5:15 PM - 6:45 PM
[U11-P03] Impacts of Various Atmospheric Carbon Species on Climates of Terrestrial Planets
Keywords:Planetary Atmospheres, Climate, Water Loss
Atmospheric carbon compounds have various impacts on terrestrial planets, namely on climate, feedback, and potentially the emergence of life, but the focus has mainly been on the oxidized gas CO2. However, a wide range of carbon species abundances are possible as a result of outgassing, photochemistry, and biological activity. Previous studies have shown that the early Earth’s mantle may have been more reduced than today (Stagno and Aulbach, 2016; Nicklas et. al, 2019), which would allow for higher levels of the reduced gasses CO and CH4 to build up. It has also been shown that many different abundances of carbon species are possible from different outgassing fluxes under various stellar conditions (Watanabe and Ozaki, 2023). Each of the compounds CO, CO2, and CH4, behave differently in the atmosphere, so it is important to understand each of their roles. In the past, Haqq-Misra et. al (2008) studied the influence of CO2 and CH4 on climate, showing that increasing either compound will cause increased surface temperatures because of their greenhouse effects. Additionally, CO2 and CO have been studied to show the influence of pressure broadening (Bouanich and Blanquet, 1988), whereby warming from CO can occur when the CO2 levels approach and exceed 1 bar (Aoki, 2022). However, no study has investigated the influence of both CO and CH4 on climate and other atmospheric species, which we now believe may have been in high abundance on the early Earth and other terrestrial planets. Furthermore, the greenhouse, pressure broadening, and scattering effects will dictate feedback in the atmosphere. In scenarios where these gasses cause increased temperatures, it will raise water vapor levels, which in turn increases the oxidation rates of CO and CH4, then lowering temperatures; creating a negative feedback that can stabilize the atmosphere. Conversely, the scattering from CO may cause a positive feedback, where the decreased temperatures lower oxidation rates, allowing for further buildup of reducing gasses. In addition to climate and feedback, we also extend our study to model hydrogen escape on such planets, to determine what abundances of carbon compounds can cause significant levels of water loss, as water is a prerequisite to life.
To investigate this, we utilize a modified version of the 1D climate model Clima, which is a module of the Atmos code (Kasting et. al, 1984; Arney et. al, 2016) that includes CO (Aoki, 2022). Our model considers a diverse extent of CO/CH4/CO2 abundances that may be possible on lifeless Earth-like planets orbiting the Sun and the M-type star GJ876. Our results show that as we increase pCH4/pCO2, there is a peak surface temperature that occurs around pCH4/pCO2 = 10, regardless of CO levels. Above this ratio, surface temperatures decrease, which has not been found in previous studies. Unlike CH4, however, CO behaves differently depending on pCH4/pCO2 ratio. At low methane abundances, CO will cause warming by as much as 20 K from its pressure broadening effect, but at higher ratios it has no impact on climate (except as a scatterer when pCO is greater than pCH4). The discovery of warming in such a scenario has implications for planets orbiting M-type stars, where it is inefficient for CO to be turned back into CO2, allowing CO to build up in the atmosphere. While CO runaway may be possible, it is also possible that CO2 may be stabilized by water vapor photochemistry. Regardless of the feedback initiated, when atmospheres become sufficiently thick it will allow for significant amounts of water to be lost because of the increased temperatures. This suggests that on planets orbiting G- and M-type stars, thinner atmospheres are more favorable for surface temperatures that do not allow substantial water loss, with the M-type star case being much more sensitive.
To investigate this, we utilize a modified version of the 1D climate model Clima, which is a module of the Atmos code (Kasting et. al, 1984; Arney et. al, 2016) that includes CO (Aoki, 2022). Our model considers a diverse extent of CO/CH4/CO2 abundances that may be possible on lifeless Earth-like planets orbiting the Sun and the M-type star GJ876. Our results show that as we increase pCH4/pCO2, there is a peak surface temperature that occurs around pCH4/pCO2 = 10, regardless of CO levels. Above this ratio, surface temperatures decrease, which has not been found in previous studies. Unlike CH4, however, CO behaves differently depending on pCH4/pCO2 ratio. At low methane abundances, CO will cause warming by as much as 20 K from its pressure broadening effect, but at higher ratios it has no impact on climate (except as a scatterer when pCO is greater than pCH4). The discovery of warming in such a scenario has implications for planets orbiting M-type stars, where it is inefficient for CO to be turned back into CO2, allowing CO to build up in the atmosphere. While CO runaway may be possible, it is also possible that CO2 may be stabilized by water vapor photochemistry. Regardless of the feedback initiated, when atmospheres become sufficiently thick it will allow for significant amounts of water to be lost because of the increased temperatures. This suggests that on planets orbiting G- and M-type stars, thinner atmospheres are more favorable for surface temperatures that do not allow substantial water loss, with the M-type star case being much more sensitive.