STRUCTURE AND DYNAMICS OF COLD WATER SUPER-EARTHS: THE CASE OF OCCLUDED CH₄AND ITS OUTGASSING

Abstract

In this work, we study the transport of methane in the external water envelopes surrounding water-rich super-Earths. We investigate the influence of methane on the thermodynamics and mechanics of the water mantle. We find that including methane in the water matrix introduces a new phase (filled ice), resulting in hotter planetary interiors. This effect renders the super-ionic and reticulating phases accessible to the lower ice mantle of relatively low-mass planets (∼5 M E ) lacking a H/He atmosphere. We model the thermal and structural profile of the planetary crust and discuss five possible crustal regimes which depend on the surface temperature and heat flux. We demonstrate that the planetary crust can be conductive throughout or partly confined to the dissociation curve of methane clathrate hydrate. The formation of methane clathrate in the subsurface is shown to inhibit the formation of a subterranean ocean. This effect results in increased stresses on the lithosphere, making modes of ice plate tectonics possible. The dynamic character of the tectonic plates is analyzed and the ability of this tectonic mode to cool the planet is estimated. The icy tectonic plates are found to be faster than those on a silicate super-Earth. A mid-layer of low viscosity is found to exist between the lithosphere and the lower mantle. Its existence results in a large difference between ice mantle overturn timescales and resurfacing timescales. Resurfacing timescales are found to be 1 Ma for fast plates and 100 Ma for sluggish plates, depending on the viscosity profile and ice mass fraction. Melting beneath spreading centers is required in order to account for the planetary radiogenic heating. The melt fraction is quantified for the various tectonic solutions explored, ranging from a few percent for the fast and thin plates to total melting of the upwelled material for the thick and sluggish plates. Ice mantle dynamics is found to be important for assessing the composition of the atmosphere. We propose a mechanism for methane release into the atmosphere, where freshly exposed reservoirs of methane clathrate hydrate at the ridge dissociate under surface conditions. We formulate the relation between the outgassing flux and the tectonic mode dynamical characteristics. We give numerical estimates for the global outgassing rate of methane into the atmosphere. We find, for example, that for a 2 M E planet outgassing can release 10 27 -10 29 molecules s −1 of methane to the atmosphere. We suggest a qualitative explanation for how the same outgassing mechanism may result in either a stable or a runaway volatile release, depending on the specifics of a given planet. Finally, we integrate the global outgassing rate for a few cases and quantify how the surface atmospheric pressure of methane evolves over time. We find that methane is likely an important constituent of water planets' atmospheres.

The Abundance of Atmospheric CO2 in Ocean Exoplanets: a Novel CO2 Deposition Mechanism

“…However, considering solid CO 2 is more dense than ice VI (see fig.16) gravity will probably limit the extent of such layers considerably. We argue though that such high deep ocean temperatures are less likely since a thick water ice mantle underlying the ocean translates to low heat fluxes at the ocean's bottom (see section 5 in Levi et al 2014). …”

“…The incorporation of CH 4 in filled-ice aids in its transport across the ice mantle, if solid state convection is established (Levi et al 2014). Therefore, it is likely that CH 4 locked in the deep ice mantle would reach the bottom of the ocean, in ocean exoplanets, and then the atmosphere.…”

“…This value for the solubility is higher than the value for the solubility when it is controlled by the presence of CO 2 clathrate hydrates (see solid curves in fig.9). The low heat fluxes in our water planets of interest (see 2 Earth mass planets in Levi et al 2014) suggest the deep ocean reservoir of CO 2 is indeed in the form of clathrates, which would control the value of n deep co 2 . Thus, although the ascending water parcels are relatively enriched with CO 2 , their volatile content is stable against degassing.…”

“…We use the 290 K isotherm as an example (see fig.12). In addition, we assume a gravitational acceleration of 10 3 cm s −2 and a planetary radius of 8000 km for our water planet (Levi et al 2014). For 290 K the CO 2 SI clathrate hydrate thermodynamic stability field lies between the pressures of 0.55 GPa and 104 MPa corresponding to a layer width of 38 km.…”

“…where u g = 0.1 m s −1 is the geostrophic velocity (see chapter 10 in Stewart (2008) and chapter 5 in Olbers et al (2012)), R p = 8000 km is the planetary radius (Levi et al 2014) and ∆λ = 30…”