The Evolution and Internal Structure of Jupiter and Saturn With Compositional Gradients

Abstract

The internal structure of gas giant planets may be more complex than the commonly assumed core-envelope structure with an adiabatic temperature profile. Different primordial internal structures as well as various physical processes can lead to non-homogenous compositional distributions. A non-homogenous internal structure has a significant impact on the thermal evolution and final structure of the planets. In this paper, we present alternative structure and evolution models for Jupiter and Saturn allowing for non-adiabatic primordial structures and the mixing of heavy elements by convection as these planets evolve. We present the evolution of the planets accounting for various initial composition gradients, and in the case of Saturn, include the formation of a helium-rich region as a result of helium rain. We investigate the stability of regions with composition gradients against convection, and find that the helium shell in Saturn remains stable and does not mix with the rest of the envelope. In other cases, convection mixes the planetary interior despite the existence of compositional gradients, leading to the enrichment of the envelope with heavy elements. We show that non-adiabatic structures (and cooling histories) for both Jupiter and Saturn are feasible. The interior temperatures in that case are much higher that for standard adiabatic models. We conclude that the internal structure is directly linked to the formation and evolution history of the planet. These alternative internal structures of Jupiter and Saturn should be considered when interpreting the upcoming Juno and Cassini data.

The Fuzziness of Giant Planets’ Cores

“…However, the primordial distribution of heavy elements could change during the planetary long-term evolution due to core erosion and/or convective mixing (e.g., Guillot et al 2004;Wilson & Militzer 2012;Vazan et al 2016). We suggest that the mass and composition of giant planet cores depend on their exact formation history.…”

“…Note that this is different from the model of Lozovsky et al (2017), where the regions with composition gradients that were found to be convective according to the Ledoux convection criteria were assumed to homogenize instantaneously due to mixing. However, homogenizing convective regions with composition gradients requires a fairly long time 10 10 years 7 9  -formation timescale (see Vazan et al 2016). The exact timescale depends on the mixing model, particularly, on the mixing length when using mixing length theory.…”

“…This means the heavy-element distribution postaccretion (at an age of several million years) is a direct reflection of how the planet accreted. This is not necessarily the present-day structure because convective mixing can spread the heavy elements further (Leconte & Chabrier 2012;Vazan et al 2016). Accordingly, the primordial fuzziness of the core of a giant planet is a direct consequence of the gradual change in the accretion ratio of heavy elements and hydrogen and helium over time.…”

Characterization of exoplanets from their formation

“…This is because it is currently unclear whether a centrally concentrated distribution of heavy elements (in a solid core or a strongly enriched inner region) at the beginning of detachment could shift to a more homogeneous distribution over time as heavy elements are soluble in H/He under conditions typical for giant planet interiors (e.g., Soubiran & Militzer 2015), or alternatively, that heavy elements sink over long timescale to the center. The results of Vazan et al (2016) indicate that no full mixing occurs if the initial compositional gradient is strong enough. If the heavy element mass fraction as a function of enclosed mass Z(M) in a planet today (see the Juno spacecraft data, Wahl et al 2017) therefore reflects at least partially the structure during build-up, this would open an interesting avenue to constrain the ratio of the solid accretion to the gas accretion rate as a function of planet mass,Ṁ Z /Ṁ XY (M).…”

“…For the other two, namely Saturn which is brighter relative to these models by about 60% ) and Uranus which is fainter by at least one order of magnitude (Guillot & Gautier 2014), additional physical effects must play a role that we do not consider here. These effects could be: a demixing of different chemical species followed by gravitational settling causing a net rearrangement of matter like a helium rain, which could explain Saturn (Stevenson & Salpeter 1977), compositional gradients (e.g., Vazan et al 2016) that can lead to semiconvection and nonadiabatic interiors, which could explain both Saturn (Leconte & Chabrier 2013) and Uranus , and finally core erosion where a part of the planet's luminosity is used to dredge up core material (e.g., Guillot et al 2004). …”