Thermal evolution of Pluto and implications for surface tectonics and a subsurface ocean

We explore the long-term evolution of Pluto&#8217;s subsurface ocean in the absence of an insulating clathrate hydrate layer. Numerical simulations of the thermal history of the interior are performed using a 1D model assuming Pluto was initially differentiated into an outer hydrosphere (H2O shell) and an inner rocky core. We consider two endmember initial conditions: the hydrosphere was either entirely molten or frozen. We also consider different radiogenic heating rates, core sizes, ice reference viscosities, and ammonia concentrations. Our results indicate that the present-day Pluto can possess a subsurface ocean if the ice shell is purely conductive or only weakly convective. Our results also indicate that the initial state affects only little on the evolution scenario. These results strengthen previous conclusions obtained based on thermal evolution studies with limited calculation conditions. The thickness of the present-day ocean can be up to ~130 km, depending on the radiogenic heating rate and ice reference viscosity. The reference viscosity of ice required to maintain an ocean until today for the case of a CI chondritic core is approximately an order of magnitude higher than that for the case of an ordinary chondritic core. We also find that a thick subsurface ocean can be maintained until relatively recently for a dense small core case, which allows the formation of high-pressure ice at the seafloor. An inclusion of ammonia in the ocean increases the possibility of the current presence of a subsurface ocean even in the case of 1 wt% NH3 at the initial.

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Methane hydrates achieve both thick oceans and thick lithospheres in Enceladus and Mimas

10.21203/rs.3.rs-63051/v2 ◽

2021 ◽

Author(s):

Ryusuke Nishitani ◽

Jun Kimura ◽

Atsushi Tani ◽

Sho Sasaki

Keyword(s):

Heating Rate ◽

Core Temperature ◽

Methane Hydrate ◽

Methane Concentration ◽

Thermal Evolution ◽

Internal Heat ◽

Active Surface ◽

Hydrate Layer ◽

Subsurface Ocean ◽

Tidal Heating

Abstract The difference between the inactive surface of Mimas and the active surface of Enceladus is puzzling. We investigate the conditions under which both have a thick subsurface ocean and the thermal lithosphere of Mimas is thicker than that of Enceladus by using a one-dimensional simulation of thermal evolution. We adopt the initial core temperature, initial methane concentration, and tidal heating rate as free parameters in the calculation. The initial methane concentration and tidal heating rate greatly affect the current ocean thickness, although the initial core temperature does not affect the thickness. Methane hydrate forms at the base of the icy shell if the initial methane concentration is not 0. The methane hydrate layer plays an insulative role in an icy shell. When the initial methane concentration is 1000 , ∼2 GW is needed to achieve more than 50 km of the subsurface ocean on Mimas and ∼7.5 GW is needed to achieve more than 25 km of the subsurface ocean on Enceladus. These values are smaller than those needed when the initial methane concentration is 0 . The existence of the methane hydrate layer promotes the survival of the subsurface ocean because it insulates internal heat. In addition, it is found that the surface heat flux is depressed if the methane hydrate layer exists, which is consistent with the unrelaxed craters in Mimas. Methane hydrate may explain the thick oceans in Mimas and Enceladus and the inactive shell of Mimas.

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Methane hydrate creates the thick oceans in Mimas and Enceladus

10.21203/rs.3.rs-63051/v1 ◽

2020 ◽

Author(s):

Ryusuke Nishitani ◽

Jun Kimura ◽

Atsushi Tani ◽

Sho Sasaki

Keyword(s):

Heating Rate ◽

Core Temperature ◽

Methane Hydrate ◽

Methane Concentration ◽

Thermal Evolution ◽

Internal Heat ◽

Active Surface ◽

Hydrate Layer ◽

Subsurface Ocean ◽

Tidal Heating

Abstract The difference between the inactive surface of Mimas and the active surface of Enceladus is puzzling. We investigate the conditions under which the both have a thick subsurface ocean and the thermal lithosphere of Mimas is thicker than that of Enceladus by using a one-dimensional simulation of thermal evolution. We adopt the initial core temperature, initial methane concentration, and tidal heating rate as free parameters in the calculation. The initial methane concentration and tidal heating rate greatly affect the current ocean thickness, although the initial core temperature does not affect the thickness. Methane hydrate forms in a subsurface ocean if the initial methane concentration is not 0. The methane hydrate layer plays an insulative role in an icy shell. When the initial methane concentration is 1000 \(\text{m}\text{o}\text{l}\hspace{0.17em}{\text{m}}^{-3}\), ∼3 GW is needed to achieve more than 50 km of the subsurface ocean on Mimas and ∼10 GW is needed to achieve more than 25 km of the subsurface ocean on Enceladus. These values are smaller than those needed for when the initial methane concentration is 0 \(\text{m}\text{o}\text{l}\hspace{0.17em}{\text{m}}^{-3}\). The existence of the methane hydrate layer promotes the survival of the subsurface ocean because it insulates internal heat. In addition, it is found that the surface heat flux is depressed if the methane hydrate layer exists, which is consistent with the unrelaxed craters in Mimas. Methane hydrate may explain the thick oceans in Mimas and Enceladus and the inactive shell of Mimas.

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