Presentation Information
[U17-P05]Retention of an NH3-rich Surface Environment on Early Mars via Ocean Dissolution
*Hiroto Kizu1, Kirara Arima2, Shungo Koyama1, Tatsuya Yoshida3, Naoki Terada1, Yuki Nakamura4 (1.Tohoku University, 2.Hokkaido University, 3.Institute of Science Tokyo, 4.The University of Tokyo)
Keywords:
Mars,Ammonia,Photochemistry,Dissolution,Impact
Mars is currently a cold and dry planet with a surface environment that is considered uninhabitable. In contrast to present conditions, geomorphological and geochemical evidence suggests that early Mars likely sustained an active hydrological cycle under a temperate surface environment (e.g., Wordsworth, 2016). Furthermore, recent in-situ investigations by the Curiosity rover have detected diverse organic molecules and nitrogen-bearing compounds (e.g., Stern et al., 2015). These discoveries suggest an environment conducive to nitrogen-based prebiotic chemistry on early Mars, offering profound implications for its habitability and the origin of life on Earth.
Ammonia (NH3) is crucial for prebiotic chemistry, serving as a vital precursor for the Strecker reaction and formose-type reactions. Although the present-day Martian atmosphere is generally oxidized, NH3 could have been produced by impacts of iron-rich meteorites on early Mars. However, despite its importance, a critical gap remains in understanding its availability. Because NH3 is photochemically unstable, it is rapidly converted to N2 via photodissociation by solar ultraviolet radiation in a few years (Kasting, 1982), likely rendering atmospheric inventories insufficient for prebiotic chemical evolution. This study evaluates a mechanism to extend the lifetime of ammonia through atmosphere-ocean interactions following large-scale impact events, specifically focusing on oceanic storage of NH3 and its subsequent release processes.
To evaluate the atmospheric evolution and stability of NH3, we employed three distinct numerical models: (1) a line-by-line radiative-convective model (Wordsworth et al., 2021) to calculate temperature profiles under different atmospheric compositions, (2) a dissolution equilibrium model (Arima et al., in prep) to calculate the partitioning of NH3 between the atmosphere and a hypothesized 550 m depth ocean (Di Achille and Hynek, 2010), and (3) a 1-D photochemical model (PROTEUS; Nakamura et al., 2023) to calculate the time-dependent density profile of each species by solving the vertical continuity-transport equations along with chemical reactions. The chemical network for the photochemical model was constructed based on Tian et al. (2011) and Adams et al. (2021). We simulated a post-impact environment characterized by a 2-bar CO2 atmosphere containing 6% H2, with NH3 and CH4 as variable parameters (Kamada et al., 2020; Koyama et al., 2024).
Our analysis indicates that the ocean serves as a significant reservoir for ammonia. Dissolution equilibrium calculations indicate that, due to the high solubility of NH3, a substantial fraction of the initial inventory partitions from the atmosphere into the ocean. Furthermore, the ocean maintains a pH range from neutral to slightly acidic, buffered by dissolved carbonate species.
To assess the long-term stability of the system, we calculated photochemical loss rates of NH3 using the photochemical model. Based on these calculated rates, we estimate that an initial inventory of 1020 mol of ammonia can be maintained within this atmosphere-ocean system for approximately 105 years. This duration is orders of magnitude longer than estimates considering atmospheric processes alone. These findings suggest that impact-induced ocean-atmosphere systems could provide a stable supply of NH3 over timescales on the order of 105 years, potentially satisfying the temporal requirements for the emergence of prebiotic precursor chemistry.
Adams et al. (2021) Astrobiology, 21, 968–980.
Di Achille & Hynek (2010) Nat. Geosci., 3, 459–463.
Kamada et al. (2020) Icarus, 338, 113567.
Kasting (1982) J. Geophys. Res., 87, 3091–3098.
Koyama et al. (2024) Sci. Rep., 14, 2397.
Nakamura et al. (2023) Earth Planets Space, 75, 140.
Stern et al. (2015) Proc. Natl. Acad. Sci., 112, 4245–4250.
Tian et al. (2011) Earth Planet. Sci. Lett., 308, 417–423.
Wordsworth (2016) Annu. Rev. Earth Planet. Sci., 44, 381–408.
Wordsworth et al. (2021) Nat. Geosci., 14, 127–132
Ammonia (NH3) is crucial for prebiotic chemistry, serving as a vital precursor for the Strecker reaction and formose-type reactions. Although the present-day Martian atmosphere is generally oxidized, NH3 could have been produced by impacts of iron-rich meteorites on early Mars. However, despite its importance, a critical gap remains in understanding its availability. Because NH3 is photochemically unstable, it is rapidly converted to N2 via photodissociation by solar ultraviolet radiation in a few years (Kasting, 1982), likely rendering atmospheric inventories insufficient for prebiotic chemical evolution. This study evaluates a mechanism to extend the lifetime of ammonia through atmosphere-ocean interactions following large-scale impact events, specifically focusing on oceanic storage of NH3 and its subsequent release processes.
To evaluate the atmospheric evolution and stability of NH3, we employed three distinct numerical models: (1) a line-by-line radiative-convective model (Wordsworth et al., 2021) to calculate temperature profiles under different atmospheric compositions, (2) a dissolution equilibrium model (Arima et al., in prep) to calculate the partitioning of NH3 between the atmosphere and a hypothesized 550 m depth ocean (Di Achille and Hynek, 2010), and (3) a 1-D photochemical model (PROTEUS; Nakamura et al., 2023) to calculate the time-dependent density profile of each species by solving the vertical continuity-transport equations along with chemical reactions. The chemical network for the photochemical model was constructed based on Tian et al. (2011) and Adams et al. (2021). We simulated a post-impact environment characterized by a 2-bar CO2 atmosphere containing 6% H2, with NH3 and CH4 as variable parameters (Kamada et al., 2020; Koyama et al., 2024).
Our analysis indicates that the ocean serves as a significant reservoir for ammonia. Dissolution equilibrium calculations indicate that, due to the high solubility of NH3, a substantial fraction of the initial inventory partitions from the atmosphere into the ocean. Furthermore, the ocean maintains a pH range from neutral to slightly acidic, buffered by dissolved carbonate species.
To assess the long-term stability of the system, we calculated photochemical loss rates of NH3 using the photochemical model. Based on these calculated rates, we estimate that an initial inventory of 1020 mol of ammonia can be maintained within this atmosphere-ocean system for approximately 105 years. This duration is orders of magnitude longer than estimates considering atmospheric processes alone. These findings suggest that impact-induced ocean-atmosphere systems could provide a stable supply of NH3 over timescales on the order of 105 years, potentially satisfying the temporal requirements for the emergence of prebiotic precursor chemistry.
Adams et al. (2021) Astrobiology, 21, 968–980.
Di Achille & Hynek (2010) Nat. Geosci., 3, 459–463.
Kamada et al. (2020) Icarus, 338, 113567.
Kasting (1982) J. Geophys. Res., 87, 3091–3098.
Koyama et al. (2024) Sci. Rep., 14, 2397.
Nakamura et al. (2023) Earth Planets Space, 75, 140.
Stern et al. (2015) Proc. Natl. Acad. Sci., 112, 4245–4250.
Tian et al. (2011) Earth Planet. Sci. Lett., 308, 417–423.
Wordsworth (2016) Annu. Rev. Earth Planet. Sci., 44, 381–408.
Wordsworth et al. (2021) Nat. Geosci., 14, 127–132