Vertical mixing in hot Jupiter atmospheres

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An outline of the paper: 3D mixing in hot Jupiter atmospheres I:
application to the day/night cold trap in HD 209458b
, by Parmentier, Showman and Lian

This paper examines the vertical mixing in a 3-D model of the atmospheric circulation of hot Jupiters – using the output of one of the latest models by the group of Adam Showman.

The results are important for two pressing issues concerning hot Jupiter atmospheres:

(1) strong vertical mixing is required to lift titanium oxide vapors up in the atmosphere of hot Jupiters.

Titanium oxide is expected to be the dominant source of opacity in the visible and responsible for the stratospheric temperature inversion observed in hot Jupiters. However, it condensates into grain and will rain out of the atmosphere if it is not brought back at a sufficient rate by vigorous vertical mixing.

(2) vertical transport is also crucial to the formation of silicate and iron clouds.

Titanium oxide, silicon and iron grains are known to form clouds in brown dwarfs with temperatures comparable to hot Jupiters. In brown dwarfs however, the vertical mixing is done by thermal convection, because the dominant energy transfer is the leakage of internal heat towards space. In hot Jupiters however, the dominant transport is the day-side to night-side redistribution of the energy from the irradiation of the host star. Thermal convection is suppressed, and horizonal motions are 100 to 1000 faster than vertical motions.

The paper by Vivien Parmentier and collaborators examines the amplitude and distribution of the vertical motions in a 3-D circulation model -tuned to the parameters of HD209458b- and compare the mixing timescale to the settling timescale of condensate grains. The result in a nutshell is that vertical mixing from the global circulation is rather vigorous, sufficient to keep small grains afloat (grains smaller than a micron). The mixing is sufficient to keep titanium oxide in the atmosphere, and to lift a haze of silicate grains that formed on the cold night side  – what would be required to explain the haze observed in HD 189733b if it is indeed made of silicate grains.

Interestingly, the strongest vertical mixing occurs at two specific points in the planet: near the Equator on the morning side, a strong updraft forms and lifts air over many atmospheric scale heights, with a corresponding downwards draft on the evening side.

Figure: Map of temperature (left) and vertical velocity (right) in the simulation (the arrows show the dominant wind directions). Notice the fast downdraft and updraft near the Equator at 40oW and 160oE, with velocities up to 300 m/s (1000 km/h). Grains that form on the cold night side can be lifted in the high atmosphere by the “morning” updraft. If the grains are small enough, they will not have time to rain down before reaching the night side again.

There is a strong caveat to interpreting the results of this study in terms of what is actually happening in hot Jupiter atmospheres: in the simulations, the condensates are injected only as tracers of the flow, like invisible dust. Actual clouds are not transparent, so their formation triggers a strong feedback effect because they change the transfer of radiation, and therefore modify the temperature profile and the circulation. The next step for this kind of simulations is thus to include the effect of clouds on the radiative transfer.

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About Author

I am a professor of planetary science at the University of Exeter. My specialty is the study of exoplanets, in particular the observation and modelling of exoplanet atmospheres. I have done my PhD a the University of Geneva and worked in Chile, France and Switzerland.