14:15 〜 14:30
[PPS05-13] Climate evolution of early Mars due to long-term obliquity variations
キーワード：火星、古火星、バレーネットワーク、GCM、ミランコビッチサイクル
The evolution of the Martian climate is one of the great mysteries of planetary exploration. Decades of geomorphological and mineralogical data from spacecraft missions have shown that early Mars would have been very different from its present very thin (~6.1 mbar) and extremely cold atmosphere (~215 K) with a dry surface environment and obliquity value (~25.2°) that is similar to the Earth (~23.4°). Early Mars had an extensive ocean, lakes, and valley networks (VNs) on its surface before 3.8-3.6 billion years ago, suggesting the presence of a dense atmosphere and abundant amounts of liquid surface water. Recent GCM studies of Mars have shown that the early climate would have been warm with rain-fed surface runoff, cool with subglacial meltwater channels, or cold with no liquid water activity depending on the amount of atmospheric hydrogen (Kamada et al., 2021, 2022). While the general framework of early Martian climate and fluvial activity has been clarified, variation in early Martian orbital parameters has not been sufficiently considered. A celestial-mechanical model for modern Mars indicates that the planet would be subject to Milankovitch cycles, including quasi-periodic variations in obliquity, eccentricity, and precession. As a result, solar insolation varies on timescales of 105-106 years. Applied to early Mars, such variations would affect the long-term climate evolution. Our previous studies have assumed a constant value of obliquity over 105-106 years (Kamada et al., 2022). However, to estimate the realistic timescale of VN formation, we should consider the possible long-term obliquity variations.
Here, we performed fully coupled GCM simulations of the atmosphere, hydrosphere and cryosphere as a function of possible obliquity cycles. The complicated obliquity variation of early Mars was simply assumed based on a Fourier series with a mean obliquity value of 41.8°, and short-term and long-term oscillation periods of ~105 and ~106 years, respectively, based on Fanale et al. (1986). We assumed a CO2/H2O/H2 mixed atmosphere with surface pressures of between 1 bar and 2 bar, H2 mixing ratios of between 0% and 6%, and geothermal heat flux of 55 mW/m2. We assumed the existence of a northern ocean and lakes in our model with the amount corresponding to a 500 m Global Equivalent Layer (GEL) in the initial state and implemented a pre-True Polar Wander topography to investigate the global water cycle of early Mars before the late Tharsis formation. We iterated the runs of the coupled GCM over the course of 106 Martian years to obtain the long-term equilibrium state for each condition of surface pressure and H2 mixing ratio.
We found that climate on early Mars can be classified into 3 types depending on the atmospheric conditions. First, early Mars had a cold climate characterized by global mean temperature much below 273 K and widespread cold-based ice sheets when surface pressures were less than 1.5 bar or H2 mixing ratio was less than 1%. Second, early Mars had a cool climate characterized by global mean temperature slightly below 273 K and widespread temperate-based ice sheets, whose subglacial meltwater channels carved valleys over 105 years when surface pressures were close to 2 bar and H2 mixing ratio was ~2-5%. Finally, early Mars had a warm climate characterized by global mean temperature above 273 K and rain-fed river systems carving valleys over 104 years when surface pressures were 2 bar and H2 mixing ratio was above 6%. In warm or cool climates, with the variation in obliquity, VNs could have formed continuously by subaerial or subglacial mechanisms in areas where the majority of VNs are observed. When obliquities were greater than 40°, which is the most probable obliquity value in the past, VNs would be formed more efficiently, suggesting that early Martian environment would be preferable for VN formation.
Here, we performed fully coupled GCM simulations of the atmosphere, hydrosphere and cryosphere as a function of possible obliquity cycles. The complicated obliquity variation of early Mars was simply assumed based on a Fourier series with a mean obliquity value of 41.8°, and short-term and long-term oscillation periods of ~105 and ~106 years, respectively, based on Fanale et al. (1986). We assumed a CO2/H2O/H2 mixed atmosphere with surface pressures of between 1 bar and 2 bar, H2 mixing ratios of between 0% and 6%, and geothermal heat flux of 55 mW/m2. We assumed the existence of a northern ocean and lakes in our model with the amount corresponding to a 500 m Global Equivalent Layer (GEL) in the initial state and implemented a pre-True Polar Wander topography to investigate the global water cycle of early Mars before the late Tharsis formation. We iterated the runs of the coupled GCM over the course of 106 Martian years to obtain the long-term equilibrium state for each condition of surface pressure and H2 mixing ratio.
We found that climate on early Mars can be classified into 3 types depending on the atmospheric conditions. First, early Mars had a cold climate characterized by global mean temperature much below 273 K and widespread cold-based ice sheets when surface pressures were less than 1.5 bar or H2 mixing ratio was less than 1%. Second, early Mars had a cool climate characterized by global mean temperature slightly below 273 K and widespread temperate-based ice sheets, whose subglacial meltwater channels carved valleys over 105 years when surface pressures were close to 2 bar and H2 mixing ratio was ~2-5%. Finally, early Mars had a warm climate characterized by global mean temperature above 273 K and rain-fed river systems carving valleys over 104 years when surface pressures were 2 bar and H2 mixing ratio was above 6%. In warm or cool climates, with the variation in obliquity, VNs could have formed continuously by subaerial or subglacial mechanisms in areas where the majority of VNs are observed. When obliquities were greater than 40°, which is the most probable obliquity value in the past, VNs would be formed more efficiently, suggesting that early Martian environment would be preferable for VN formation.