Clouds asymmetry and temperature contrast between the morning and evening terminators of WASP-39b

Simon Delisle ( Université de Montréal )

Thermal phase-curve observations of close-in exoplanets present a powerful tool to probe longitudinal variations of atmospheric temperature and molecular abundances, thereby providing fundamental insights into the atmospheric dynamics and chemical kinematics under extreme atmospheric conditions. Measurements of the phase curves, however, generally require expensive continuous observations over a full orbital period of the planet, are limited to wavelength with favorable planet-to-star flux ratios, and only coarsely resolve the terminator region where the temperature and chemical gradients are expected to be the steepest. Here, we present a novel 3D spectroscopic terminator mapping technique that via modeling of the radiative transfer through a 3D planetary atmosphere of a rotating planet during transit enables the mapping of the temperature field, molecular abundances, and cloud top pressure across different regions of the terminator based on transit observations only. Applied to the JWST NIRSPEC/PRISM and NIRSPEC/G395H transits of the hot Saturn WASP-39b, we find a highly asymmetric terminator with the cloud top pressure at the evening terminator differing significantly from the morning terminator. We identify a equator temperature contrast with the equator hotter on the morning terminator and the planet's poles at intermediate temperatures. This first study opens the door to spectroscopic terminator mapping of temperature and molecular abundances of dozens of exoplanets observed with JWST during transit, providing spatial information for a large sample of planets and highly complementary to thermal phase curve observations for those planets for which phase curves are available.