Haze Aerosols in the Atmosphere of Early Earth: Manna from Heaven

Abstract

An organic haze layer in the upper atmosphere of Titan plays a crucial role in the atmospheric composition and climate of that moon. Such a haze layer may also have existed on the early Earth, providing an ultraviolet shield for greenhouse gases needed to warm the planet enough for life to arise and evolve. Despite the implications of such a haze layer, little is known about the organic material produced under early Earth conditions when both CO(2) and CH(4) may have been abundant in the atmosphere. For the first time, we experimentally demonstrate that organic haze can be generated in different CH(4)/CO(2) ratios. Here, we show that haze aerosols are able to form at CH(4) mixing ratios of 1,000 ppmv, a level likely to be present on early Earth. In addition, we find that organic hazes will form at C/O ratios as low as 0.6, which is lower than the predicted value of unity. We also show that as the C/O ratio decreases, the organic particles produced are more oxidized and contain biologically labile compounds. After life arose, the haze may thus have provided food for biota.

Organic haze on Titan and the early Earth

“…This rate is also comparable with the carbon burial rate in the Archean, which is estimated to have been approximately the same as the current carbon burial rate of 5 ϫ 10 13 g of C per year (43). Thus, the analysis in this work suggests that, on the postbiotic Earth, haze formation may substantially impact the temperature and habitability of the planet (22,24,44), the cycling of organic material within the biosphere (28), and the geologic record (45,46). The value for the CH 4 concentration on the prebiotic Earth is not certain, because the magnitude of abiotic CH 4 production from possible sources such as mid-Atlantic ridges (47) is unknown.…”

“…Although the mass changes only slightly when the C͞O ratio Ͼ1, there was an increase in particle production observed when the CH 4 and CO 2 concentrations were approximately equal. This is an unexpected result, because aerosols were not predicted to form below a C͞O ratio of 1 (38), and previous experiments with an electric discharge reported a decrease in aerosol production with added CO 2 (28). According to chemical schemes presented in the literature, as CH 4 and CO 2 are photolyzed, they are dissociated into the molecular and radical species H 2 , CO, CH 2 , CH, H, and O (12,39).…”

“…where 0 is the unit density (1 g⅐cm Ϫ3 ), m is the material density of the particle, and S is the Jayne shape factor (28,(31)(32)(33). Eq.…”

“…Some workers have studied organics formed in environments including CO, N 2 , and CH 4 , but these have either been focused on Titan conditions (25) or have looked only at amino acid production for early Earth (26,27). Previous work by our group examined CH 4 ͞CO 2 hazes using an electric discharge source (28), but a UV energy source is needed to interpret the results for the early Earth's atmosphere. Here, we use the results from CH 4 ͞CO 2 ͞N 2 photolysis to determine how similar the atmospheric chemistry on the early Earth may have been to Titan's current organic production and to explore the possibility of an early Earth haze layer (Fig.…”

Space as a Tool for Astrobiology: Review and Recommendations for Experimentations in Earth Orbit and Beyond

“…Several complementary sources are considered, among them the coupled system ocean/atmosphere (the primitive soup theory) (see e.g. Oparin 1953;Trainer et al 2004). A key question resides in the capacity of the primitive atmosphere to produce large organic molecules enriched by nitrogen and oxygen chemical functional groups, representative of prebiotic molecules.…”