Presenter: Jérémy Leconte
Abstract:
Transmission spectroscopy provides us with information on the atmospheric properties at the limb, which is often intuitively assumed to be a narrow annulus around the planet. This is why all existing retrieval algorithms used so far to constrain the atmospheric composition from data rely either on i) a single 1D forward model, thus assuming a uniform limb or ii) a linear combination of 1D models to account for heterogeneities between different regions of the limb (e.g. east vs. west). Even full three-dimensional atmospheric models (GCMs) commonly use only the atmospheric columns at the terminator to predict the observable transmission spectrum for a given simulation.
Here, we will demonstrate that the region probed in transmission actually extends significantly toward the day and night sides of the planet and that, as a result, the real transmission spectrum computed from a GCM simulation with our new fully 3D radiative transfer differs significantly from results obtained with the usual assumptions. This comes from the fact that the terminator of hot, synchronously rotating planets is a region exhibiting sharp thermal and compositional gradients. Finally using both real-planet examples and more idealized case, we will demonstrate how this effect can lead to strong biases is the temperature and abundances retrieved from actual data—biases that will need to be addressed and corrected for if we want to be able to make robust inferences from future JWST and ARIEL data.