Observable Signatures of Wind-driven Chemistry with a Fully Consistent Three-dimensional Radiative Hydrodynamics Model of HD 209458b

Abstract

We present a study of the effect of wind-driven advection on the chemical composition of hot Jupiter atmospheres using a fully-consistent 3D hydrodynamics, chemistry and radiative transfer code, the Met Office Unified Model (UM). Chemical modelling of exoplanet atmospheres has primarily been restricted to 1D models that cannot account for 3D dynamical processes. In this work we couple a chemical relaxation scheme to the UM to account for the chemical interconversion of methane and carbon monoxide. This is done consistently with the radiative transfer meaning that departures from chemical equilibrium are included in the heating rates (and emission) and hence complete the feedback between the dynamics, thermal structure and chemical composition. In this letter we simulate the well studied atmosphere of HD 209458b. We find that the combined effect of horizontal and vertical advection leads to an increase in the methane abundance by several orders of magnitude; directly opposite to the trend found in previous works. Our results demonstrate the need to include 3D effects when considering the chemistry of hot Jupiter atmospheres. We calculate transmission and emission spectra, as well as the emission phase curve, from our simulations. We conclude that gas-phase non-equilibrium chemistry is unlikely to explain the model-observation discrepancy in the 4.5 µm Spitzer/IRAC channel. However, we highlight other spectral regions, observable with the James Webb Space Telescope, where signatures of wind-driven chemistry are more prominant.

Implications of three-dimensional chemical transport in hot Jupiter atmospheres: Results from a consistently coupled chemistry-radiation-hydrodynamics model

“…However, it is also apparent that there is a slight increase in the offset of the phase curve, with the peak emission being shifted further away from secondary eclipse. This is consistent with our previous simulations of HD 189733b using much simpler chemical scheme and is due to the temperature response of the atmosphere (Drummond et al 2018c). The temperature response of the atmosphere is very similar to that already presented in Drummond et al (2018c) and in Figure 5 of that paper we show that the largest temperature increase occurs east of the substellar point, which shifts the hotspot further east.…”

“…This implies that in the particular case of HD 189733b the parameterised chemical timescale developed by Cooper & Showman (2006) gives a similar chemical evolution to the reduced chemical network of Venot et al (2019a) for CO. The results and conclusions presented in Drummond et al (2018c) are therefore validated here with the use of a more accurate chemical scheme. This important result demonstrates that, at least in some cases, simplified chemistry schemes can be used in 3D atmosphere models, greatly increasing computational efficiency while maintaining accurate results, compared with more sophisticated chemical schemes.…”

“…In Drummond et al (2018c) we conducted a simple tracer experiment that demonstrated that mass can be transported from the mid-latitudes into the equatorial region on a timescale of several hundred days. The tracer was initialised to an abundance of zero everywhere, except in a "source" region at high-pressure and high-latitudes.…”

“…For the equatorial region of HD 189733b, our simulations predict that HD 189733b remains in the COdominated regime, but that 3D vertical mixing can decrease the CO/CH 4 ratio by several orders of magnitude, due to an increase the equatorial abundance of CH 4 . This leads to temperature changes of 10% compared with the local chemical equilibrium simulation (Drummond et al 2018c).…”

“…Lines et al 2018a). Drummond et al (2018b) and Drummond et al (2018c) found that both horizontal and vertical quenching play important roles in determining the abundance of CH 4 in the observable region of the atmosphere for both HD 209458b and HD 189733b. Mendonça et al (2018) recently included an updated chemical relaxation scheme, developed in Tsai et al (2018), in a 3D atmosphere model for the case of WASP-43b.…”