09:00 〜 09:15
[U08-01] Conditions required for the CO runaway on the early Earth
キーワード：Carbon monoxide、Atmospheric chemistry
One of the overarching goals of the ongoing and future exoplanetary observational surveys is to detect biosignatures—signs of life. The traditional concept of planetary habitability relies on the presence of liquid water on the planet. However, it would not be a sufficient condition for the planet to actually be habitable. The planetary habitability is also affected by several environmental factors, such as the climatic state and the availability of bioessential elements. From this perspective, carbon is of particular interest, because it is an essential element for life, and major carbon species in the planetary atmosphere (CO2, CO, and CH4) play a key role in the planetary environment. More specifically, CO2 and CH4 are potent greenhouse gases that control the global climate, and CO is a source of energy and carbon for microorganisms. Thus, the quantitative understanding of factors controlling the relative abundance of these carbon species in the planetary atmosphere has far reaching implications in the search for life in the universe.
The abundance of CO in the anoxic atmosphere of the early Earth would have been controlled by the CO2 photolysis and OH availability. Previous studies have revealed that when CO production via the photolysis of CO2 increases to the same level as the rate of OH radical formation from H2O photolysis, CO can overwhelm the photochemical controls on their accumulation and rapidly build-up in the atmosphere (CO runaway). Such photochemical instability has a far-reaching implication for the origin of life, given that the organic matter, such as aldehydes and carboxylic acids, can be easily formed in the CO-rich atmosphere. However, the systematic examination on the conditions required for the CO runaway has not yet been conducted.
Here, we employ a 1D numerical model of atmospheric photochemistry to investigate the conditions required for the CO runaway for the early Earth. The model includes a series of photochemical reactions in the anoxic atmosphere. We explored the impact of varying key model parameters, such as atmospheric CO2 levels and volcanic outgassing rate of H2, on atmospheric chemistry and search for possible solutions which allow for CO-rich atmospheres. We also evaluate biogeochemical processes (e.g., depositional flux of formaldehyde and its maximum concentration in the ocean) under the CO runaway conditions.
Our model demonstrates that increasing CO2 increases the importance of CO2 photolysis relative to H2O photolysis and that at a certain threshold value of CO2, the CO runaway is triggered. Additional sensitivity experiments with respect to volcanic H2 flux demonstrate that higher H2 fluxes cause the CO runaway at smaller CO2 levels. Once CO runaway is triggered, atmospheric CO levels increase to >~0.1 bar. Our results also indicate the clear gap in the phase diagram of atmospheric CH4/CO2 vs. CO/CO2 at which atmospheric CO level is highly dependent on CH4. These results have ramifications for the search for habitable planets with reducing atmospheres that may promote the origin of life.
The abundance of CO in the anoxic atmosphere of the early Earth would have been controlled by the CO2 photolysis and OH availability. Previous studies have revealed that when CO production via the photolysis of CO2 increases to the same level as the rate of OH radical formation from H2O photolysis, CO can overwhelm the photochemical controls on their accumulation and rapidly build-up in the atmosphere (CO runaway). Such photochemical instability has a far-reaching implication for the origin of life, given that the organic matter, such as aldehydes and carboxylic acids, can be easily formed in the CO-rich atmosphere. However, the systematic examination on the conditions required for the CO runaway has not yet been conducted.
Here, we employ a 1D numerical model of atmospheric photochemistry to investigate the conditions required for the CO runaway for the early Earth. The model includes a series of photochemical reactions in the anoxic atmosphere. We explored the impact of varying key model parameters, such as atmospheric CO2 levels and volcanic outgassing rate of H2, on atmospheric chemistry and search for possible solutions which allow for CO-rich atmospheres. We also evaluate biogeochemical processes (e.g., depositional flux of formaldehyde and its maximum concentration in the ocean) under the CO runaway conditions.
Our model demonstrates that increasing CO2 increases the importance of CO2 photolysis relative to H2O photolysis and that at a certain threshold value of CO2, the CO runaway is triggered. Additional sensitivity experiments with respect to volcanic H2 flux demonstrate that higher H2 fluxes cause the CO runaway at smaller CO2 levels. Once CO runaway is triggered, atmospheric CO levels increase to >~0.1 bar. Our results also indicate the clear gap in the phase diagram of atmospheric CH4/CO2 vs. CO/CO2 at which atmospheric CO level is highly dependent on CH4. These results have ramifications for the search for habitable planets with reducing atmospheres that may promote the origin of life.