Steamworlds: atmospheric structure and critical mass of planets accreting icy pebbles

Steamworlds: atmospheric structure and critical mass of planets accreting icy pebbles

Author:

Chambers

Abstract:

In the core accretion model, gas-giant planets first form a solid core, which then accretes gas from a protoplanetary disk when the core exceeds a critical mass. Here, we model the atmosphere of a core that grows by accreting ice-rich pebbles. The ice fraction of pebbles evaporates in warm regions of the atmosphere, saturating it with water vapor. Excess water precipitates to lower altitudes. Beneath an outer radiative region, the atmosphere is convective, following a moist adiabat in saturated regions due to water condensation and precipitation. Atmospheric mass, density and temperature increase with core mass. For nominal model parameters, planets with core masses (ice + rock) between 0.08 and 0.16 Earth masses have surface temperatures between 273 K and 647 K and form an ocean. In more massive planets, water exists as a super-critical convecting fluid mixed with gas from the disk. Typically, the core mass reaches a maximum (the critical mass) as a function of the total mass when the core is 2-5 Earth masses. The critical mass depends in a complicated way on pebble size, mass flux, and dust opacity due to the occasional appearance of multiple core-mass maxima. The core mass for an atmosphere of 50 percent hydrogen and helium may be a more robust indicator of the onset of gas accretion. This mass is typically 1-3 Earth masses for pebbles that are 50 percent ice by mass, increasing with opacity and pebble flux, and decreasing with pebble ice/rock ratio.

Solubility of Rock in Steam Atmospheres of Planets

Solubility of Rock in Steam Atmospheres of Planets

Authors:

Fegley et al

Abstract:

Extensive experimental studies show all major rocky elements (Si, Mg, Fe, Ni, Ca, Al, Na, K) dissolve in steam to a greater or lesser extent. We use these results to compute chemical equilibrium abundances of rocky element-bearing gases in steam atmospheres equilibrated with silicate magma oceans. Rocky elements partition into steam atmospheres as volatile hydroxide gases and via reaction with HF or HCl as volatile halide gases in much larger amounts than expected from their vapor pressures over volatile-free solid or molten rock at the same temperature. We compute the extent of fractional vaporization by defining gas to magma partition coefficients and show Earth's sub-solar Si to Mg bulk elemental ratio may be due to loss of a primordial steam atmosphere. We conclude hot rocky exoplanets that are undergoing or have undergone escape of steam atmospheres may experience fractional vaporization and loss of Si, Mg, Fe, Ni, Ca, Al, Na, and K. This loss may modify their bulk composition, density, heat balance, and internal structure.