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, 17, 1710-1723

Atmospheric Prebiotic Chemistry and Organic Hazes

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Atmospheric Prebiotic Chemistry and Organic Hazes

Melissa G Trainer. Curr Org Chem.

Abstract

Earth's atmospheric composition at the time of the origin of life is not known, but it has often been suggested that chemical transformation of reactive species in the atmosphere was a significant source of prebiotic organic molecules. Experimental and theoretical studies over the past half century have shown that atmospheric synthesis can yield molecules such as amino acids and nucleobases, but these processes are very sensitive to gas composition and energy source. Abiotic synthesis of organic molecules is more productive in reduced atmospheres, yet the primitive Earth may not have been as reducing as earlier workers assumed, and recent research has reflected this shift in thinking. This work provides a survey of the range of chemical products that can be produced given a set of atmospheric conditions, with a particular focus on recent reports. Intertwined with the discussion of atmospheric synthesis is the consideration of an organic haze layer, which has been suggested as a possible ultraviolet shield on the anoxic early Earth. Since such a haze layer - if formed - would serve as a reservoir for organic molecules, the chemical composition of the aerosol should be closely examined. The results highlighted here show that a variety of products can be formed in mildly reducing or even neutral atmospheres, demonstrating that contributions of atmospheric synthesis to the organic inventory on early Earth should not be discounted. This review intends to bridge current knowledge of the range of possible atmospheric conditions in the prebiotic environment and pathways for synthesis under such conditions by examining the possible products of organic chemistry in the early atmosphere.

Keywords: Abiotic synthesis; Amino acid formation; Chemical evolution; Early earth atmosphere; Organic haze; Prebiotic chemistry; Primitive earth..

Figures

Fig. (1)
Fig. (1)
Absorption cross-sections (cm2 molecule-1) in the far ultraviolet for some atmospheric molecules suggested for the prebiotic Earth. The cross-section data were compiled using the MPI-Mainz-UV-VIS Spectral Atlas of Gaseous Molecules (www.atmosphere.mpg.de/spectral-atlas-mainz), and sourced from the indicated references [157-165]. The dissociation thresholds – wavelengths below which photodissociation will occur – for CO and N2 are 104 and 100 nm, respectively, and thus these molecules are not included on this graph. The solar spectrum represents the irradiation at the top of the current Earth atmosphere (~600 km) [166].
Fig. (2)
Fig. (2)
Recent assessment of organic aerosol produced from Far-UV irradiation of CH4/CO2/N2 atmospheres using high resolution aerosol mass spectrometry shows that the aerosol is comprised of molecules containing N- and O-heteroatoms. This mass spectrum shows the elemental analysis of the photochemical aerosol produced from 1:1 CH4 and CO2 mixture, originally presented as Figure 5b in Trainer et al. [77]. Mass ratios indicated in the legend, and atomic ratios O/C and N/C are 0.23 and 0.27, respectively. Previously published fluxes for organic matter produced via this pathway [77] indicate that the aerosol could be a significant source of fixed nitrogen to the environment. Structural information cannot be determined from the mass spectrum, only chemical formulae. As an example, the region around m/z 89 has been highlighted as containing a series of ions with coincident O- and N-functionalities. A few possible structures are suggested, but cannot be distinguished.
Fig. (3)
Fig. (3)
Summary of observed chemical products from simulations of atmospheric prebiotic chemistry. The C/O ratio is used as an approximate marker for the reduced/oxidized state of the gas mixtures studied, assuming N2 or NH3 contributions, with more details presented in the text or cited works. Note the scale on the horizontal axis is arbitrary. The shading in the bars indicates the direction of increasing production (lighter à darker). Note the lower limit on C/O of 0.5 represents the ratio where CO2 is the only carbon source. Although amino acid and nucleobase synthesis has been extensively reported for samples collected over water baths and/or with subsequent hydrolysis [131-134, 152] (marked on graph with *), the results indicated by bars on this graph are for direct atmospheric synthesis of amino acids. Sulfur compounds are given for irradiated atmospheres with either SO2 or H2S with CH4.

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