16:30 〜 16:45
[MIS17-11] Nitrogen-hydrocarbon clathrate substitution and implications for the explosive formation hypothesis of small lakes on Titan
キーワード：タイタン、炭化水素湖、クラスレート置換、分子動力学シミュレーション
Titan, the biggest moon of Saturn, has unique surface environments in the Solar System, characterized by the presence of several oceans and many small lakes of liquid hydrocarbons. The small lakes are usually located near the poles and formed within deep depressions called sharp-edged depressions (SEDs) with raised rims and deep and flat floors (e.g., Hayes, 2016). Hypotheses to create SEDs include karst, thermokarst, N2 phase change explosions in the past, and cryovolcanism model (e.g., Mitri et al., 2019; Solomonidou et al., 2020; Wood and Radebaugh, 2020). However, these hypotheses may suffer from their young geomorphology and concentrations in the north pole region.
Here, we propose a new hypothesis of formation of SEDs; that is, substitution of N2 clathrate by hydrocarbon liquid and N2 degassing resulted in local pressurization and explosion in the subsurface. N2 clathrate could have formed on ancient cold Titan (Charnay et al., 2014) when liquid N2 reacted with H2O ice crust. After a climate transition to the current warm state, N2 in the clathrate could have been exchanged with liquid methane/ethane and released to the liquid phase.
We conducted molecular dynamics simulations with GROMACS version 5.1.5 (Abraham et al., 2015) and 2020.4 (Lindahl et al., 2020). Molecular models of TIP4P/Ice (Abascal et al., 2005), OPLS_AA (Kaminski et al., 1994), and TraPPE (Potoff and Siepmann, 2001) were used. The initial configuration was composed of N2 clathrate of 2 × 2 × 3 of the unit cell and hydrocarbon liquid of 500 C2H6 (pure ethane system) or 300 CH4 and 300 C2H6 molecules (mixture system) in a small box (3.4×3.4×~10 nm). The main simulations were conducted for a few microseconds in Berendsen thermostat and barostat (Abraham et al., 2015; Lindahl et al., 2020) at 3 MPa, corresponding to ~4,000 m in depth on Titan, and at 130–260 K for the pure ethane system and at 190–220 K for the mixture system.
We found that substitution of N2 clathrate occurs under all conditions of our simulations. The time variations of N2 substitution by ethane allow us to determine the activation energy of N2 diffusion in clathrate as 33 ± 12 kJ/mol. This activation energy is higher than but within the range of previous work (5–23 kJ/mol) for CH4-C2H6 clathrate substitution (Choukroun and Sotin, 2012). Using the activation energy of 33 kJ/mol, the timescale for complete N2 release from a 10-µm clathrate sphere is estimated as ~102, ~106, and ~108 years for temperatures of 130, 110, and 90 K, respectively. We also found that substitution of N2 clathrate proceeded more efficiently in interaction with methane and ethane mixtures than with pure ethane.
Our results suggest that if N2 clathrate existed in Titan’s subsurface, N2 release by interactions with liquid hydrocarbon in polar regions would have proceeded efficiently in relatively warm subsurface of Titan. This could have caused a gas accumulation and pressure increase in the subsurface, possibly leading to gas explosions that could form SEDs.
Here, we propose a new hypothesis of formation of SEDs; that is, substitution of N2 clathrate by hydrocarbon liquid and N2 degassing resulted in local pressurization and explosion in the subsurface. N2 clathrate could have formed on ancient cold Titan (Charnay et al., 2014) when liquid N2 reacted with H2O ice crust. After a climate transition to the current warm state, N2 in the clathrate could have been exchanged with liquid methane/ethane and released to the liquid phase.
We conducted molecular dynamics simulations with GROMACS version 5.1.5 (Abraham et al., 2015) and 2020.4 (Lindahl et al., 2020). Molecular models of TIP4P/Ice (Abascal et al., 2005), OPLS_AA (Kaminski et al., 1994), and TraPPE (Potoff and Siepmann, 2001) were used. The initial configuration was composed of N2 clathrate of 2 × 2 × 3 of the unit cell and hydrocarbon liquid of 500 C2H6 (pure ethane system) or 300 CH4 and 300 C2H6 molecules (mixture system) in a small box (3.4×3.4×~10 nm). The main simulations were conducted for a few microseconds in Berendsen thermostat and barostat (Abraham et al., 2015; Lindahl et al., 2020) at 3 MPa, corresponding to ~4,000 m in depth on Titan, and at 130–260 K for the pure ethane system and at 190–220 K for the mixture system.
We found that substitution of N2 clathrate occurs under all conditions of our simulations. The time variations of N2 substitution by ethane allow us to determine the activation energy of N2 diffusion in clathrate as 33 ± 12 kJ/mol. This activation energy is higher than but within the range of previous work (5–23 kJ/mol) for CH4-C2H6 clathrate substitution (Choukroun and Sotin, 2012). Using the activation energy of 33 kJ/mol, the timescale for complete N2 release from a 10-µm clathrate sphere is estimated as ~102, ~106, and ~108 years for temperatures of 130, 110, and 90 K, respectively. We also found that substitution of N2 clathrate proceeded more efficiently in interaction with methane and ethane mixtures than with pure ethane.
Our results suggest that if N2 clathrate existed in Titan’s subsurface, N2 release by interactions with liquid hydrocarbon in polar regions would have proceeded efficiently in relatively warm subsurface of Titan. This could have caused a gas accumulation and pressure increase in the subsurface, possibly leading to gas explosions that could form SEDs.