11:15 〜 11:30
[PPS09-09] Simulation of formaldehyde and sediment transport on early Mars using a global climate and hydrological model
キーワード：火星、全球気候モデル、全球河川モデル、光化学モデル、ホルムアルデヒド、RNA
Present-day Mars is cold and arid environment, yet a variety of geological evidence suggests that early Mars prior to 3.6 Ga was warm and wet, with an active atmospheric as well as surface and subsurface water cycle. Furthermore, the Curiosity rover recently detected bulk organic carbon in sedimentary rocks at Gale crater. To assess the possibility of early Martian habitability, it is crucial not only to establish the stable presence of liquid water but also to understand the synthesis and concentration processes of organic compounds. One of the most prominent organic species is formaldehyde - a simple organic molecule that participates in aqueous processes such as the formose reaction to produce more complex organic including ribose, a fundamental building block of RNA, and a key molecule in theories regarding the origin of life.
In this study, we updated our previous global climate models and developed a self-consistent model of water and material cycles on early Mars, incorporating the transport of organics and sediments via rivers and their subsequent accumulation in ocean and lakes. To diagnostically determine the atmospheric concentration of formaldehyde in the global climate model, Koyama and Kamada et al. (in prep) created a look-up table of formaldehyde production based on surface atmospheric pressure, temperature, water vapor content, and UV radiation flux using a photochemical model. We also implemented the processes of formaldehyde dissolution and adsorption on raindrops and snowflakes during large-scale condensation and cumulus convection, as well as infiltration on land and runoff to rivers. Simulations were performed with obliquities ranging from 20° to 60°, and an atmospheric H2 mixing ratio of 6% to generate a warm early Martian environment. In addition, ancient topography map from before the Tharsis loading was used, and ocean and lakes were assumed to initially exist at elevations below -2.3 km.
The simulation results showed that the global mean temperatures were 270.9, 272.5 and 272.9 K for obliquities of 20°, 40° and 60°, respectively, with corresponding the annual precipitation (rainfall plus snowfall) amounts of 161.5, 155.0 and 167.7 mm. Over the course of a Mars year, formaldehyde deposition on the surface amounted to 2.9×108, 2.9×108 and 3.1×108 kg, while snowfall contributed 1.2×106,1.1×106 and 1.4×106 kg for obliquities of 20°, 40° and 60°, respectively. Formaldehyde deposited on land seeps into groundwater, eventually flowing as rivers into the ocean and lakes. The amount of formaldehyde transported into the ocean and lakes via these rivers were 8.2×106, 1.3×107 and 2.5×107 kg per Mars year. Consequently, the molar concentrations of formaldehyde in the northern ocean increased by 1.5×10-10, 1.4×10-10 and 1.4×10-10 mol/kg per Mars year for obliquities of 20°, 40° and 60° respectively. Assuming a conversion rate of formaldehyde into ribose of 3.5×10-6 mol/mol (as estimated from the previous formose reaction experiments), the annual ribose production in the northern ocean is estimated to be 5.5×103, 5.1×103 and 5.1×103 kg per Mars year.
In addition to formaldehyde, rivers transport sand and gravel downstream and into the ocean and lakes through riverbed erosion. Assuming a representative grain size of 1 mm for sand, the sediment flow into ocean and lakes were 3.7×1011, 3.8×1011 and 5.5×1011 kg per Mars year. Assuming this formaldehyde and sediment were deposited in layers near the estuaries, the mass ratio of carbon to sediment is about 8.9, 13.7 and 18.2 µg C (carbon)/g. When direct atmospheric deposition of formaldehyde into the ocean and lakes is also considered, this ratio increases to about 113.4, 115.4 and 93.3 µgC/g. Notably, this organic mass ratio is consistent with the ~ 200 µgC/g of organic matter observed by the Curiosity rover. A significant fraction of the observed Martian organic matter is likely of formaldehyde origin, suggesting that active organic synthesis took place on early Mars.
In this study, we updated our previous global climate models and developed a self-consistent model of water and material cycles on early Mars, incorporating the transport of organics and sediments via rivers and their subsequent accumulation in ocean and lakes. To diagnostically determine the atmospheric concentration of formaldehyde in the global climate model, Koyama and Kamada et al. (in prep) created a look-up table of formaldehyde production based on surface atmospheric pressure, temperature, water vapor content, and UV radiation flux using a photochemical model. We also implemented the processes of formaldehyde dissolution and adsorption on raindrops and snowflakes during large-scale condensation and cumulus convection, as well as infiltration on land and runoff to rivers. Simulations were performed with obliquities ranging from 20° to 60°, and an atmospheric H2 mixing ratio of 6% to generate a warm early Martian environment. In addition, ancient topography map from before the Tharsis loading was used, and ocean and lakes were assumed to initially exist at elevations below -2.3 km.
The simulation results showed that the global mean temperatures were 270.9, 272.5 and 272.9 K for obliquities of 20°, 40° and 60°, respectively, with corresponding the annual precipitation (rainfall plus snowfall) amounts of 161.5, 155.0 and 167.7 mm. Over the course of a Mars year, formaldehyde deposition on the surface amounted to 2.9×108, 2.9×108 and 3.1×108 kg, while snowfall contributed 1.2×106,1.1×106 and 1.4×106 kg for obliquities of 20°, 40° and 60°, respectively. Formaldehyde deposited on land seeps into groundwater, eventually flowing as rivers into the ocean and lakes. The amount of formaldehyde transported into the ocean and lakes via these rivers were 8.2×106, 1.3×107 and 2.5×107 kg per Mars year. Consequently, the molar concentrations of formaldehyde in the northern ocean increased by 1.5×10-10, 1.4×10-10 and 1.4×10-10 mol/kg per Mars year for obliquities of 20°, 40° and 60° respectively. Assuming a conversion rate of formaldehyde into ribose of 3.5×10-6 mol/mol (as estimated from the previous formose reaction experiments), the annual ribose production in the northern ocean is estimated to be 5.5×103, 5.1×103 and 5.1×103 kg per Mars year.
In addition to formaldehyde, rivers transport sand and gravel downstream and into the ocean and lakes through riverbed erosion. Assuming a representative grain size of 1 mm for sand, the sediment flow into ocean and lakes were 3.7×1011, 3.8×1011 and 5.5×1011 kg per Mars year. Assuming this formaldehyde and sediment were deposited in layers near the estuaries, the mass ratio of carbon to sediment is about 8.9, 13.7 and 18.2 µg C (carbon)/g. When direct atmospheric deposition of formaldehyde into the ocean and lakes is also considered, this ratio increases to about 113.4, 115.4 and 93.3 µgC/g. Notably, this organic mass ratio is consistent with the ~ 200 µgC/g of organic matter observed by the Curiosity rover. A significant fraction of the observed Martian organic matter is likely of formaldehyde origin, suggesting that active organic synthesis took place on early Mars.