14:30 〜 14:45
[U11-04] Atmospheric production of formaldehyde and its carbon isotope evolution on early Mars
★Invited Papers
キーワード：初期火星、ハビタビリティー、炭素同位体、有機物、ホルムアルデヒド
Present-day Mars is extremely cold and dry, but geomorphological and geochemical evidence suggests a past environment warm enough to sustain liquid water on its surface (Wordsworth, 2016). These observations have increased interest in its habitability. Beyond the presence of water, organic synthesis is another important requirement in evaluating the chemical evolution to potential ancient Martian life. Formaldehyde (H2CO), a highly soluble and reactive molecule, plays a significant role in transferring carbon from the atmosphere to the surface and forming various organic compounds, including bio-important molecules. Therefore, determining whether the surface environment on early Mars was suitable for producing H2CO is crucial for understanding its habitability.
Recent Mars explorations have discovered several types of organic matter on the surface (e.g., Eigenbrode et al., 2018). The Curiosity rover has reported highly variable and 13C-depleted carbon isotope ratios of the organic matter in early Martian sediments (House et al., 2022). However, its origin is unknown. One of the potential sources is the deposition of H2CO, formed by photochemical reduction of CO2 in the atmosphere (Ueno et al., 2022).
Here, we investigate the production and the carbon isotope composition of H2CO in the early Martian atmosphere. First, we calculated the atmospheric production of H2CO on early Mars using a 1-D photochemical model assuming a thick CO2-dominated atmosphere with H2 and CO (Koyama et al., 2024). Our results show that H2CO was continuously formed on Noachian Mars, and this continuous supply of atmospheric H2CO can be used to form various organic compounds, including amino acids and sugars. Given the previously reported conversion rate from H2CO into ribose, the calculated H2CO deposition flux suggests a continuous supply of bio-important sugars on early Mars, particularly during the Noachian and early Hesperian periods.
Next, to explore the carbon isotope composition of H2CO, we developed a coupled photochemistry-climate evolution model that includes carbon isotope fractionation induced by CO2 photolysis, C escape, and volcanic outgassing. This model calculates the evolution of the carbon isotope composition in H2CO in the early Martian atmosphere, beginning with an atmosphere composed of mantle-derived CO, CO2, and H2. The calculated evolution of the carbon isotopic ratio in H2CO covers the extensive range observed in Martian organic matter, with its minimum δ¹³C value of ~-200‰. If Mars was frozen but episodically warm in the late Noachian, surface ice containing the atmospheric H2CO (δ¹³C < -140‰) might have dissolved into Martian water bodies, leading to diverse carbon isotopic ratios in the resulting organic matter.
Our findings suggest that organic matter in early Martian sediments could have originated from photochemically produced H2CO, and its deposition could have led to the continuous formation of bio-important molecules in a warm climate on early Mars.
Recent Mars explorations have discovered several types of organic matter on the surface (e.g., Eigenbrode et al., 2018). The Curiosity rover has reported highly variable and 13C-depleted carbon isotope ratios of the organic matter in early Martian sediments (House et al., 2022). However, its origin is unknown. One of the potential sources is the deposition of H2CO, formed by photochemical reduction of CO2 in the atmosphere (Ueno et al., 2022).
Here, we investigate the production and the carbon isotope composition of H2CO in the early Martian atmosphere. First, we calculated the atmospheric production of H2CO on early Mars using a 1-D photochemical model assuming a thick CO2-dominated atmosphere with H2 and CO (Koyama et al., 2024). Our results show that H2CO was continuously formed on Noachian Mars, and this continuous supply of atmospheric H2CO can be used to form various organic compounds, including amino acids and sugars. Given the previously reported conversion rate from H2CO into ribose, the calculated H2CO deposition flux suggests a continuous supply of bio-important sugars on early Mars, particularly during the Noachian and early Hesperian periods.
Next, to explore the carbon isotope composition of H2CO, we developed a coupled photochemistry-climate evolution model that includes carbon isotope fractionation induced by CO2 photolysis, C escape, and volcanic outgassing. This model calculates the evolution of the carbon isotope composition in H2CO in the early Martian atmosphere, beginning with an atmosphere composed of mantle-derived CO, CO2, and H2. The calculated evolution of the carbon isotopic ratio in H2CO covers the extensive range observed in Martian organic matter, with its minimum δ¹³C value of ~-200‰. If Mars was frozen but episodically warm in the late Noachian, surface ice containing the atmospheric H2CO (δ¹³C < -140‰) might have dissolved into Martian water bodies, leading to diverse carbon isotopic ratios in the resulting organic matter.
Our findings suggest that organic matter in early Martian sediments could have originated from photochemically produced H2CO, and its deposition could have led to the continuous formation of bio-important molecules in a warm climate on early Mars.