Energy Sources and Atmospheric Organic Production

Chyba and Sagan (1992, 1997) have summarized a number of endogenous sources of organics on early Earth, in effect updating the decades-earlier compilation of Miller and Urey (1959). The approach of these estimates is to multiply an experimentally derived efficiency for organic molecule production (e.g., in moles or kg organic production per joule of a particular energy source inputted into the candidate atmosphere) by an estimate of the globally averaged amount of that type of energy available on early Earth. Obviously critical to these estimates is the nature of the early atmosphere. As long as the exact nature of the early atmosphere is unknown, comparisons are best made for a variety of atmospheric models (Chyba and Sagan, 1992).

Chyba and Sagan (1991) updated the Miller and Urey (1959) estimates for global and electrical discharge on early Earth. Miller and Urey (1959) had used the best estimates then available for global coronal and lightning discharges - estimates that derived from Schonland (1953) from his pioneering

Fig. 6.2. Yields (mol J-1) of organic molecules produced by input spark discharge energy, as a function of atmospheric composition and H2 abundance. From Stribling and Miller (1987).

experimental work in 1928 (Schonland, 1928). Of course, such estimates could be made more reliably by 1991, subsequent to the advent of global monitoring - yet, remarkably, up until that date the field of the origin of life continued to base estimates of organic production on early Earth by coronal discharge on experiments dating to 1928. Chyba and Sagan (1991), in their review of contemporary data for lightning and coronal energy dissipation, found values about 20 and 120 times smaller, respectively, than those suggested by Miller and Urey (1959). However, the greatest uncertainty in these estimates remains the extrapolation from contemporary rates of electrical energy discharge back to the primitive Earth - an "extrapolation" that amounts to the assumption that the rates 4 Gyr ago were simply the same as those today (Miller and Urey, 1959; Chyba and Sagan, 1991).

Apart from electrical energy sources, a principal source of energy for organic production in the early Earth atmosphere would have been ultraviolet light from the Sun. Chyba and Sagan (1992) used earlier work by Zahnle and Walker (1982) on young solar analogue stars to estimate the evolution of solar ultraviolet energy at the Earth as a function of wavelength interval and time. These results are shown in Fig. 6.3. Production of organics from UV energy can then be estimated. These results assume that there is no significant UV shielding in the atmosphere; a UV shield would act to reduce organic production.

Fig. 6.3. Evolution of the Sun's ultraviolet luminosity through time, beginning with a —108-yr-old Sun at 4.5 Gyr ago, and extending to today. Dashed lines beyond 3.6 Gyr ago indicate qualitative evolution to current values. From Chyba and Sagan (1992).

Results for organic production in reducing (Miller-Urey) and neutral ([H2]/[CO2] = 0.1) atmospheres are given in Table 6.1. The case [H2]/[CO2] = 0.1 may overestimate the H2 abundance implied by most early Earth models, but the available data (see, e.g., Fig. 6.2) do not extend to much lower values so we choose this level, even at the risk of exaggerating endogenous production for this "end-member" atmosphere. Except where indicated, these data are taken from the compilation in Chyba and Sagan (1997), which updates Chyba and Sagan (1992). (Chyba and Sagan (1992) do, however, consider a number of production mechanisms not included here.)

For comparison, we also include a Tian et al. (2005) atmosphere for which we set [H2]/[CO2] = 3, the optimum case for organic production (see Fig. 6.2) in a CO2-rich atmosphere. Results for electrical discharge synthesis in this atmosphere are scaled from that for the Miller-Urey atmosphere according to Fig. 6.2; on the basis of these data, we estimate organic synthesis by electrical discharge to be a factor of 10 lower for the Tian et al. atmosphere than for the Miller-Urey atmosphere. Tian et al. (2005) present results for UV-driven synthesis in their model atmosphere as a function of changing [H2]/[CO2] ratio; we cite their value (about 1 x 1010 kg year-1) here.

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