydrogen [1 x 1010 cm–2 s–1 (13)] cannotbalance the Archean hydrogen outgassing rate (1 x 1011 cm–2s–1). Therefore, hydrodynamic escape has to occur.
The hydrodynamic escape required in (15) to explain the Xe isotopeabundance started 50 million years after the formation of Earth.The strength of solar extreme ultraviolet (EUV) flux neededis several hundred times that of today. The length of this "extreme"escape episode is only about 200 to 300 million years. The initialhydrogen escape flux is 1 to 5 x 1014 cm–2 s–1.Hence, the "close to upper limit" hydrogen escape suggestedby Catling is irrelevant to the origin-of-life problem becauseit occurred much earlier in the history of Earth. The hydrodynamicescape values calculated in (1), extrapolated to an EUV fluxseveral hundred times that of today, yield a hydrogen escapeflux similar to that required to explain Xe abundance in (15).
The field of prebiotic atmospheric chemistry is ripe with possibilitiesfor further research, including understanding the fractionationof heavy elements, the composition and thermal structure ofthe early atmosphere, and the origin of life. However, our assumptionof a cold exobase is supported, and we stand by the main conclusionsin (1). The nonthermal escape rate of hydrogen from early Earth'satmosphere should have been lower than the hydrodynamic escaperate and the rate of outgassing of hydrogen. Hence, the ancientatmosphere was hydrogen rich.
References and Notes
1. F. Tian, O. B. Toon, A. A. Pavlov, H. De Sterck, Science308, 1014 (2005).[Abstract/Free Full Text]
2. B. D. Shizgal, G. G. Arkos, Rev. Geophys.34, 483 (1996). [CrossRef]
4. Titan, with an atmosphere of N2, methane, and associated hydrocarbons, has an exobase temperature near 150 K (15), which is approximately twice its emission to space temperature. Venus and Mars also have very low exobase temperatures, lower even than in our H2 escape model, because CO2 radiates energy efficiently to space.
5. G. Visconti, J. Atmos. Sci.32, 1631 (1975). [CrossRef]
6. The mixing ratios of CO2 and O2 in (4) are 300 parts per million (ppm) and 10–3, respectively. The O2 concentration in prebiotic Earth's atmosphere is determined by the H2 concentration. For an H2 concentration of 10–3, the O2 concentration should be 10–15 (16). The CO2 concentration in today's atmosphere is 360 ppm. The Archean atmosphere should contain substantially more CO2. It is known that nonlocal thermodynamic equilibrium (non-LTE) is important for CO2 15 µm cooling, and only LTE is considered in (4).
8. R. G. Roble, R. E. Dickinson, Geophys. Res. Lett.16, 1441 (1989).
9. J. F. Kasting, J. B. Pollack, Icarus53, 479 (1983). [CrossRef] [ISI]
10. In a cold anoxic atmosphere, atomic hydrogen can be formed from H2 photodissociation. It is found that the production rate of atomic hydrogen from photodissociation of H2 in the atmosphere of extrasolar planet HD209458b (orbit 0.05 astronomical units) is 9.5 x 1011 cm–2 s–1 (17). By extrapolating this production rate to early Earth (5 times solar EUV flux today), an H production rate of 1.2 x 1010 cm–2 s–1 is obtained. This rate is about one order of magnitude smaller than the hydrogen degassing rate for early Earth (1 x 1011 cm–2 s–1), so it should be unimportant. It is possible that there were other sources of atomic hydrogen in the early Earth's atmosphere, such as CH4 or H2O. However, the abundance of these gases would have been much lower than that of H2 in the upper atmosphere. We have performed photochemical simulations of the H abundance in the lower atmosphere of early Earth and find that the H abundance is negligibly small. To better understand the H escape rate requires a multifluid model with photochemistry.
11. T. Torgerson, Chem. Geol.79, 1 (1989).
12. J. W. Chamberlain, D. M. Hunten, Theory of Planetary Atmospheres: An Introduction to Their Physics and Chemistry (Academic Press, 1987).
13. S. Kumar, D. M. Hunten, J. B. Pollack, Icarus55, 369 (1983). [CrossRef] [ISI]
14. K. Seki, R. C. Elphic, M. Hirahara, T. Terasawa, T. Mukai, Science291, 1939 (2001).[Abstract/Free Full Text]
1000 times as high on prebiotic Earth as it is today (
