14:45 〜 15:00
[U11-05] Impacts of recurrent solar energetic particle precipitation on the concentrations of CO and H2 in the early Martian atmosphere
キーワード：火星、ホルムアルデヒド、太陽高エネルギー粒子
Understanding the effects of solar energetic particles (SEPs) on the atmospheric chemistry on early Mars is of astrobiological interest because the precipitation of SEPs into the early Martian atmosphere could have facilitated the prebiotic chemistry (Lingam et al., 2018; Adams et al., 2021). The flare observations of solar-type G stars by the Kepler mission suggested that our Sun should have been much more active, and intense SEP events could have hit the planetary atmospheres repeatedly 4 billion years ago (e.g., Lingam et al., 2018). A denser solar wind density and a faster coronal mass ejection speed from the young Sun could have promoted a higher intensity and a hard spectrum of SEPs (Hu et al., 2022), leading to higher ionization rates in the lower atmosphere than present-day SEP events. Combining such frequent and hard-spectra SEP events in the past with the high efficiency in the production of amino acids suggested by previous laboratory experiments, SEPs are one of the key energy sources for prebiotic chemistry (Kobayashi et al., 2023).
Concentration of CO and H2 in the early Martian atmosphere is an important factor determining the production rates of formaldehyde (H2CO), a precursor of amino acids and ribose. Koyama et al. (2024) showed a continuous supply of H2CO on the Martian surface in a dense CO2-dominated early Martian atmosphere with a higher CO and H2 concentrations. Higher concentration of H2 of more than a few percent could have warmed the early Martian atmosphere via collision-induced absorptions and produced a hospitable environment with a liquid water on the surface (Kamada et al., 2021). Recent model study by Nakamura et al. (2023) suggested that the precipitation of SEPs into the present-day Martian atmosphere cause an enhancement of HOx via ion chemistry and change the HOx-related chemical composition such as ozone. However, the effects of such SEP-induced HOx enhancement on the composition of CO and H2 in the dense early Martian atmosphere have not yet been evaluated.
In this study, we explored the impacts of SEPs on the composition of the early Martian atmosphere using a one-dimensional photochemical model (Nakamura et al., 2023). The ionization rates of atmospheric molecules are calculated by a continuous slowing down approximation using a proton stopping power data, and an incident proton flux spectrum calculated by Hu et al. (2022) under 3.9 Ga solar wind parameters. We derived analytic expressions for the SEP-induced production rates of H and OH via ion chemistry under a photochemical equilibrium approximation. We found that the dissociation of water vapor by SEP-induced ion chemistry is more than three orders of magnitudes higher than the photolysis rate. In various H2 degassing rate cases, the CO mixing ratio decreased by 1-2 orders of magnitudes due to an enhanced OH density, while the response of the H2 mixing ratio depends on the H2 degassing rate. Without H2 degassing, the H2 mixing ratio increases because the recombination of atomic hydrogen excesses the loss of H2 by OH, which results in the enhancement of the H2 mixing ratio from 20 ppm to 100 ppm. With a high H2 degassing rate of 5×1011 cm-2 s-1, the recombination of atomic hydrogen is not enough to already existing dense H2 and the loss of H2 by OH becomes dominant, which results in the decrease of H2 mixing ratio from 2% to 0.2%.
References:
Adams et al. (2021). Astrobiology. 21:8, 968–980, doi:10.1089/ast.2020.2273.
Hu et al. (2022). Sci. Adv .8, eabi9743, doi:10.1126/sciadv.abi9743.
Kamada et al. (2021). Icarus, 368, 114618, doi:10.1016/j.icarus.2021.114618.
Kobayashi et al. (2023). Life, 13, 1103, doi:10.3390/life13051103.
Koyama et al. (2024). Sci. Rep., 14, 2397, doi:10.1038/s41598-024-52718-9.
Lingam et al. (2018). The Astrophysical Journal, 853:10, doi:10.3847/1538-4357/aa9fef.
Nakamura et al. (2023). J. Geophyss Res.: Space Physics, 128, 12, doi:10.1029/2022JA031250.
Concentration of CO and H2 in the early Martian atmosphere is an important factor determining the production rates of formaldehyde (H2CO), a precursor of amino acids and ribose. Koyama et al. (2024) showed a continuous supply of H2CO on the Martian surface in a dense CO2-dominated early Martian atmosphere with a higher CO and H2 concentrations. Higher concentration of H2 of more than a few percent could have warmed the early Martian atmosphere via collision-induced absorptions and produced a hospitable environment with a liquid water on the surface (Kamada et al., 2021). Recent model study by Nakamura et al. (2023) suggested that the precipitation of SEPs into the present-day Martian atmosphere cause an enhancement of HOx via ion chemistry and change the HOx-related chemical composition such as ozone. However, the effects of such SEP-induced HOx enhancement on the composition of CO and H2 in the dense early Martian atmosphere have not yet been evaluated.
In this study, we explored the impacts of SEPs on the composition of the early Martian atmosphere using a one-dimensional photochemical model (Nakamura et al., 2023). The ionization rates of atmospheric molecules are calculated by a continuous slowing down approximation using a proton stopping power data, and an incident proton flux spectrum calculated by Hu et al. (2022) under 3.9 Ga solar wind parameters. We derived analytic expressions for the SEP-induced production rates of H and OH via ion chemistry under a photochemical equilibrium approximation. We found that the dissociation of water vapor by SEP-induced ion chemistry is more than three orders of magnitudes higher than the photolysis rate. In various H2 degassing rate cases, the CO mixing ratio decreased by 1-2 orders of magnitudes due to an enhanced OH density, while the response of the H2 mixing ratio depends on the H2 degassing rate. Without H2 degassing, the H2 mixing ratio increases because the recombination of atomic hydrogen excesses the loss of H2 by OH, which results in the enhancement of the H2 mixing ratio from 20 ppm to 100 ppm. With a high H2 degassing rate of 5×1011 cm-2 s-1, the recombination of atomic hydrogen is not enough to already existing dense H2 and the loss of H2 by OH becomes dominant, which results in the decrease of H2 mixing ratio from 2% to 0.2%.
References:
Adams et al. (2021). Astrobiology. 21:8, 968–980, doi:10.1089/ast.2020.2273.
Hu et al. (2022). Sci. Adv .8, eabi9743, doi:10.1126/sciadv.abi9743.
Kamada et al. (2021). Icarus, 368, 114618, doi:10.1016/j.icarus.2021.114618.
Kobayashi et al. (2023). Life, 13, 1103, doi:10.3390/life13051103.
Koyama et al. (2024). Sci. Rep., 14, 2397, doi:10.1038/s41598-024-52718-9.
Lingam et al. (2018). The Astrophysical Journal, 853:10, doi:10.3847/1538-4357/aa9fef.
Nakamura et al. (2023). J. Geophyss Res.: Space Physics, 128, 12, doi:10.1029/2022JA031250.