Modelling the Global Cloud Cover of HD 189733b
Abstract
Context.Each rain droplet on Earth contains a microscopic condensation seed of sand or ash at its center, leading to more efficient water droplet formation than supersaturated vapour alone. Similarly, in hot Jupiter atmospheres, the formation of seed particles from the gas phase allows the condensation of solid mineral compounds such as Iron, Quartz and Enstatites. Recent observations of the atmosphere on HD 189733b suggest that clouds form in its atmosphere. Clouds have a large effect on the local thermodynamic and chemical properties of the atmosphere. The different thermodynamic conditions on the dayside and nightside will result in locally different cloud structures.Aims.Inspired by mineral cloud modelling efforts for Brown Dwarf atmospheres, we present spatially varying kinetic cloud model structures for HD 189733b. We investigate the resulting cloud properties at the dayside, nightside and terminator regions of the planet.Methods.We apply a 2-model approach, using results from a 3D radiative-hydrodynamic (RHD) simulation of the atmosphere of HD 189733b and our kinetic cloud formation model. 1D Tgas-pgas profiles at the equator and +45 latitude covering the dayside, nightside and terminator regions of the simulation were chosen as input for the cloud formation model. Effective medium theory and Mie theory are applied to calculate the wavelength dependent opacity of the cloud materials.Results.We present results of the local cloud properties such as number density, size, composition and opacity at the chosen locations. The mean grain size varies between nm at the cloud deck to mm sizes at the cloud base. Cloud particle sizes in the upper atmosphere were found to be one order of magnitude larger on the dayside compared to the nightside. More efficient nucleation of cloud particles on the nightside leads to larger number densities compared to the dayside. The solid composition of cloud particles throughout the atmosphere is dominated by a combination of Silicate-Oxides (~70%) with a significant fraction of Iron (~15%) content. Iron grains dominate at higher temperature regions such as the cloud base. We show that the gas phase can be depleted of elements by several orders of magnitude due condensation of material onto grains. Our results suggest that the thermodynamic conditions on HD 189733b are suitable for efficient cloud formation and that the majority of the atmosphere is covered in thick mineral clouds.
- Publication:
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AAS/Division for Extreme Solar Systems Abstracts
- Pub Date:
- December 2015
- Bibcode:
- 2015ESS.....311110H