4:45 PM - 5:00 PM
[MIS14-18] Oceanic retention of ammonia and thermal stability of the atmosphere-ocean system on the primitive earth
Keywords:primitive earth, ammonia, atmosphere-ocean system, reduced atmosphere, faint young Sun paradox
One of the hypotheses solving the faint young Sun paradox on the ancient earth is the greenhouse effect of NH3 that has strong infrared absorptions (Sagan and Mullen, 1972). NH3 might be important for the synthesis of prebiotic organic compounds like amino acids on primitive earth (Bada and Miller, 1968). Although NH3 tends to be removed from the atmosphere rapidly due to photochemical conversion to N2 (Kasting, 1982), Sagan and Chyba (1997) estimate that the lifetime of NH3 (10-5 bar) with the amount required for enough greenhouse effect may be as long as 50 million years as organic haze photochemically derived from CH4 shields NH3 from UV. They also argue that it might extend to 500 million years if NH3 was supplied continuously to the atmosphere from a reservoir initially containing NH3 with the amount equivalent to the total of current atmospheric N2. However, it has been little discussed what could be the possible reservoirs of NH3.
NH3 has high solubility to water; it dissolves 200 mol/kg in pure water under standard temperature and 1 bar of NH3. Its solubility increases when CO2 dissolved in water because H+ and therefore NH4+ concentrations increase in water associated with the ionization of CO2. Therefore, a large amount of NH3 might be dissolved in the primitive ocean if NH3 was present in the primitive earth atmosphere. On the other hand, NH3 solubility has strong temperature dependence which releases NH3 from water to gas under high temperatures. Thus, the greenhouse effect of NH3 and the release of NH3 from oceans might have a positive feedback relationship. Here we assess whether thermal stability could be established in the NH3-bearing atmosphere-ocean system and the possible strength of greenhouse effect and the NH3 partitioning between atmosphere and ocean at stable states if exist. Based on those analyses, we clarify whether or not the primitive ocean could function as a long-term reservoir for NH3. We used a one-dimensional radiative-convective equilibrium atmospheric model for the atmosphere and the solution equilibria model for the isothermal, uniform ocean taking NH3 and CO2 as solvents.
We take N2, NH3, CO2, and CH4 as atmospheric composition and apply 75% of the present solar constant. The vertical NH3 profile in the atmosphere is given taking into account the effect of rainout. The outgoing planetary radiation to space is calculated by using a line-by-line model and the surface temperature balanced with the absorption of solar radiation is obtained as a function of atmospheric composition, especially NH3 partial pressure. Here the planetary albedo is fixed at 0.3. The solution equilibria model shows the amount of NH3 dissolution to the ocean with the present oceanic mass as a function of NH3 and CO2 partial pressures. For simplicity, other dissolved components are ignored. The thermal stability of the atmosphere-ocean system can be judged by comparing the temperature dependence of NH3 partial pressure at solution equilibria under a fixed total amount of NH3 in the system and the dependence of surface temperature on NH3 partial pressure in the radiative-convective equilibrium atmosphere.
As the reference case, we suppose 1 bar of N2 atmosphere with 0.1 vol% of CO2 and CH4, respectively, and 1.0×1019 mol (equivalent to 3% of N2 in the present atmosphere) for the total amount of NH3. In this case, a thermally stable solution is uniquely obtained. At this solution, NH3 is partitioned 5.8×1016 mol (2.0×10-4 bar) to the atmosphere and the surface temperature is 330 K which exhibits a strong NH3 greenhouse. Thermally stable solutions are found when the total amount of NH3 and partial pressures of N2, CO2, CH4 are varied. Surface temperatures at thermally stable solutions little depend on the CH4 volume fraction. We obtained single stable solutions with surface temperatures from 273 K to 373 K when the CO2 volume fraction is smaller than to 10-5. On the other hand, two stable solutions with high and low surface temperatures appear under larger CO2 volume fractions. The surface temperature at thermally stable solution increases with increasing the total amount of NH3 and N2 partial pressure. Our result suggests that thermally stable states with surface temperatures above 273 K were possibly established when NH3 existed in the ancient atmosphere in solution equilibrium with the ocean. If NH3 loss from the atmosphere-ocean system is mostly caused by photolysis, a sufficient greenhouse effect by NH3 might have lasted 500 million years because the ocean could store a large amount of NH3 with replenishing NH3 into NH3 into the atmosphere.
NH3 has high solubility to water; it dissolves 200 mol/kg in pure water under standard temperature and 1 bar of NH3. Its solubility increases when CO2 dissolved in water because H+ and therefore NH4+ concentrations increase in water associated with the ionization of CO2. Therefore, a large amount of NH3 might be dissolved in the primitive ocean if NH3 was present in the primitive earth atmosphere. On the other hand, NH3 solubility has strong temperature dependence which releases NH3 from water to gas under high temperatures. Thus, the greenhouse effect of NH3 and the release of NH3 from oceans might have a positive feedback relationship. Here we assess whether thermal stability could be established in the NH3-bearing atmosphere-ocean system and the possible strength of greenhouse effect and the NH3 partitioning between atmosphere and ocean at stable states if exist. Based on those analyses, we clarify whether or not the primitive ocean could function as a long-term reservoir for NH3. We used a one-dimensional radiative-convective equilibrium atmospheric model for the atmosphere and the solution equilibria model for the isothermal, uniform ocean taking NH3 and CO2 as solvents.
We take N2, NH3, CO2, and CH4 as atmospheric composition and apply 75% of the present solar constant. The vertical NH3 profile in the atmosphere is given taking into account the effect of rainout. The outgoing planetary radiation to space is calculated by using a line-by-line model and the surface temperature balanced with the absorption of solar radiation is obtained as a function of atmospheric composition, especially NH3 partial pressure. Here the planetary albedo is fixed at 0.3. The solution equilibria model shows the amount of NH3 dissolution to the ocean with the present oceanic mass as a function of NH3 and CO2 partial pressures. For simplicity, other dissolved components are ignored. The thermal stability of the atmosphere-ocean system can be judged by comparing the temperature dependence of NH3 partial pressure at solution equilibria under a fixed total amount of NH3 in the system and the dependence of surface temperature on NH3 partial pressure in the radiative-convective equilibrium atmosphere.
As the reference case, we suppose 1 bar of N2 atmosphere with 0.1 vol% of CO2 and CH4, respectively, and 1.0×1019 mol (equivalent to 3% of N2 in the present atmosphere) for the total amount of NH3. In this case, a thermally stable solution is uniquely obtained. At this solution, NH3 is partitioned 5.8×1016 mol (2.0×10-4 bar) to the atmosphere and the surface temperature is 330 K which exhibits a strong NH3 greenhouse. Thermally stable solutions are found when the total amount of NH3 and partial pressures of N2, CO2, CH4 are varied. Surface temperatures at thermally stable solutions little depend on the CH4 volume fraction. We obtained single stable solutions with surface temperatures from 273 K to 373 K when the CO2 volume fraction is smaller than to 10-5. On the other hand, two stable solutions with high and low surface temperatures appear under larger CO2 volume fractions. The surface temperature at thermally stable solution increases with increasing the total amount of NH3 and N2 partial pressure. Our result suggests that thermally stable states with surface temperatures above 273 K were possibly established when NH3 existed in the ancient atmosphere in solution equilibrium with the ocean. If NH3 loss from the atmosphere-ocean system is mostly caused by photolysis, a sufficient greenhouse effect by NH3 might have lasted 500 million years because the ocean could store a large amount of NH3 with replenishing NH3 into NH3 into the atmosphere.