10:45 〜 12:15
[MIS20-P05] Formation conditions of methane clathrate that caps Pluto's ocean
キーワード：クラスレートハイドレート、冥王星、氷火山活動、分子動力学シミュレーション、内部海
Pluto has a global subsurface ocean despite of its small size and lack of strong tidal heating (Nimmo et al., 2016). To avoid freezing the subsurface ocean over 4.5 billion years, CH4 clathrate layer with low thermal conductivity is suggested to be present at the base of the icy crust to thermally insulate the subsurface ocean (Kamata et al., 2019). However, the formation conditions, such as required temperature and CH4 contents in oceanic water, are not well constrained. Given the high abundance of NH3 in comets (Mumma and Charnley, 2011), and the occurrence of H2O-NH3 ice along extensional fracture (Cruikshank et al., 2019), Pluto's ocean may contain high concentrations of NH3 in addition to CH4. Although the presence of NH3 in liquid can decrease the dissociation temperature of clathrate (Vu et al., 2014; Petuya et al., 2020), the formation conditions of CH4 clathrate layer for the H2O-CH4-NH3 system have been addressed experimentally at pressures less than 20 MPa (Petuya et al., 2020), which is far from the pressure conditions achieved in Pluto (e.g., 100–200 MPa: Kamata et al., 2019).
Here we performed molecular dynamics simulations to understand the formation conditions (i.e., temperature and CH4 contents in liquid) of CH4 clathrate. We investigated the stability of CH4 clathrate coexisting with H2O-NH3-CH4 fluids with different NH3 and CH4 concentrations at pressures of 20, 40, and 200 MPa and temperatures of 200–320 K. Our results show that dissociation temperatures of CH4 clathrate monotonically increase with pressure and decrease with NH3 concentration in liquid. For a given NH3 concentration in liquid, the dissociation temperature at 200 MPa is systematically 18 ± 8 K higher than the dissociation temperature at 20 MPa. At the maximum NH3 concentration in our simulation (30% NH3 relative to H2O in liquid), the dissociation temperature at 200 MPa is 293 ± 8 K, decreased from 312 K without NH3 (Sloan and Koh, 2007). However, all the clathrate dissociation temperatures at 200 MPa are still above the melting point of H2O-NH3 mixtures, indicating that the clathrate layer is thermodynamically stable at the interface between the subsurface ocean and icy crust of Pluto. In the presentation, we will also report the required CH4 concentration in liquid to stabilize CH4 clathrate. Based on the required CH4 concentrations as functions of pressure and NH3 concentration, we will discuss formation timing of CH4 clathrate layer on early Pluto and possibilities of replenishment of surface CH4 through eruptions of the subsurface liquid on recent Pluto (Cruikshank et al., 2019; Martin and Binzel, 2021).
Here we performed molecular dynamics simulations to understand the formation conditions (i.e., temperature and CH4 contents in liquid) of CH4 clathrate. We investigated the stability of CH4 clathrate coexisting with H2O-NH3-CH4 fluids with different NH3 and CH4 concentrations at pressures of 20, 40, and 200 MPa and temperatures of 200–320 K. Our results show that dissociation temperatures of CH4 clathrate monotonically increase with pressure and decrease with NH3 concentration in liquid. For a given NH3 concentration in liquid, the dissociation temperature at 200 MPa is systematically 18 ± 8 K higher than the dissociation temperature at 20 MPa. At the maximum NH3 concentration in our simulation (30% NH3 relative to H2O in liquid), the dissociation temperature at 200 MPa is 293 ± 8 K, decreased from 312 K without NH3 (Sloan and Koh, 2007). However, all the clathrate dissociation temperatures at 200 MPa are still above the melting point of H2O-NH3 mixtures, indicating that the clathrate layer is thermodynamically stable at the interface between the subsurface ocean and icy crust of Pluto. In the presentation, we will also report the required CH4 concentration in liquid to stabilize CH4 clathrate. Based on the required CH4 concentrations as functions of pressure and NH3 concentration, we will discuss formation timing of CH4 clathrate layer on early Pluto and possibilities of replenishment of surface CH4 through eruptions of the subsurface liquid on recent Pluto (Cruikshank et al., 2019; Martin and Binzel, 2021).