Effects of Bulk Composition on the Atmospheric Dynamics on Close-in Exoplanets

Abstract

Super Earths and mini Neptunes likely have a wide range of atmospheric compositions, ranging from low molecular mass atmospheres of H 2 to higher molecular atmospheres of water, CO 2 , N 2 , or other species. Here we systematically investigate the effects of atmospheric bulk compositions on temperature and wind distributions for tidally locked sub-Jupiter-sized planets, using an idealized 3D general circulation model (GCM). The bulk composition effects are characterized in the framework of two independent variables: molecular weight and molar heat capacity. The effect of molecular weight dominates. As the molecular weight increases, the atmosphere tends to have a larger day-night temperature contrast, a smaller eastward phase shift in the thermal phase curve, and a smaller zonal wind speed. The width of the equatorial super-rotating jet also becomes narrower, and the "jet core" region, where the zonal-mean jet speed maximizes, moves to a greater pressure level. The zonal-mean zonal wind is more prone to exhibit a latitudinally alternating pattern in a higher molecular weight atmosphere. We also present analytical theories that quantitatively explain the above trends and shed light on the underlying dynamical mechanisms. Those trends might be used to indirectly determine the atmospheric compositions on tidally locked sub-Jupiter-sized planets. The effects of the molar heat capacity are generally small. But if the vertical temperature profile is close to adiabatic, molar heat capacity will play a significant role in controlling the transition from a divergent flow in the upper atmosphere to a jet-dominated flow in the lower atmosphere.

Demarcating Circulation Regimes of Synchronously Rotating Terrestrial Planets within the Habitable Zone

“…For both of these regimes, the day-night heat transport in the free atmosphere of synchronously rotating planets is strongly influenced by radiation and zonally propagating waves Perez-Becker & Showman 2013;Wordsworth 2015;Koll & Abbot 2015Komacek & Showman 2016;Komacek et al 2017;Zhang & Showman 2017). The free-atmosphere day-night temperature contrast can be predicted based on a scaling theory developed by Komacek & Showman (2016); Komacek et al (2017) and Zhang & Showman (2017),…”

“…The free-atmosphere day-night temperature contrast can be predicted based on a scaling theory developed by Komacek & Showman (2016); Komacek et al (2017) and Zhang & Showman (2017),…”

Results from a set of three-dimensional numerical experiments of a hot Jupiter atmosphere

“…Although TF simulations are less physically accurate than their RT counterparts (see discussion in Showman et al 2009;Amundsen et al 2016), they have still proved useful in the study of hot Jupiter (and other) atmospheres. Recent examples of the use of TF simulations include exploration of the efficiency of dynamical redistribution of heat (Komacek & Showman 2016) and the dependence on the atmospheric flow on bulk composition (Zhang & Showman 2017). For the RT simulation we do not include TiO and VO formation as evidence for its presence has only been suggested for Wasp-121b (Evans et al 2016) so far, and not HD 209458B.…”

Atmospheric Circulations of Hot Jupiters as Planetary Heat Engines

“…This orbital spin state creates a strong day-night forcing which excites standing waves that flux angular momentum toward the equator and drive equatorial superrotation (Showman & Polvani 2011). The strength of superrotation should therefore depend on the ratio between horizontal wave propagation and radiative cooling timescales (Koll & Abbot 2015;Komacek & Showman 2016;Zhang & Showman 2017). This basic expectation is complicated, however, by results that show that the jet's state depends on both horizontal standing waves and vertical eddies (Tsai et al 2014;Showman et al 2015), and it is still unclear how the two mechanisms jointly determine the jet's magnitude.…”