The Impact of Non-Uniform Thermal Structure on the Interpretation of Exoplanet Emission Spectra

TL;DR

This paper investigates how assuming a single 1D thermal profile in atmospheric models can bias the interpretation of exoplanet emission spectra, especially when the atmosphere has multiple thermal components, emphasizing the need for more complex modeling.

Contribution

It demonstrates the biases introduced by 1D assumptions in spectral retrievals of hot Jupiter atmospheres and highlights the importance of considering multi-thermal profiles for accurate atmospheric characterization.

Findings

1D models can significantly bias molecular abundance estimates.

Two thermal profiles are necessary for accurate interpretation when contrast exceeds 40%.

H2O abundance remains robust, but CH4 estimates are biased under 1D assumptions.

Abstract

The determination of atmospheric structure and molecular abundances of planetary atmospheres via spectroscopy involves direct comparisons between models and data. While varying in sophistication, most model-spectra comparisons fundamentally assume "1D" model physics. However, knowledge from general circulation models and of solar system planets suggests that planetary atmospheres are inherently "3D" in their structure and composition. We explore the potential biases resulting from standard "1D" assumptions within a Bayesian atmospheric retrieval framework. Specifically, we show how the assumption of a single 1-dimensional thermal profile can bias our interpretation of the thermal emission spectrum of a hot Jupiter atmosphere that is composed of two thermal profiles. We retrieve upon spectra of unresolved model planets as observed with a combination of $HST$ WFC3+$Spitzer$ IRAC as well as $JWST$ under varying differences in the two thermal profiles. For WFC3+IRAC, there is a significantly biased estimate of CH$_4$ abundance using a 1D model when the contrast is 80%. For $JWST$, two thermal profiles are required to adequately interpret the data and estimate the abundances when contrast is greater than 40%. We also apply this preliminary concept to the recent WFC3+IRAC phase curve data of the hot Jupiter WASP-43b. We see similar behavior as present in our simulated data: while the H$_2$O abundance determination is robust, CH$_4$ is artificially well-constrained to incorrect values under the 1D assumption. Our work demonstrates the need to evaluate model assumptions in order to extract meaningful constraints from atmospheric spectra and motivates exploration of optimal observational setups.