Inhomogeneous Cloud Coverage and Altitude-Dependent Heat Transport on the Hot-Jupiter NGTS-10 Ab from its Optical-to-Infrared Phase Curve
Hot-Jupiters, gas giant planets with equilibrium temperatures above 1,000 K, host large temperature gradients between their permanent day- and nightsides, resulting in circulation regimes that have no Solar System analogs. Past Kepler and Spitzer measurements show that the phase curves of these objects typically peak westward of the substellar point at optical wavelengths and eastward in the infrared, indicative of an interaction between circulation and cloud coverage. However, few hot-Jupiters have joint reflected light and thermal emission measurements, preventing a definitive statement as to the link between these phenomena. Here, we present the first phase curve of a hot-Jupiter that separates thermal emission and reflected light through JWST observations of NGTS-10 Ab with the NIRSpec PRISM instrument ($λ$=0.5-5.5 $μ$m). Using the spectrally-resolved phase curve, we jointly retrieve the planet's thermal and reflected light maps. We show that the temperature and reflectance distributions are anti-correlated, best explained by clouds evaporating from the eastern substellar region ($φ\approx$-17 to 48$^\circ$) where temperatures are highest. This is confirmed by comparisons of the phase-resolved spectra with three-dimensional circulation models, which show that NGTS-10 Ab's atmosphere hosts inhomogeneous cloud coverage, likely made-up of $μ$m-sized silicate particles, and weak atmospheric drag ($τ_\mathrm{drag}\gt 10^6$ s). Finally, by measuring the spectral variation of the thermal phase curve offsets, we infer a slope of 7.1$\pm$1.9 degrees per pressure dex, indicative of heat transport that becomes more efficient at depth. Future optical and infrared hot-Jupiter phase curve measurements over a wide range of equilibrium temperatures will enable a complete mapping of the interplay between heat transport and cloud formation in highly-irradiated exoplanet atmospheres.