11:00 AM - 1:00 PM
[BCG05-P05] Factors controlling production of organics synthesized from CO atmosphere
Keywords:CO atmosphere, Photochemistry, Origin of life
For life to emerge on the early Earth, a system that provides a continuous supply of organic matter from the inorganic environment is necessary. It is known that organic matter is produced by photochemical reactions in a reducing atmosphere containing carbon monoxide (CO). Therefore, it is very important for the study of the origin of life to understand quantitatively how the flux of organic matter produced from CO by photochemical reactions in the primitive atmosphere is affected by any conditions.
The photochemical reaction between CO and H2O proceeds as follows. First, H2O photodissociates to produce hydrogen radicals (H) and hydroxyl radicals (OH). Some of CO reacts with OH to form CO2 (CO+OH→CO2+H), and other reacts with H to formyl radicals (HCO) (CO+H+M→HCO+M, M: the third body of the reaction). And all organic matter is considered to be produced via HCO. Therefore, the rate ratio of HCO production to the rate of CO2 production, HCO/CO2, determines the final amount of organic matter produced. The larger this ratio is, the more efficiently organic matter is expected to be produced. Two factors that can change the production rate ratio are the total atmospheric pressure and the redox state. In this study, we focused on the formation rate ratio HCO/CO2, and experimentally investigated how (1) total atmospheric pressure, (2) hydrogen partial pressure, and (3) water vapor pressure change the total organic matter/CO2. After irradiating a gas whose carbon source was only CO with UV light for 3 hours, the total organic acids and the CO2 were measured, and the production rate ratio R = total organic acids/CO2 was calculated.
As a result of the experiment, it was confirmed that the higher the total atmospheric pressure Ptotal[bar] and the higher the hydrogen partial pressure PH2[bar], the larger the production rate ratio R. The relationship R=(1+0.293PH2)×0.248Ptotal/(Ptotal+0.858) was found. The reason why the total atmospheric pressure changed R is due to the fact that the formation of HCO is a three-body reaction and the reaction rate constant increases monotonically with the total atmospheric pressure. The reason why the partial pressure of hydrogen changed R is considered to be that the OH consumption reaction (H2+OH→H2O+H) was promoted by the hydrogen inclusion, and the H/OH ratio became higher. Experiments with varying water vapor pressure showed that the higher the water vapor pressure, the faster the rate of organic matter formation and the higher the rate ratio. However, it is unclear why the vapor pressure changed the production efficiency.
Based on the above experimental results, we calculated the global organic matter flux, Forg, when CO is in a steady state in the atmosphere. As a result, it was confirmed that the atmospheric total pressure is the most important parameter for the organic flux, followed by the CO2 partial pressure. It was also found that the Forg increased monotonically with total atmospheric pressure and reached a maximum value when the CO2 partial pressure at the surface was 0.1 bar. The organic matter flux was estimated to be about 1×1012 mol/yr at a total atmospheric pressure of 10 bar and a CO2 partial pressure of 0.1 bar. This is larger than the organic flux of 2.5×1011 mol/yr estimated by Chyba & Sagan (1992) for lightning discharges in a reducing atmosphere. These results suggest that the photochemical reaction of CO was an important source of organic matter in the primitive atmosphere.
The photochemical reaction between CO and H2O proceeds as follows. First, H2O photodissociates to produce hydrogen radicals (H) and hydroxyl radicals (OH). Some of CO reacts with OH to form CO2 (CO+OH→CO2+H), and other reacts with H to formyl radicals (HCO) (CO+H+M→HCO+M, M: the third body of the reaction). And all organic matter is considered to be produced via HCO. Therefore, the rate ratio of HCO production to the rate of CO2 production, HCO/CO2, determines the final amount of organic matter produced. The larger this ratio is, the more efficiently organic matter is expected to be produced. Two factors that can change the production rate ratio are the total atmospheric pressure and the redox state. In this study, we focused on the formation rate ratio HCO/CO2, and experimentally investigated how (1) total atmospheric pressure, (2) hydrogen partial pressure, and (3) water vapor pressure change the total organic matter/CO2. After irradiating a gas whose carbon source was only CO with UV light for 3 hours, the total organic acids and the CO2 were measured, and the production rate ratio R = total organic acids/CO2 was calculated.
As a result of the experiment, it was confirmed that the higher the total atmospheric pressure Ptotal[bar] and the higher the hydrogen partial pressure PH2[bar], the larger the production rate ratio R. The relationship R=(1+0.293PH2)×0.248Ptotal/(Ptotal+0.858) was found. The reason why the total atmospheric pressure changed R is due to the fact that the formation of HCO is a three-body reaction and the reaction rate constant increases monotonically with the total atmospheric pressure. The reason why the partial pressure of hydrogen changed R is considered to be that the OH consumption reaction (H2+OH→H2O+H) was promoted by the hydrogen inclusion, and the H/OH ratio became higher. Experiments with varying water vapor pressure showed that the higher the water vapor pressure, the faster the rate of organic matter formation and the higher the rate ratio. However, it is unclear why the vapor pressure changed the production efficiency.
Based on the above experimental results, we calculated the global organic matter flux, Forg, when CO is in a steady state in the atmosphere. As a result, it was confirmed that the atmospheric total pressure is the most important parameter for the organic flux, followed by the CO2 partial pressure. It was also found that the Forg increased monotonically with total atmospheric pressure and reached a maximum value when the CO2 partial pressure at the surface was 0.1 bar. The organic matter flux was estimated to be about 1×1012 mol/yr at a total atmospheric pressure of 10 bar and a CO2 partial pressure of 0.1 bar. This is larger than the organic flux of 2.5×1011 mol/yr estimated by Chyba & Sagan (1992) for lightning discharges in a reducing atmosphere. These results suggest that the photochemical reaction of CO was an important source of organic matter in the primitive atmosphere.