Planetary Climate Dynamics

Abstract:

Atmospheric convection behaves differently in hydrogen-rich atmospheres compared to higher mean molecular weight atmospheres due to compositional gradients of tracers. Previous 1D studies predict that when a condensable tracer exceeds a critical mixing ratio in H2-rich atmospheres, convection is inhibited, leading to the formation of radiative layers where the temperature decreases faster with height than in convective profiles. We use 3D convection-resolving simulations to test whether convection is inhibited in H2-rich atmospheres when the tracer mixing ratio exceeds the critical threshold, while including processes neglected in 1D, e.g., turbulent mixing and evaporation. We run two sets of simulations. First, we perform simulations initialized on saturated isothermal states and find that compositional gradients can destabilize isothermal atmospheres. Second, we perform simulations initialized on adiabatic profiles, which show distinct, stable inhibition layers form when the condensable tracer exceeds the critical threshold. Within the inhibition layer, only a small amount of energy is carried by latent heat flux, and turbulent mixing transports a small amount of tracer upward, but both are generally too weak to sustain substantial tracer or heat transport. The thermal profile gradually relaxes to a steep radiative state, but radiative relaxation timescales are long. Our results suggest stable layers driven by condensation-induced convective inhibition form in H2-rich atmospheres, including those of sub-Neptune exoplanets.

Abstract:

This dataset provides horizontal flow fields (velocity, relative vorticity, divergence, and shear strain rate) from nine rotating turbulence experiments carried out at TurLab at the University of Turin between November 2016 and February 2017 by a team led by Oxford scientists in collaboration with others at the time from Università degli Studi di Torino, University of South Florida, Università del Piemonte Orientale and Sapienza Università di Roma. The experimental device was a rotating cylinder, 5 m in diameter filled with fresh water to a depth of 56 cm. The tank had a conical bottom, sloping upwards towards the centre from the outer edge over a radius of 2.25 m at an angle of 11.1◦. The flow was mechanically forced by a moving comb of vertical paddles partially immersed in the water, which moved backwards and forwards along a near-radial line over 100–150 s. The resulting flow was visualised using a laser sheet and microscopic particles, imaged using two cameras, and then processed into horizontal velocity fields and derived quantities using UVMAT/CIV (http://servforge.legi. grenoble-inp.fr/projects/soft-uvmat). The main quantity varied between the experiments included in this dataset is the rotation period of the tank. There were minor differences in the comb configuration at each rotation period. †

Abstract:

We present JWST NIRSpec/PRISM integral field unit time-resolved observations of 2M1207 A and b (TWA 27), an ∼10 Myr binary system consisting of an ∼2500 K substellar primary hosting an ∼1300 K companion. Our data provide 20 time-resolved spectra over an observation spanning 12.56 hr. We provide an empirical characterization for the spectra of both objects across time. For 2M1207 A, nonlinear trend models are statistically favored within the ranges 0.6-2.3 μm and 3.8-5.3 μm. However, most of the periods constrained from sinusoidal models exceed the observing window, setting a lower limit of 12.56 hr. We find the data at Hα and beyond 4.35 μm show a moderate time correlation, as well as a pair of light curves at 0.73-0.80 μm and 3.36-3.38 μm. For 2M1207 b, light curves integrated across 0.86-1.77 μm and 3.29-4.34 μm support linear trend models. Following the interpretation of Z. Zhang et al., we model the 2M1207 b data with two 1D atmospheric components, both with silicate and iron condensates. The model of time variability due to changes in the cloud filling factor shows broad consistency with the variability amplitudes derived from our data. Our amplitudes, however, disagree with the models at ≈0.86-1 μm. While an additional model component such as rainout chemistry may be considered here, our analysis is limited by low signal-to-noise ratio. Our results demonstrate the capability of JWST to simultaneously monitor the spectral variability of a planetary-mass companion and host at low contrast.

Abstract:

Large-amplitude variations are commonly observed in the atmospheres of directly imaged exoplanets and brown dwarfs. VHS 1256B, the most variable known planet-mass object, exhibits a near-infrared flux change of nearly 40%, with red color and silicate features revealed in recent JWST spectra, challenging current theories. Using a general circulation model, we demonstrate that VHS 1256B's atmosphere is dominated by planetary-scale dust storms persisting for tens of days, with large patchy clouds propagating with equatorial waves. This weather pattern, distinct from the banded structures seen on solar system giants, simultaneously explains the observed spectra and critical features in the rotational light curves, including the large amplitude, irregular evolution, and wavelength dependence, as well as the variability trends observed in near-infrared color-magnitude diagrams of dusty substellar atmospheres.

Abstract:

Highly irradiated giant exoplanets known ‘ultrahot Jupiters’ are anticipated to exhibit large variations of atmospheric temperature and chemistry as a function of longitude, latitude and altitude. Previous observations have hinted at these variations, but the existing data have been fundamentally restricted to probing hemisphere-integrated spectra, thereby providing only coarse information on atmospheric gradients. Here we present a spectroscopic eclipse map of an extrasolar planet, resolving the atmosphere in multiple dimensions simultaneously. We analyse a secondary eclipse of the ultrahot Jupiter WASP-18b observed with the Near Infrared Imager and Slitless Spectrograph instrument on the JWST. The mapping reveals weaker longitudinal temperature gradients than were predicted by theoretical models, indicating the importance of hydrogen dissociation and/or nightside clouds in shaping global thermal emission. In addition, we identify two thermally distinct regions of the planet’s atmosphere: a ‘hotspot’ surrounding the substellar point and a ‘ring’ near the dayside limbs. The hotspot region shows a strongly inverted thermal structure due to the presence of optical absorbers and a water abundance marginally lower than the hemispheric average, in accordance with theoretical predictions. The ring region shows colder temperatures and poorly constrained chemical abundances. Similar future analyses will reveal the three-dimensional thermal, chemical and dynamical properties of a broad range of exoplanet atmospheres.