Layered semi-convection and tides in giant planet interiors

Abstract

Context. Recent Juno observations have suggested that the heavy elements in Jupiter could be diluted throughout a large fraction of its gaseous envelope, providing a stabilising compositional gradient over an extended region of the planet. This could trigger layered semi-convection, which, in the context of giant planets more generally, may explain Saturn’s luminosity excess and play a role in causing the abnormally large radii of some hot Jupiters. In giant planet interiors, it could take the form of density staircases, which are convective layers separated by thin stably stratified interfaces. In addition, the efficiency of tidal dissipation is known to depend strongly on the planetary internal structure. Aims. We aim to study the resulting tidal dissipation when internal waves are excited in a region of layered semi-convection by tidal gravitational forcing due to other bodies (such as moons in giant planet systems, or stars in hot Jupiter systems). Methods. We adopt a local Cartesian model with a background layered density profile subjected to an imposed tidal forcing, and we compute the viscous and thermal dissipation rates numerically. We consider two sets of boundary conditions in the vertical direction: periodic boundaries and impenetrable, stress-free boundaries, with periodic conditions in the horizontal directions in each case. These models are appropriate for studying the forcing of short-wavelength tidal waves in part of a region of layered semi-convection, and in an extended envelope containing layered semi-convection, respectively. Results. We find that the rates of tidal dissipation can be enhanced in a region of layered semi-convection compared to a uniformly convective medium, where the latter corresponds with the usual assumption adopted in giant planet interior models. In particular, a region of layered semi-convection possesses a richer set of resonances, allowing enhanced dissipation for a wider range of tidal frequencies. The details of these results significantly depend on the structural properties of the layered semi-convective regions. Conclusions. Layered semi-convection could contribute towards explaining the high tidal dissipation rates observed in Jupiter and Saturn, which have not yet been fully explained by theory. Further work is required to explore the efficiency of this mechanism in global models.

Convective turbulent viscosity acting on equilibrium tidal flows: new frequency scaling of the effective viscosity

“…Wu 2005;Ogilvie & Lin 2007;Goodman & Lackner 2009;Papaloizou & Ivanov 2010;Favier et al 2014;Barker 2016), and this mechanism may be important for tidal circularisation and spin synchronisation. In giant planets, the role of stably-stratified (or semi-convective) layers is also being explored (Fuller et al 2016;André et al 2017;André et al 2019;Pontin et al 2020) with possible application to the orbital migration of the moons of Jupiter and Saturn (e.g. Lainey et al 2009Lainey et al , 2012Lainey et al , 2017Lainey et al , 2020.…”

Multi-cavity gravito-acoustic modes in stars: A general analytical resonance condition

“…Finally, we also note that the present general formulation can be applied to study the transmission of progressive waves through layered media in stellar interiors or planetary atmospheres since it offers the possibility to easily retain and remove the boundary conditions according to the configuration. This can be useful in addressing the problem of the transport of angular momentum, heat, or chemical elements by waves in these objects and the present results provide a general formalism for tackling such issues (e.g., André et al 2019;Cai et al 2021) As shown by Takata (2016b), the reflection and transmission coefficients in a base wave transmission-reflection problem are related to those of the adjoint wave transmission-reflection problem (see Sect. 2.3 for a description of the two configurations).…”

The impact of tidal friction evolution on the orbital decay of ultra-short period planets

“…Nevertheless, a more accurate estimation of the internal energy dissipation in stars must include coupled effects with the stellar wind (magnetic braking), chemical differentiation of the fluid layers, differential rotation, and non-linear processes such as the turbulence in the fluid envelope. Indeed, the presence of double-diffusive convective layers and/or differential rotation could affect strongly the excitation, the propagation, and the dissipation of tidal waves (Baruteau & Rieutord 2013;Favier et al 2014;Guenel et al 2016;André, Barker & Mathis 2017;Lin & Ogilvie 2018;André, Mathis & Barker 2019).…”