An exploration of double diffusive convection in Jupiter as a result of hydrogen–helium phase separation

Abstract

Jupiter's atmosphere has been observed to be depleted in helium (Yatm ∼ 0.24), suggesting active helium sedimentation in the interior. This is accounted for in standard Jupiter structure and evolution models through the assumption of an outer, He-depleted envelope that is separated from the He-enriched deep interior by a sharp boundary. Here we aim to develop a model for Jupiter's inhomogeneous thermal evolution that relies on a more self-consistent description of the internal profiles of He abundance, temperature, and heat flux. We make use of recent numerical simulations on H/He demixing, and on layered double diffusive (LDD) and oscillatory double diffusive (ODD) convection, and assume an idealized planet model composed of an H/He envelope and a massive core. A general framework for the construction of interior models with He rain is described. Despite, or perhaps because of, our simplifications made we find that self-consistent models are rare. For instance, no model for ODD convection is found. We modify the H/He phase diagram of Lorenzen et al. to reproduce Jupiter's atmospheric helium abundance and examine evolution models as a function of the LDD layer height, from those that prolong Jupiter's cooling time to those that actually shorten it. Resulting models that meet the luminosity constraint have layer heights of ≈0.1–1 km, corresponding to ≈10 000–20 000 layers in the rain zone between ∼1 and 3–4.5 Mbar. Present limitations and directions for future work are discussed, such as the formation and sinking of He droplets.